Engine Rebuild Guide

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Ewan67
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Engine Rebuild Guide

Post by Ewan67 » 24 Oct 2016, 20:39

Hello all,
Last couple of rebuilds I've done I referenced a guide on offwidth.co.uk that I downloaded. Now had a hard drive fail and although essentials were backed up, this guide wasn't. Went to download it again and offwidth.co.uk has disappeared. Don't suppose any one else has a copy they could let me have?

Cheers
Mk 2. Full DEP system. Boyesen Rad Valve and PWK33. Stan Stephens stage 2 tune. Wiseco piston. No balance shaft. Trailtech Vapor.

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Pete
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Re: Engine Rebuild Guide

Post by Pete » 29 Oct 2016, 22:45

Without a manual I have done most of this by simply following the description below of how to disassemble and reassemble. Copied and pasted without the pics and exploded diagrams. Lots of useful info about what to do and in what order, and stuff to watch out for.


Honda CRM250R Mark 2 Engine Rebuild Project
A detailed guide to rebuilding an off-road two-stroke single

Although this site deals specifically with the Honda CRM250 Mark II, since that is what I have, all two-stroke off-roaders all have very similar designs. The instructions here will see you through for almost any bike.

If you find this page useful, if you find errors, if you have more hints or if you just want a chat please do get in touch.

Introduction
In July 2003, for reasons that are not clear even to me, I bought a Honda CRM250 crosser from a boy-racer type for a shade over a grand.

Now, there are plenty of places on the internet and elsewhere that offer sensible, practical advice on what to look for when you are buying a second-hand bike. Perhaps if I had followed at least one of these tips, I would not have spent all winter in the shed. But then again I would not have the knowledge and skill to rebuild a (simple) engine, nor the satisfaction of riding a bike restored from an abused wreck to, well, to whatever it becomes when I have finished the work.

One problem I encountered with this job is the almost complete lack of literature available. The CRM is an import meaning the official service manual is in Japanese and is unobtainable anyway. There is a basic version with lots of exploded diagrams available here: Workshop manual (PDF 3.5M) (Happily available to download again thanks to a new web hosting package that doesn't complete suck.). If you find this manual useful, please do me a really quick easy favour but clicking on any of the google ads on this page that interest you. Doing this helps to pay for the hosting of this site!

No decent books seem to exist for beginners, most being aimed at tuners with a lot of mechanical experience, which is the main reason I am putting together this page. One investment I did make was a copy of Eric Gorr's Motocross and Off-Road Performance Handbook, which contains heaps of useful advice, but nothing about engine rebuilds that I won't cover here.


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Before you start
Is a rebuild necessary?
Sometimes it is obvious when you need to rebuild your engine; a seizure for example, or knocking sounds or metal shavings in the oil. Sometimes it is not so clear. But the fact that you are reading this means your crosser probably needs a rebuild. You may not have consciously realised this yet, because the brain is great at suppressing highly traumatic memories, such as child abuse and that time you were told your big end had gone. So first of all, be sure you need to rebuild. If you are in doubt, you probably do. By waiting, you make the job harder and more expensive.

Is it worth it?
So, is "is it worth it?" A failed engine has a depressing, terminal feel to it. The bike has suddenly lost most of its value, you can't even ride it and you probably want to chuck it in the nearest skip. This reaction is entirely normal, but once you have got over it take an objective look at the situation. Your options are:

Sell the bike as a non-runner
Break the bike and sell it as parts
Get the engine rebuilt professionally
Rebuild it yourself
Set fire to it and claim on the insurance
Options 1. and 2. will lose you a lot of money, and you may fail to find buyer(s) anyway. Option 3. is also expensive, perhaps £400 - £600 depending on the mechanic and there is always a risk they won't do a good job. Option 5. will cost you the excess plus increased premiums and could put you in prison.

Option 4, IMHO, is the best. It will cost you perhaps £200 to rebuild the engine, which is cheaper than any of the other options. It will take up a lot of time though you will also learn a great deal and improve your mechanical skill.


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What you need
A place to work
You are embarking on a long and complicated job, which is probably the most ambitious mechanical project to date and takes around 4-6 full days to complete. Do not rush it.

It is important that you have a place to work that is comfortable, spacious, warm, well-organised and bright. You will need a robust clear workbench and shelf space for the parts as you remove them. If you don't, make clearing out the garage/shed and building a bench your first task. Get a heater, kettle and one or two portable lamps in there.

Remember that mechanical work is potentially dangerous. Cuts and bruises are common, so wear gloves and keep a first-aid kit handy. Make sure someone knows what you are up to and can check on you from time to time, preferably with a cup of tea.

Tools
Surprisingly few special tools are needed, you will probably have most of these already, but if your existing tools are cheap or old and battered consider borrowing or buying a high quality set. Cheap tools are dangerous, frustrating and a false economy.

A metric socket set. From 8mm upwards. The largest nut on the CRM is 28mm.
A torque wrench
A set of decent screwdrivers
A plastic or rubber mallet
A special flywheel puller, or a general two- or three-legged puller about 4" / 100mm
An impact driver. I got away without this but it makes life easier.
A stand
The torque wrench and puller will set you back £30 - £50 so you may want to borrow those. If you don't have a stand (the type that fits under the engine and lifts both wheels, not a swingarm stand), you can easily knock one up out of wood or just use a milk crate or breeze blocks stacked to the right height with a piece of wood on top.

'Soft' tools
Workshop manual (PDF 3.5M). If you find this manual useful, please do click on any of the google ads on this page - this helps to pay for the hosting of this site!
Plenty of rags
Engine degreasant eg. 'Gunk'
A tube of 'Hylomar Blue ' gasket sealing compound
A bottle of fresh gearbox oil
1.5 - 2 litres ready-mixed coolant
Some 2-stroke oil
A tube of semi-permanent Locktite / thread locking liquid
WD-40
Some engine grease
Household items
Pad and pen
Many plastic bags of various sizes
Permanent marker
Gaffer tape
Lots of disposable latex / nitrile gloves
Fresh paraffin (kerosene / Avgas)
There's nothing there you won't find in your local supermarket and car/bike parts shop, and none of it should cost much money. Do wear gloves, or your hands will end up in a right state. Beware that paraffin dissolves latex gloves.

Parts
I would recommend not buying parts in advance. What you need to replace depends on what you find as you dismantle the engine. It is even possible that you will find the engine damaged beyond repair so let's get it knocked down before investing in the (overpriced) new parts.

CRM riders can get any and all parts from Leisure Trail, Oakleye Yard, Beech Ave, Long Eaton, Nottingham, NG10 2AX. 0115 973 2466. They are the CRM experts and although fairly expensive are very helpful. Others, try Crosserparts.com.

Parts that may be needed, with approximate prices:

Full gasket sets (top and bottom ends) £60
Piston kit (PROX) £80
Small end bearing £10
Crank rebuild with new con-rod & bearings ~ £150
Crank main bearings £20
Crank seals £20
Other smaller bearings, around £15 per pair plus seals.
Misc bits & bobs £20

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Preparing the bike
Start your notebook. It is a good idea to note down everything you do as you go along. This reduces the chances of forgetting a step when you reassemble the bike. Start now.

As you go along, put all the nuts, bolts and small parts you remove in plastic bags and gaffer-tape them onto the appropriate part, or label the bag and put it in a box. Tag it and bag it! If you have a digital camera you might want to take pictures as you go.

Clean. Give the bike a really good wash. Apply lots of 'Gunk' degreaser to the engine, frame, swingarm and other greasy areas with an old brush, then rinse with water. The aim here is to get the engine and neighbouring parts as clean as possible before you remove them from the frame. This makes the job a lot less unpleasant and reduces the chance of crap getting into the engine.

WD40 everything. Spray good old WD40 generously onto every nut. bolt and fastener that you may conceivably have to undo during the job. Pay special attention to the exhaust studs and swingarm pivot nuts. To be sure, do all of them. In fact the only place not to spray it is the brakes.

Remove panels. Remove the side panels (10mm bolts at top, screw at bottom), and rad scoops (2x 10mm nuts into tank, large screw at bottom).

Remove seat and tank. Remove seat by sliding it backwards and lifting off. Turn off fuel tap, detach fuel pipe from tank by loosening the jubilee clip and easing it off the fuel tap with a flat blade screwdriver. Prepare to catch any fuel that leaks out.

Drain trans oil. Place an old container under the engine. Loosen and remove the oil drain plug, which is located right on the bottom of the engine. Keep upward pressure on the bolt as you remove it to reduce the amount of oil running down your arm. Give it ten minutes or so to drain, then rock the bike from side to side to get out all you can. Replace the plug. Careful not to over-tighten or you can strip the crankcase threads.


Drain the carb. Loosen and remove the carb drain screw, which is located right at the bottom of the carb float chamber. Fuel will drain out so catch it with an old rag. Replace screw.

Drain coolant. Place a bucket below the water pump (front bottom right of engine). Remove radiator filler cap then loosen the coolant drain plug on the water pump (see pic). Coolant will gush all over your shoes. Rock the bike from side to side to drain as much as you can, then replace the plug.


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Disconnecting everything
Remove exhaust. Undo the nuts holding the exhaust onto the engine (2 x 8mm). These have a tendency to shear. If they seem stuck, soak thoroughly in WD40, tap gently with a hammer, leave for an hour and try again. Patience is better than a broken stud. Release the clamp holding the two parts of the exhaust together (if fitted), remove remaining exhaust mounting bolts and winkle the header pipe off the bike.

Remove spark plug. Pull the HT lead off the spark plug and secure it out of the way with a cable tie. Remove the spark plug, and stuff a small piece of rag into the hole.

Detach clutch cable. Put the bike in first, then loosen the clutch cable right off at the lever end. Align the slots in the adjuster to allow the cable to be pulled right out of the lever assembly. Loosen the 8mm nut securing the other end of the cable to the engine case, and unclip the inner cable.

Detach throttle cable. Trace the throttle cable back to the point where it splits in two. Slide the rubber boot out of the way and release the cable leading to the carb. You may have to slacken the throttle cable near the bar to do this.

Remove carb. Detach any small drain or breather pipes from the carb. Loosen the large jubilee clips on each side of the carb and gently winkle it out. The fuel line and throttle cable can be left attached.

Disconnect coolant pipes. Detach all coolant pipes leading to the engine, noting how they fit. Beware more coolant will probably gush onto your shoes.

Disconnect oil pump. Remove the plastic cover from the premix oil pump (front right of engine). Clamp the oil feed pipes and detach them from the pump. Use a suitably-sized bolt to blank off the pipes and remove the clamps. Detach the second throttle cable from the oil pump, and replace the cover.

Disconnect exhaust valve cables. Remove the cover from the exhaust valve (the thing labelled 'RS Valve', 2 x 8mm bolts). Detach the two cables, noting which way round they go. Replace cover and secure the cables out of the way.

Disconnect genny wiring. Trace the wiring coming out of the generator (big round bit on left of engine) up to two block connectors and detach. You may have to loosen the left radiator to get them free.

Detach gear lever. Before removing it, score across the end so you can fit it back at the same angle. Remove the clamp bolt and slide the lever off its splines.

Detach kick-start lever. Undo the bolt, not the screw.

Remove everything else. You may still have things connected to the engine on your bike, eg. choke cable, temperature sender and other wiring. Detach it all using your common sense! There should now be nothing attached to the engine except the chain and the mounting bolts.


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Disassembly in frame
If your tool collection includes such fancy things as impact drivers and rotor holding tools, you can go right ahead and remove the engine at this point, doing the rest of the disassembly on the bench.

The rest of us must partially disassemble the engine in the frame. This is to allow certain bolts to be undone by leaving the bike in gear and locking the back wheel. It's ugly but it works.

Left side
Remove sprocket cover. Remove the small cover concealing the front sprocket (2 x 8mm bolts).

Remove genny cover Evenly undo the many bolts holding on the plastic generator cover (the big round bit marked 'Honda') and ease off the cover.

At this point, grab the flywheel (the large metal rotor), and thrust it violently up and down and side to side. If you can feel any movement the crank main bearings will need to be replaced. If there is significant play, the bearings are already on their way out and may have taken the seals and big end with them.

Remove flywheel nut. Tricky. The large metal rotor is the flywheel, and it is held onto the crankshaft by a large nut. If you try to loosen it, the engine simply goes round. If you have a rotor holding tool, great, otherwise you need to put the bike in first gear and lock the back wheel so you have something to turn against.

You could get a volunteer to stand on the back brake. I put a rachet handle through one of the holes in the rear sprocket so that it rested on both sides of the swingarm. Take care not to bend the spokes if you do this.

Either way, be careful. The nut will be very tight and you will tend to wind up the suspension as you turn, threatening to ping the wrench across the garage. If it won't budge, soak it in WD40, tap it gently with a hammer and try again later. If all else fails, large electric or pneumatic impact drivers exists that will dow the job.

Pull flywheel. The fun is not over yet. Now you need to pull the flywheel off the crankshaft, and you can bet it is very well stuck on. A proper flywheel puller is best, but a 4" two- or three-legged puller will also work. Resist the temptation to wallop the flywheel, you could damage the crank or stator. If no joy leave it on and take the whole engine to a garage. Try not to lose the little chunk of metal that aligns the flywheel on the shaft (known as a Woodruff Key).

Remove stator. The stator is the crazy jumble of wiring behind the flywheel. Carefully remove it, along with the ignition pickups (small black boxes) and the rest of the wiring. The single black wire on the right is the neutral switch, which simply pulls straight off the engine.

Check the back of the stator for any signs of fuel or oil. This indicates that the crank seal has been leaking. and should be replaced.

Right side
Remove clutch cover. Evenly undo the many 8mm bolts holding on the clutch cover. Some are longer than others, so note which goes where. Carefully remove the cover. Do not attempt to prise it off as you could damage the seal. If it is stubborn, first check whether you have missed any bolts. Then tap firmly all the way round with a rubber or plastic mallet to break the seal. There is no need to remove the water pump or oil pump from the cover.


Here's what lurks behind the clutch cover...

Loosen crank end nut. The nut holding those three gears onto the end of the crankshaft (see pic above) needs to be loosened in the same way as you did the flywheel nut.

Disassemble clutch. Evenly undo the five bolts that hold in the clutch springs, and remove the springs. Pull off the cover to expose the plates. Note the spacer and needle bearing that lies behind the cover. It's a good idea to thread all the plates and other parts onto a large zip-tie to keep them in the right order.

Remove clutch centre nut. Yep, you guessed it, the 28mm clutch centre nut is another one of those buggers where you have to lock the back wheel. First flatten the lock washer tabs with a flat-blade screwdriver. Remove the nut and clutch basket and add it to your collection of clutch parts on the zip-tie.

Remove kick start mech. Pull the end of the kickstart spring out of its hole and withdraw the kick start mech as a complete unit. Thread a zip-tie through to keep it all together. Also withdraw the gear below that connects it to the clutch.

Remove balancer shaft. One final puzzle; how to remove the balancer shaft nut now the clutch has gone? I did this by jamming a socket between the balancer end weight and the crankcase (left end of shaft). However I am not officially recommending that method as you could damage the crankcase. Once free, pull off the gear and withdraw the shaft from the left side of the engine.

Remove front sprocket. Ease out the circlip that retains the front sprocket (or whatever device you have), and slide it off its splines. Winkle the sprocket out of the chain and remove.


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Removing the engine
Remove the right-side radiator and expansion tank. This just makes it easier to get the engine out. Note which pipes go where before removing them. Leave the expansion tank attached to the rad.

Undo engine mounting bolts. Remove the mounting nuts, bolts and brackets located at the front, bottom and top of engine. The engine will sit happily in place without the bolts.

At this point, the bike needs to go onto a stand as the weight needs to come off the rear wheel. It's a good idea to 'crack' the swingarm pivot nut first though.

Remove swingarm pivot bolt. Undo the 18mm nut on the left (it will be tight), and tap out the bolt with a suitable drift. Be ready for the swingarm to fall off when the bolt comes out.

At last!!. Clear a space on your bench for the engine. Lift the engine out of the right-side of the frame. You may want help here is quite heavy and it is quite fiddly to get it out. Sodde's Law says there is something you have forgotten to disconnect or remove, so don not wrench the engine out if you meet resistance, find out what is stopping it.


Give yourself a pat on the back


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Stripping down the motor
Withdraw clutch push rod and lever. There is a rod that passes through the middle of the clutch shaft. If it has not fallen out already, tip it out now. You can now lift the little clutch actuator lever from the left side crankcase (the bit the clutch cable attached to).

Remove crank end nut and gears. Don't lose the Woodruff key.

Remove gear selector shaft and mech. OK, I admit I am making up names for these parts now! Anyway, pull out the shaft in the bottom left corner, noting how it fits onto the shift mechanism to its right. Examine the mechanism carefully and note how it fits together. Remove the bolt securing the mechanism and remove it, watch out as there are two spring loaded pawls in there.


Removing the reeds and reed box

Remove reeds. Undo the 8mm bolts holding the reed box onto the engine remove it. Withdraw the reeds Take care not to damage them.

Remove head (optional). Removing the head is not essential at this stage, but it is easier to do it now while the cylinder is still attached to the crankcases. Undo the bolts evenly and note the way the gasket fits.

Remove cylinder. Evenly undo the four nuts holding the cylinder onto the crankcase. Strike the cylinder diagonally upwards with a rubber mallet all the way around, aiming to lift the cylinder off evenly. Do not pry it off as you could damage the seals. Just be patient and keep tapping it until it comes off. Lift the cylinder squarely off the piston.

Remove piston. The piston pin is held in place by a circlip on each end. Pop these out from their grooves and gently push or tap the piston pin out of the piston. Lift the piston off the little end and slide out the little end bearing. Try not to disturb the carbon deposits on the piston crown.

This is a good point to do a quick check on the crank. Before proceeding, slot the gear change lever back on and put the gearbox into neutral (confirm by checking that the front sprocket now turns freely). Now rotate the crank by grabbing hold of the little end and pumping it up and down as if you were the piston. Does it feel 100% smooth? Can you hear any scraping or scratching noises at all? If so, either the crank main bearings need replacing (fairly easy) or the big end bearing is worn (difficult). If there is any detectable side-to-side movement of the crank fly weights as they spin, it may have gone out of true and must be rebuilt.


The cylinder, piston and crankcases


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Splitting the cases
Now you are ready to split the crankcase halves and really get a good look inside the motor.

Remove crankcase bolts. Looking in the left side of the engine, you can see a large number of 8mm bolts holding the two crank halves together. Evenly undo them, noting where the longer ones go.

Split the cases. This step can be tricky, as the cases are held together by the friction fit of the crankshaft in its bearings as well as the old gasket. Use a rubber mallet again and strike the join apart all the way around. Once you have a significant crack, turn the engine onto its right side to try to keep all the gears in place. Keep easing open the cases with your fingers or a plastic or wooden lever. Some gearbox parts are bound to fall out so don't panic, but aim to keep everything together in the right engine case if possible. Gingerly lift the left case off the right and separate the halves.

At this point you may be interested in a slight diversion. Ever wondered how bike gearboxes work? How come they can turn when all the gears are engaged? Well, the answer is right in front of you and it's a cunning and elegant piece of design. Why not have a fiddle around and see if you can figure out how it works? If you have trouble, try looking here.

Remove gear shafts. If you were luckily, all that fell out when you split the cases were the gear selector forks. Put these, and the two shafts they sit on, in a bag. Carefully lift off both gear shafts, trying not to let anything fall off either end. Wrap them in string or elastic band to keep everything together. If you were unlucky and everything fell apart, see the diagrams in the service manual to help you get it all back together correctly. Email me for a copy of the manual.

Pull the selector drum out of its bearing (the cylinder with the weird grooves in it).

Extract crank. The crank shaft will be stuck in one or other of the crank cases and will probably need to be driven out. Support the case on pieces of wood and drive the crank shaft out with a hammer. DO NOT strike the end of the shaft directly, instead thread the nut most of the way back onto the end of the shaft and hold a piece of wood between the nut and the hammer. If you make a mess of the end of the shaft it will be very hard for an engineer to true the crank.

That's it! You have just finished stripping down the engine. Time for another cup of tea.


The gearbox just before disassembly
NOTE: A reader has correctly pointed out that I have stupidly labelled the gears the wrong way around. From the top of the picture downwards they should be: 1st, 5th, 3rd, 4th, 2nd, 6th.


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Inspection
I've already mentioned about a few simple tests you can do to evaluate what needs replacing. I'm no expert though, so I recommend taking the cylinder, piston and crank (and cases too if you can be bothered) to a good bike mechanic for inspection.

Starting from the top...

Cylinder head
Carefully inspect the head, head gasket and the top of the cylinder for any evidence of leaks or discolouration. If you find streaks of carbon or corrosion, is possible that the head and cylinder need to be re-ground to get them to mate exactly. This can be caused by the engine overheating, a nasty crash, or bolting the motor ruthlessly into a bent frame. Check that all the waterways are clear and not fouled by old gasket sealant or sludge.

Cylinder
The CRM250 (at least the mark II) does not have a plated cylinder, which is good news as you can have it rebored if necessary. Take a really close look at the inside surface of the cylinder for any marks, scorches or scratches. If in any doubt, and certainly if the piston seized, get it professionally inspected.

It is a good idea to always have the cylinder honed when rebuilding. This is not the same as reboring, it just scratches a very fine key into the surface which helps oil retention (or so I'm told). Hones are expensive so get a trustworthy mechanic to do this for you.

If you have an accurate set of calipers (to 0.05mm), measure the cylinder bore in several places. Otherwise get a mechanic to do it when you order your piston kit. The standard bore is 66.00mm for all marks of CRM250. Honda recommend a wear limit of 66.15mm, after which you should consider getting the cylinder rebored to the first oversize (66.50mm I believe). If you have an older bike you may well find that this has already been done.

Piston
The state of the piston can tell you a lot about your engine. But don't take my word for it, click here.

Measure the piston diameter, as this will tell you if the cylinder has been bored out in the past. If you look carefully at the piston crown you may see a figure eg. 1.00 that indicates whether the piston is oversize and by how much (in mm). It also says 'IN' or has an arrow so you can assemble it the right way around.

Looking underneath there are more markings. Mine says 'CR25088 PROX 1', which indicates it is made by Pro-X in to fit a CR250 bored out to 1mm oversize. Handy note: Pro-X do not sell pistons specifically for the CRM, but a CR250 or TRX-250 of the appropriate size is perfectly good. In my case I have a 0.5mm overbore which fits a 1.0m oversize CR250 piston.

The piston ring end gap can tell you how worn out the rings are. Pop out the top ring from the piston, and slide it squarely into the cylinder bore. Use a feeler gauge to measure the tiny gap between the ends of the ring. It it is more than 0.6mm the rings should be replaced. However, given the amount of effort we're putting in here and the cost of a piston kit, I'd say replace the whole shebang anyway.

Genuine Honda piston kits come in at around £110, but you can get equally good ones from Pro-X for about £80. These kits include the pin, rings and circlips but not the little end bearing. Since this is only a tenner, it's worth replacing it as a matter of course.

Crankshaft
Whether or not you need to have the crank rebuilt is the single biggest decision and cost of the job. If there was any indication that the big end bearings were worn (see above), or any indication that it is out of true then you will have to bite the bullet and get it done, or buy a reconditioned one if you can find one. Leisure trail can do the rebuild for you, I belive the cost is around £150. It's really not a DIY job.

Another worthwhile check is the clearence between the bigend and the tion that the big end bearings were worn (see above), or any indication that it is out of true then you will have to bite the bullet and get it done, or buy a reconditioned one if you can find one. Leisure trail can do the rebuild for you, I belive the cost is around £150. It's really not a DIY job.

Another worthwhile check is the clearence between the bigend and the thrust washers on either side of it. If you have a feeler gauge, measure this gap. Unfortunately I'm not sure what it is supposed to be, but excess clearance is bad news.

I advise getting the crank checked out by at least one good mechanic before deciding whether to go for the rebuild or not.

Reeds
Check out the reeds for any splits or frayed ends, and make sure they all close evenly. Reeds do have a limited life but are pretty easy to replace with the engine in the frame so new ones are not crucial at this stage.

Gear box
Take a close look at the gear selector forks. Are they perfectly straight and flat? Are there any grooves worn in them? Look at the gear shafts. Are the sides of the gears where the forks go nice and smooth? Are the gear dogs and dog slots square or showing signs of wear?

Worn parts should obviously be replaced, but they are difficult to find. Really there's no reason why the gearbox should be worn unless the oil was badly neglected or someone had a very heavy left foot.

Crank cases
Give the cases a really good clean. Use Gunk to get off grease from the outside, then rinse them in plently of clean paraffin and let them dry. Look over the cases for any sign of cracks or warpage. Don't be surprised if you see gouges where the crank flyweights turn, this is evidence of a previous piston / con rod / bearing faliure and isn't necessarily a problem.

Check all the bearings in the cases by turning them manually and feeling for any roughness or free play. Bearings are remarkably cheap, so if you are suspicious about any of them replace them. Some are friction fit like the mains, others are retained by screws and brackets.

Make a list of the parts you need as you go along, and don't hesitate to get professional advice. It's a waste of money to replace good parts, and a complete disaster if you don't replace parts that need it only to find out when everything is back together.


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Reassembly
OK, so you have your carefully organised parts, nice clear bench, all the new bits and plenty of time free? Time to put it all back together!

At this point the Haynes manual would say something really helpful like 'Assembly is the reverse of disassembly.' Well, yes, it is, but there are a lot of little things to watch out for. Use your notebook to make sure you don't miss out any steps or parts as you go.

Bearings
If you decided to replace the crank main bearings and seals you will need to drive out the old ones from their interference fit in the crank case. First prise out the old seals. Then support each crank case on pieces of wood and drive out the old bearings from the outside with a drift (metal rod) and hammer. They can be pretty hard to shift so be patient, and as my mechanic says one big hit is better than lots of small ones. Make sure you drive them out squarely, and if possible strike the outer race not the inner race or they might come to bits.

Getting the new ones in is also fun. Put the new bearings in the freezer for a few hours to make them contract, and if possible gently warm up the crankcases in an oven (but be careful not to melt anything). Wipe a thin flim of oil over the housing. Drop in the cold bearings (markings facing outwards) and quickly push them into place. If you are luckly they will push right in, but if not they need to be quickly tapped home. Use your rubber mallet and try not to strike the inner race or you could damage the bearing.

A reader has written in with this very helpful info on bearing replacement:

"To get the bearings out of the crank case just give it a good clean and pop it in the oven at about 120 to 150 degrees C. The bearings will just fall out - perhaps you will need just a gentle tap, but nothing major.

"If you plan everything correctly, then you can put the new ones in whilst the cases are still hot (especially if you put the bearings overnight in a freezer).

"You can then assemble the gearbox and (if you have put the crank in the freezer at the same time as the bearings) by the time you put the crank in, the main bearings will have heated up enough to allow everything to slide together really easily.

"If you drive the mains out when the cases are cold then you risk either damaging (cracking) the cases or you actually opening out bearing housing, over a period of time you can then get to the point that the bearings spin the their housings. - Dominic Webb

Gears, crank and cases
First make sure your cases are completely clean. Remove every last trace of the old gaskets with a plastic or wooden scraper or a fingernail. Flush them with clean paraffin and let dry. The tiniest speck of grit or swarf could get into a bearing and ruin it. Lighly oil all bearings and gear box parts with transmission oil.

Place the right side crank case face up on the bench, and reassemble the gear selector drum, gear shafts, selector forks and fork shafts. The forks are labelled 'L', 'C' and 'R' for 'left', 'centre', and 'right', but they are different sizes and will only fit one way anyway. Check that everything spins and you can select all the gears. See pic.

Drop the crank shaft into place and tap it gently into the main bearing. Fit the new gasket and make sure that the alignment dowels are still in place.

Now lower the left side case into place. This can be tricky as there are a lot of things to line up. Make sure the con rod doesn't get trapped. Tap it down gently and evenly, until it is far enough closed to refit the crank case bolts. Gradually tighten the bolts, keeping the spacing around the gasket even. You will find that it takes a fair amount of force to close the cases as you are pushing the crankshaft into the main bearings.

Before you completely close the gap, stand the engine upright and check the gearbox is working properly. You may find the selector drum quite hard to turn particularly between netural and second, but if you twiddle both shafts it should change. If in doubt, open the cases and check.

Once all is well, tighten the bolts evenly in a criss-cross pattern to close the cases fully. Remove the bolts again, place a drop of thread lock on each and refit. Torque them up carefully and evenly. I'm not sure what the official torque setting is, I tightened them up to 8 lbs-feet (11 Nm) and only one broke! Try 5 lbs-feet (6.8 Nm).

You will probably find the crankshaft will not turn at this stage -- don't panic as it needs to be aligned in the cases. Screw the nuts onto each end of the shaft so they protrude slightly. Using a piece of wood and a hammer, bang on the crank ends until it is central in the cases and turns freely.

If you are replacing the crank seals, tap them into place at this stage. The markings face outwards. Note that the genny side seal only needs to go in far enough to be flush with the housing, don't bash it all the way in against the bearing or it won't seal.

Trim off the new gasket where it crosses the reed box opening and protrudes into the cylinder.

Check your gears again for good luck!

Piston
If you have bought a new piston or rings, you will need to fit the rings onto the piston. Take a very close look at the piston rings. On one side, near the gap, you should see some markings, 'T' for top and maybe a number eg. '100' for 1.00mm oversize. These markings must face upwards. Looking even more carefully you can make out that one ring has a slighly tapered section, this is the top ring. Lube the rings with 2-stroke oil and fit them into the grooves carefully, don't open them up any more than necessary. If you have them the right way around they will move freely in the grooves.

The service manual indicates that genuine Honda pistons have a third ring nestling behind the bottom ring, but I have not played with a genuine piston as yet.

Now, look at the inlet side of the piston (marked 'IN' and having a window in the skirt). You should see a small notch in each groove, this is where the ring end gaps must be aligned. When you have it right you will be able to compress the rings all the way into the grooves with your fingers.

Take one of the two little wire circlips (always use new ones!) and insert it into into the groove on the left side of the piston. Make sure it is fully home, and rotate it so that the end gap faces straight down.

Oil the little end bearing with 2-stroke oil and slip it into the little end. Place the piston over the little end making sure you have it the right way around. The ring end gaps, port window or 'IN' markings should face the inlet side (back) or if there is an arrow on the piston crown it should face the exhaust side (front).

Take the piston pin and slide it through the piston and little end from the right. You may have to tap it gently home. Finally, insert the second circlip taking great care not to drop it into the engine. The piston should now swivel freely on the conrod.


Piston detail. This old piston was in good shape and I nearly refitted it, but it is now doing fine service as an ashtray.

Cylinder and cylinder head
Flush out the cylinder with fresh paraffin and let it dry before fitting it. This ensures there are no metal particles left from the honeing. Put a very thin layer of Hylomar blue gasket sealer over both faces of the cylinder-crankcase union and let it dry. Make sure you don't block any of the waterways. Drop the new gasket into place.

Oil the cylinder lightly with 2-stroke oil then lower it over the piston, exhaust port towards the front. Fitting it over the piston is quite fiddly, as you need to fully compress the rings into their grooves. An assistant is helpful here. Keep it straight and don't force it, and make sure the ring end gaps stay in the right place.

Once the cylinder is home, bolt it down lightly and check that the crank rotates smoothly. Some resistance is normal due to friction of the piston in the cylinder and the crank in the seals. If all is well, torque it down evenly to 4.3 kg-m (32 lbs-feet).

Put a very thin layer of Hylomar blue gasket sealer over both faces of the cylinder-cylinder head union and let it dry. Make sure you don't block any of the waterways. Drop the new head gasket into place, noting the little tab that is marked 'UP'.

Drop the head into position, thermostat towards the front, and torque down evenly to 2.7 kg-m (20 lbs-feet).

Gear selector mech
This is quite complicated and fiddly. Put the bike in neutral. The knob on the end of the selector drum and the dot on the detent rotor should both face straight up. The wheel on the spring-loaded arm should rest in the small neutral detent on the detent rotor. Watch out for the spring-loaded pawls. The bolt should be thread-locked and torqued to 2.2 kg-m (16 lbs-feet), but do a dry practice run first. Slide on the gear change lever and check you can select all the gears.


Gear change mech detail.

Everything else
That's the difficult stuff over! Now we get to the 'assembly is the reverse of disassembly' part. Just follow your notes or the description above. Here are the basic stages with a few pointers:

Replace reeds and reed box preferably with new gaskets
Replace crankshaft gears, woodruff key and nut and torque up to 8.0 kg-m (59 lbs-feet).
Insert clutch push rod and lever
Put the engine back into the frame
Replace swingarm pivot bolt and mounting bolts. Grease all bolts before fitting.
Replace radiator and expansion tank
Replace sprocket
Replace balancer shaft and torque up to 5.5 kg-m (40 lbs-feet). IMPORTANT NOTE: There is dot on the gear on the crankshaft that must align with a marking on the balancer shaft gear.
Replace kick-start mech
Replace clutch basket and plates, and torque up center nut to 7.5 kg-m (55 lbs-feet). Bend down the lock washer tabs.
Assemble rest of clutch parts
Replace stator and wiring
Replace flywheel and nut. Torque to 7.0 kg-m (52 lbs-feet). Handy hint: using two wrenches you can torque the flywheel nut and crank end nut against each other
Replace clutch cover. A new gasket is preferable but you might get away without.
Replace genny cover
Replace sprocket cover
Reattach kickstart. Check the motor kicks over smoothly with good compression.
Reconnect exhaust valve cables
Reconnect oil pump. Bleed the pump and make sure both pipes are full of oil and not air. If in doubt, start the bike with a small amount of fuel-oil premix in the fuel tank until you are sure the oil is getting through.
Reattach all coolant pipes
Replace carb, carb heater pipes and breather/drain pipes
Reattach throttle cable
Reattach clutch cable
Reconnect all electrics
Replace spark plug with a new one and torque to 1.8 kg-m (13 lbs-feet)
Replace exhaust. Use a new exhaust gasket if possible. Put plenty of copper grease on the studs.
Refill coolant. Fill the radiator first, up to the neck, then the expansion tank. Breifly detach the coolant pipe from the top of the cylinder head to check there is no air in the system.
Add transmission oil (about 700ml). On the CRM, there is an oil drain plug on the clutch cover. When oil trickles out of here you have the correct level.
Replace seat and tank
Replace panels

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Pre-flight checks
Before you turn everything on and start cranking away on the kick start, take a moment to do a few simple checks.

Are all nuts and bolts properly tightened? Are all parts accounted for? Are all coolant pipes secured? Drive chain tension OK?

With the ignition and fuel tap OFF, kick the engine over, feeling for good strong compression. Remove the plug and kick again, this time feeling for a baby-smooth turnover.

Remove the plug but keep the cap on, and hold it so that the end touches the cylinder head. Turn the ignition on, and kick the bike over slowly at first, and gradually quicker until you see a spark. You should be able to do this easily by hand not foot. If there is no spark, check the kill switch and genny wiring until you get one.

If all is well, it's time to fire her up! Don't forget the basics -- lights and kill switch OFF, fuel tap, choke and ignition ON. Be patient at first as it will take time for the fuel to come through.

Once it fires, and you have finished cheering, use the choke to keep the revs fairly low. Stay off the throttle. You may have to tweak the idle speed. As the bike warms up, check over it for oil and coolant leaks, loose things rattling, unusual noises etc. A bit of smoke is to be expected as there is lots of oil in the cylinder at this stage. Keep an eye on the coolant level.

Once the bike is fully warmed up, start riding it gently around on the flat. Use only slight throttle at first, and increase only graudally. Give the new piston time to adapt to the cylinder. After ten minutes or so, switch off and have a close check over everything once again.

If you are suspicious about anything, don't ignore it, investigate. You don't want all your hard work to be a waste of time.

If all is well, it's time to hit the road. A rebuilt engine needs the same run-in procedure as a new bike, so go gently at first, gradually using more throttle and revs. Give it a few hundred miles to run in before really letting rip.


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Congratulations!
Well done, you are now an official grease monkey! A member of that exclusive club of insatiable mechanical tinkerers. Before you know it, it will be you down the local breakers saying 'got any damaged repairables, I want a winter project?.' Now you will be able speak authoritatively on fueling, suspension settings and in fact any other mechanical subject not acutally covered here on the ground that you have done a complete engine rebuild. Well, perhaps not, but at least you are now wiser, more skillful and if you break down you only have yourself to blame!

If you found this page helpful, if you find errors, if you have more hints or if you just want a chat please do get in touch: crm@jeremythomson.co.uk.

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Ewan67
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Re: Engine Rebuild Guide

Post by Ewan67 » 30 Oct 2016, 20:54

Thanks for that Pete.
Mk 2. Full DEP system. Boyesen Rad Valve and PWK33. Stan Stephens stage 2 tune. Wiseco piston. No balance shaft. Trailtech Vapor.

gregorious77
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Re: Engine Rebuild Guide

Post by gregorious77 » 31 Oct 2016, 20:24

so when you do the rebuild take pics and the guide will be renewed yay


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