Mk1, 2 & 3 have iron cylinder liners with an original bore of 66mm. When worn they can be rebored up to 68mm with 0.25mm intervals. There are a lot of companies capable of reboring cylinders but make sure that they shape the powervalve to fit the new bore. There have been several problems on this forum created by people not doing this. Original capacity is 246cc which rises to 261cc at 68mm (maximum bore that pistons are generally available for) bore.

Original pistons are cast and quite expensive. Most owners buy aftermarket pistons, often forged, which gives better quality. Wiseco, Prox, Wossner pistons are usually what is fitted. Prox are the cheaper, being cast, and not quite as good as the others. Though Leisure Trail use these as standard fitment when they do any piston/rebore work.

Pistons are becoming harder to find, particularly those designed for CRMs only. There are however alternatives which in fact are thought to be better, particulaly Honda TRX 250 of years '87-'89 (which I use) and some talk of CR250 '88 (Though Prox TRX 250 '87-'89 pistons are labelled CR '88 on the underside of the piston). Please do check the suitability of pistons before you buy .

AR cylinders are electroplated with a slightly larger piston (66.4mm) allowing for very close to 250cc capacity. If this bore is scored then you have to get it cut away and replated. If you have to get your AR barrel recoated then you are probably better to get it done to more common size as 66.4mm pistons are getting rarer. So say 66.5mm so you can use a TRX piston. Just make sure that the piston rings supplied can be used on an electroplated bore and you can/do source a piston before getting the barrel plated.

It is very much worthwhile to get a new piston before you get the cylinder rebore/replated so that the machining can be done to fit the piston.

If you use a TRX piston with a standard CRM con rod you need a different (from CRM) small end bearing 18x23x21.8. Wiseco part no. is B1014.

Honda don't list conrods in their spare parts manuals. They list the whole crankshaft.

Leisure Trail are still the only company offering replacement conrods. They do all the work via a subcontractor. Their option requires going over to a TRX piston due to the little end size of the conrod.

There are other combinations of conrod/piston/bore diameter available but they are not commonly used.

http://www.johnnystoybox.com/rod_comparison.htm describes how a TRX '87-'89 cod rod was used when rebuilding a CRM engine. This is however not possible using this conrod as it is the wrong length.

When you get your barrel rebored check that the powervalve has been machined to match. There have been instances of the powervalve intruding into the cylinder and catching the piston/rings after reassembly.

Apparently Mk2 & Mk3 crankshafts are interchangeable as long as you swap the methods by which the drive side is attached to the gears. I.e. Mk2 uses a nut and Mk3 uses a bolt. If you use a Mk3 crnkshaft you must use a Mk3 clutch cover has the Mk3 crank is slightly longer than the Mk2 due to the bolt on the end rather than the nut of the Mk2.

Eddie Evans has done this in a swap of various parts of a Mk2 & Mk3 engines.

The Mk1 shaft is interchangeable with the Mk2, the only difference is that the Mk1 has a 'cush' drive for the primary drive gear (you can either use the 'cush drive' or leave it out). I suppose that Honda felt the Mk2 could do away with the 'Cush' drive and replaced it with a spacer.

Full engine rebuild (Mk2) - http://www.offwidth.co.uk/bike/crm_rebuild/

Useful part numbers when doing a rebuild -

Balance shaft seal 91201-KAE-003
Mk1 clutch basket lock washer 90445-KAE-000
Clutch casing gasket 11349-KAE-740
Head gasket 12251-KAE-740
Barrel gasket 12191-KAE-740
Reedbox gasket 14132-KAE-000
Thermostat 19300-KAE-023
Right hand radiator hose (bottom one to head)19063-KAE-000
Little end bearing 91015-KAE-003
Swingarm nut and bolt 90305-GE8-003 & 90161-759-000
nuts that hold the barrel to the block 94050-10000

Athena make gaskets for CRM250s. They are cheaper than Honda but quite difficult to get hold of.

Some piston differences. From Left to right - PROX TRX 250, Wiseco TRX 250, Wiseco 'Racers' TRX 250.

Intake -


Exhaust -


Side -


Underside -


Top


Differences relevent to CRM useage -

PROX - Cast piston, no balancing machining, no piston crown sealing rings, cast sides, conventional gudgeon pin set up.

Wiseco - Forged piston (stronger), balanced, machined piston crown with antidetonation grooves (better sealing), machined sides and high silicone content (more accurate surface & more oil retention), conventional gudgeon pin but with extra lubrication holes and lubrication channel.

Wiseco 'Racers' - As above but with better flowing intake piston skirt and different balancing to allow a longer gudgeon pin with greater resistance to side and twisting movement of the piston and less surface loading. Also coating on inside and crown to decrease localised heat build up and friction.

Mk3 conrod dimensions to the best of my measuring abilities (I strongly suspect all CRM250s are the same):

Little end
Internal diameter - 23.0mm
Width - 16.5mm

Length between centres
132.5mm

Big end
Pin 25.0mm
Internal diameter - 32.0mm
Width - 16.5mm

Washer width - 1.0mm

Please someone tell me if I have measured wrongly


I've found something very close to original spec.

KTM 250/300 90-99

Wiseco WPR135 (no made any longer but still around?)
PROX PX03.6310 (I believe but please check specs as I can't find)

Little end
Internal diameter - 22.0mm
Width - 17.0mm

Length between centres
132.0mm

Big end
Internal diameter - 31.0mm
Width - 17.0mm


Whether this is actually the same centre to centre I'm not sure but could it be mounted on the crank with a 0.5mm off set given the smaller big end?

Forumite Burnout1 did this -

Hi ,
Cool site, wish i had of found it before i embarked on this mission :
These were never imported into new zealand so we had to improvise with the following parts- (I think my bike is a '91 MK2 but not 100%)

I had my Cylinder Modified to fit a 2004 ProX CR250 rod and a 1997 ProX Oversized RM250 Piston. Needed to have some major cylinder Mods done and they had to take a fair bit off the top to accomadate the new stroke length. Got all the mains replaced and a crank grind too...

But wanna know something really funny???......
if you short shift it not in the powerband it still has that knock!!!