CRM250SMR (using CR250/YZ490/KTM250SX internals)
Posted: 08 Apr 2012, 12:45
Hi Guys,
Being that my original thread is full of complete and utter crap including trash talking, disputes, vast time lapses of zero progress etc I have decided to build a new thread which will be far more concise and with a bit of luck gain a 'sticky' status in this forum area.
This build started of as me trying to buy a reliable workhorse for a road bike, I purchased a CRM250AR that I got a friend of a friend to look at but had not sighted it myself untill it was delivered.
first signs looked good but after a few engine dramas I decided to do a rebuild and was presented with some pretty horrible engine internals including a rod which was close to separating from the crank, a cracked piston, lots of rust in chassis and bearings and a host of other little nightmares.
I dont know why, but something clicked in my mind to build a bad arse SM.
my first concept with a little ill fated as I attempted to build a skinny tire motard using the stock rims and pirrelli scorpion tires.
The engine was to contain a TRX piston and a TRX rod.
After realising my errors: which were the fact that the AR had a larger bore than the TRX, I decided to abandon that idea all together and build an ever more 'stupidly mental' motard.
I decided to set my goal as "the worlds fastest CRM" and set off from there.
I started off by completely stripping down and rebuilding the bottom end while i contemplated the rest of the build.
during my investigations into CRM250 performance we concluded that no amount of porting would ever yield more than 40hp from a CRM250 due to the size of the exhaust port and being unable to build boost ports into the cylinder due to water jack positioning.
so we starting thinking about what cylinders could be used when I stumbled across this when comparing a heap of base gaskets I purchased for comparisons. The 2005 CR250 base gasket is almost identical in shape to the CRM minus the rear studs which are 3.5 and 4mm offset respectively. So it was decided that we were going to build a CR/CRM hybrid motor.
before any of this could proceed we needed to get the bottom end completed. but with MR honda wanting 899 dollars for a crank and rod, I decided to flip him the bird and build a better crank.
So I aquired the only decent rod that had a 25mm big end pin, while being light/strong and most importantly VERY LONG!
how long exactly? the rod we found was from a YZ490 which measures 137.5mm in length.
That is a wopping 12.5mm longer than a CR250 rod (125mm) and 5.5mm longer than a CRM rod (132mm). This will make for a very mid rangy engine that is also rev hungry. large rod/stroke ratios also have the benefit of putting less thrust on the piston skirts which results in longer piston and bore lives too.
So the YZ rod was ordered and shipped to Graham at Crank Rebuilding Services in Alora, Queensland, Australia. His work is bloody brilliant. He was able to widen my crank webs via grinding to accept the wider rod and rebuild the crank for $80 (AUD)
Being that my original thread is full of complete and utter crap including trash talking, disputes, vast time lapses of zero progress etc I have decided to build a new thread which will be far more concise and with a bit of luck gain a 'sticky' status in this forum area.
This build started of as me trying to buy a reliable workhorse for a road bike, I purchased a CRM250AR that I got a friend of a friend to look at but had not sighted it myself untill it was delivered.
first signs looked good but after a few engine dramas I decided to do a rebuild and was presented with some pretty horrible engine internals including a rod which was close to separating from the crank, a cracked piston, lots of rust in chassis and bearings and a host of other little nightmares.
I dont know why, but something clicked in my mind to build a bad arse SM.
my first concept with a little ill fated as I attempted to build a skinny tire motard using the stock rims and pirrelli scorpion tires.
The engine was to contain a TRX piston and a TRX rod.
After realising my errors: which were the fact that the AR had a larger bore than the TRX, I decided to abandon that idea all together and build an ever more 'stupidly mental' motard.
I decided to set my goal as "the worlds fastest CRM" and set off from there.
I started off by completely stripping down and rebuilding the bottom end while i contemplated the rest of the build.
during my investigations into CRM250 performance we concluded that no amount of porting would ever yield more than 40hp from a CRM250 due to the size of the exhaust port and being unable to build boost ports into the cylinder due to water jack positioning.
so we starting thinking about what cylinders could be used when I stumbled across this when comparing a heap of base gaskets I purchased for comparisons. The 2005 CR250 base gasket is almost identical in shape to the CRM minus the rear studs which are 3.5 and 4mm offset respectively. So it was decided that we were going to build a CR/CRM hybrid motor.
before any of this could proceed we needed to get the bottom end completed. but with MR honda wanting 899 dollars for a crank and rod, I decided to flip him the bird and build a better crank.
So I aquired the only decent rod that had a 25mm big end pin, while being light/strong and most importantly VERY LONG!
how long exactly? the rod we found was from a YZ490 which measures 137.5mm in length.
That is a wopping 12.5mm longer than a CR250 rod (125mm) and 5.5mm longer than a CRM rod (132mm). This will make for a very mid rangy engine that is also rev hungry. large rod/stroke ratios also have the benefit of putting less thrust on the piston skirts which results in longer piston and bore lives too.
So the YZ rod was ordered and shipped to Graham at Crank Rebuilding Services in Alora, Queensland, Australia. His work is bloody brilliant. He was able to widen my crank webs via grinding to accept the wider rod and rebuild the crank for $80 (AUD)