Last updated: 5/7/07
Welcome to the latest installment of my CRM250 top-end rebuild. After much trial and error, and a lot of help from Wiseco and Leisure Trail, I've gathered all the necessary parts. Next up is to send the crank off for reassembly, have the cylinder bored and exhaust valve trimmed, and have a little bit of porting work done on the cylinder to liven the girl up a bit. After a thorough search I've decided to use Scott Clough of SCR Racing to do my crank and cylinder work. Scott is quite a racer in his own right and comes very highly recommended. I'll post before and after pictures of the cylinder after I have it back.
The stock CRM piston is on the left, the Wiseco TRX250R piston on the right. Differences - the stock Honda item is cast while the Wiseco is forged, the TRX piston also benefits from an intake transfer port. I would also expect the more extensive machining on the Wiseco piston results in a more balanced piece. The Honda piston uses a 19mm pin, the Wiseco an 18mm.
The Wiseco TRX piston is over twenty grams lighter than the stock Honda item. As noted above, the pin diameters differ, as do the small end diameters of the rods for the two models. In order to use the TRX piston with the stock Honda CRM rod, use Wiseco wrist pin bearing # B1014 (18mm x 23mm x 21.8mm) to make up for the difference in the rods. If installing a new rod kit, as I am with my rebuild, you can specify a kit from Leisure Trail with a stock TRX sized small end and use bearing # B1003 from Wiseco, (18mm x 22mm x 21.8mm). A TRX rod kit will not work as the big end pin is 0.5mm smaller than the CRM crank opening. I tried unsuccessfully to locate a bearing that would allow the use of a TRX rod kit with the stock CRM big end pin, but in the end settled for the CRM rod kit from LT. The TRX rod would have saved me over $100 plus shipping.
So to summarize:
A Honda TRX250R, 1987-1989 model piston will work in a CRM250 with the stock rod by using the B1014 wrist pin bearing.
The above Wiseco piston carries part # 562M06600, where the last four digits are used to identify the bore size. Sizes are available from a stock bore of 66mm to 68mm in 0.25mm increments. The price is $130.63 for pistons below 67mm and $143.68 above. This includes a new wrist pin, clips, and rings. The bearing is ordered separately and costs $14.95.
The TRX250R piston from a 1985 - 1986 is 5mm taller from the center of the pin to the top of the crown and will not work, an important point to remember when ordering. (Thanks Wiseco for accepting my return with no restocking fees).
I also purchased a set of Boyesen two-stage reeds and an FMF silencer to help the engine breathe better. I expect to have it all back together within the next couple of weeks. I'll post pictures of the rebuild and riding impressions once it's running. The YZ250 I just acquired has set a high bar for performance, but I'm hopeful the little CRM250 will represent nicely.