Ok- Im thinking its time to put a new oversize piston in the 'old girl'- but thought that 'whilst I'm at it, I might as well go the whole hog and put a big bore in. I have heard that a good one will increase torque and a bit more hp and gives a smoother power spread, for not a lot more cost than a new oversize piston and bore. Any thoughts positive or negative??

never heard of a big bore kit for a crm

increased size will put alot more strain on the bottom end also theres jetting etc to sort out if you do find a piston that will fit.

My adivce dont bother just get hold of another 250 piston and then your ready to go

CRMs will bore out to 68mm (266 cm3) but after that you aree on your own. Stickman of this site has done a 290 as I remember but that needs a lot of work.

yeah dont bother doing it...

the hardest thing is finding an aftermarket piston that has the right gudgeon size...

there are many other simpler ways to extra more power than that, so its not worth it.


how damaged is your bore? could you just get it honed and put an 1st oversize in it +01mm (MK1/2/3 only) without replating?

http://wossneronline.com/forged_pistons ... cts_id=390

when my crm mk3 nipped on the bottom of the piston skirt I had it rebored and put a wossner piston in it and I am still blown away by the improvement in power its not the cheapest job but is easy enough to do. I would do it to a standard barrel if I had the money and wanted to tune the bike up a bit.

plus I didnt change the jets as it still ran fine so re jetting may not be needed but that was just my bike.

How much did the job cost superbob? Did you get a rebore and an oversized wossner piston??

Sorry Superbob but a Wossner piston on its own won't give a noticable gain in power/performance over any other well produced piston. What gave you the benefit was the rebore and new piston & rings because of a drop in performance from original due to a combination of shagged bore, rings and piston.

Yes I know what you mean I think a was tired when I typed it as I meant the the wossner is piston is good but as you said its the rest of the work what has made it really good.