Cylinder bore query

[shanejohn]

Hello , I'm hoping to get some information regarding my CRM250 , I have what I believe is the MK2 however I live in South Africa and spares are unobtainable I'm in the process of totally restoring the bike and I managed to find what might be the only other one in the country so I bought it to use as spares , the two bikes seem identical however upon removing the cylinder from each engine I found one has a 66mm bore and the other a 72mm bore , finding a piston and rings for the better condition 72mm bore seems impossible and I can't seem to find any info on the bike with a 72mm bore . Is there anyone out there who might be able to enlighten me as to why I might have a 72mm ?

[davecee92]

The cylinder has been overbored in the past. Heres a link to a company with all sizes im not sure on availability you would need to contact them
http://www.forged-pistons.co.uk/webshop ... iston-kit/

[knackeredMk1]

Overboring to 72mm would require some serious work and making a bespoke liner. Maximum piston size for a CRM is 68mm I took mine out to 68.5 or 68.25 (can't remember) and that required a special racing piston. If you bore out more than ~70mm you have no liner in places.

You should be able to get the 66mm bore rebored and use a TRX 250 piston as in the FAQs ................

[shanejohn]

Thank you for the response guys much appreciated, so obviously someone has done some work to this cylinder , I must say it looks absolutely factory which is why I thought it was, I will have to move forward on my project using the standard cylinder that I have , one last question if anyone can offer advice. The standard cylinder I have looks to be in great condition however there is one very small vertical scratch about 20mm in length just above the exhaust port I can hardly feel it , I know this is not ideal but getting re nickeled is not an option for me , I'd like to know if the bike will run like this ?
you guys talk about boring , but as far as I understand the crm has the nickel plated liner and if I was to bore this I would need to have it nickel plated again (not an option for me)
Please excuse my ignorance with regards to these things but that's why I'm on this forum

[davecee92]

Mk2 crms dont have nickel plated barrels its just a case of getting it rebored which is much cheaper.

[knackeredMk1]

Shane please read the FAQs before you ask questions. If they don't answer your need at least you will be better informed to ask a more pertinent question. - viewtopic.php?f=20&t=5267

[shanejohn]

I shall do that , thank you once again for your responses I've actually never joined a forum before so I'm unaware of the appropriate protocol

[shanejohn]

Okay I have read the FAQ's and have gleaned quite a bit but before I purchase a piston and prepare for a rebore I would just like to confirm something , I intend to use the TRX piston as suggested in the FAQ my bore is currently the standard 66mm , what size TRX piston would I need ? would it be a 66.25mm ?