over bore

[k2000x]

what size is a standard bore on a vr250 cylinder and is it possible to bore it to take a 75mm piston

[cappra]

Standard bore size is 70.00mm.  Taking it out to 75mm might be a bit dicey!
73mm is the largest I have seen on a VR.

[Lozza]

Does the iron liner press out?

[cappra]

Yep, all Montesa's prior to 1985 had press in liners.

[runutz698]

Solo montesa has quite a few NOS liners!!! Just thought it would help if you wanted to take back to orginal.

Cheers
Nutz

[Lozza]

Does the liner form the inner wall of the transfer or is there aluminium there?

[Nathan S]

The liner is the inner wall of the transfer.

[JC]

OD of VR liner is 80.5mm. Running oversize pistons to 3mm liner thickness was common in the day. Its what Wiseco seemed to work on. That would mean 74.5mm piston. Liner material of VR is extrememly hard when you try to bore it, which probably indicates good quality. So it may be strong enough to take 75mm piston.  (Lyall O'Brien once told me its Rockwell hardness rating but I've long since forgotten it)

[k2000x]

hi guys just wanted to know as ive got my hands on a jet ski piston and with few mods looks like it could work its 75mm has 16mm pin and the pin centre to deck/top of piston is almost the same..i have a old cylinder that i was going to use so should i try it

[Nathan S]

Gotta be worth a try!

[JC]

Yeh, I reckon so too, esp since you have a spare barrell. And the liner can be replaced anyway.
Ossa liners were 2.75mm thick on std bore.
VR liner is full circumference under the transfer cut-outs, which will give a little more support where its needed too.
75 bore gives 283cc

[cappra]

Don't forget to have the head fly cut to the new bore size.

[mx250]

75 bore gives 283cc

[k2000x]

found a better piston to try...440 jet ski big bore piston 72.5mm this sounds alot safer than the 75mm

[Lozza]

Have you got the piston or just sifting through specs? Something to check is that the piston has no cutaways in the rear skirt.

I better say it

"but will it work?"