What bore should the barrel be, on a Sherpa 250 198a? The barrel and head on mine are from a much older bike and I’m looking at getting a replacement. Need to make sure I get the right thing. Thanks for any advice

Standard was 70mm which was 238cc. The earlier "250" Sherpa T engine was 72mm bore standard. Pistons are generally only available in 72+mm sizes now. The 198 and later cylinders can be bored to suit 72+mm pistons without needing a bigger sleeve.

One thing that is visually distinctive about the 198 and later "250" Sherpa Ts (198A and 198B) is that they use a metal head gasket instead of the earlier design that had the head seal directly against the cylinder sleeve (so the sleeve on a 198 cylinder does not protrude). The gasket type head design may have first come out on the model preceding the 198 but I don't know for sure.

I can do photos of a 198 cylinder and head disassembled if you need it. 198A is the same as 198.

I`m pretty sure they were a 71mm bore for the 238`s David. The 212 Alpina`s were also a 71mm bore. I know the M158`s are a 71mm standard bore. Or the Australian ones were anyway. The one I owned was 71mm standard. Graham.

30 minutes ago, bullylover said:

I`m pretty sure they were a 71mm bore for the 238`s David. The 212 Alpina`s were also a 71mm bore. I know the M158`s are a 71mm standard bore. Or the Australian ones were anyway. The one I owned was 71mm standard. Graham.

Sorry Graham yes I was wrong. I've been working on too many engines lately

Some great replies chaps, thanks very much. So if I change my barrel back to the correct age related, I will need to change piston too? Or would this be good practice anyway? 

That depends on a couple of things:

The "new" cylinder may come with it's own piston.

The condition of your existing piston and rings. Do they have enough life left to be worth it?

Is the "new" cylinder diameter small enough to be bored to suit your piston? It's likely that a 198 cylinder will have been rebored at least once by now.

My own choice would be that if a new piston was available to use as part of an engine rebuild, I would use a new piston to reduce the risk of having to do the job again.

Excellent. cheers