Well, IF you are also going to stroke the motor, then 74mm piston I think.

 

Could you explain how I would do that? I've rebuild engines before but not modified them.

 

What I mean is that you add a longer connecting rod for a longer stroke. For example, a lot of TW200 owners swap in The piston and barrel for a XT or TTR225. The 225 has a longer rod, than a TW200, so it won't make 225cc's, but it will make 219cc's I think. To install a longer rod involves splitting the cases and careful measure for interference for a TW, so most find a 225 and go from there.

 

I would say go to the search and have fun. I suggest just going to a 200cc. You could probably just swap the jug with a stock TW 200 piston, rings etc. Going up to the 250cc is a lot of work from what I have read.

 

I'd sell the 125 and buy a bike with a bigger engine, or just do a larger engine swap. It's not worth it to try what you'reconsidering, unless you're doing it to satisfy a craving to tackle a huge mechanical project.

 

Pretty much. Also they dont sell the TW200 here.

 

Just seen an SR250 engine up for £400... any thoughts on installing this with the TW125 output shaft and stator? I dont mind having to modify the engine mounts

 

it has been done, but that engine is quite different than tw engine. sr250 is almost the same as xt250(the old one)
I have some photos saved: - I don't think internals from this engine are interchangeable as xt/ttr225 are.

you can see the engine is longer, so you need to modify the frame, also the exhaust exits the head differently than tw does. both of these builds have the engine NOT center, because of chain allignment.

and here all the time I thought this was sr/xt250 swap too. but now I see it is possibly xt200/225 swap before placerlodes extension or trans modification. see the sprocket is offset.

 

I've also seen a couple of XT250 engines