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big bore for a 125

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I have an 2002 CR125, as of right now it has a professionally tuned/jetted carb, v-force 3 reeds, wieseco high compression single ring piston, and a full fmf exhaust. I want to go big with it this winter. I was looking around and found out that the biggest i can go is 139 on the OEM jug because of the thin cylinder walls on the 02's. This is all sort of new to me with the big bore kit. The jug on it right now only has 1 season on it. Should i sent out the jug to get it bored and a port/polish on it, or just get a brand new jug. And where would i go for either one of these. Thanks for any advice

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As with any 2T, you're limited because of the ports/passages.

A Wiseco high comp piston????? I didn't know there was such a thing.....I thought a 2T piston was a 2T piston (not having compression options).

You have the jetting , the reeds and the pipe, but still other areas that can be improved upon.

Oil type and ratio?

Most people run a 32:1 ratio, which in my experience has resulted in a blubbery/smoking engine.

I've found that MC1 at 50:1 offered the best performance.

If you change oil ratios, you might have to rejet a tad also.

Clay checking the piston-to-head clearance will verify how much you can bump the compression by milling the head. Adding more compression is the best power mod for any 2T.


Tweeking timing can have it's benefits also.

Advancing the timing a degree at a time might add a little, maybe not.

Usually a small change is all that's needed, getting greedy here can have bad results.


Swapping to a CR80 CDI box can have benefits also. The smaller engines sometimes have better timing curves which COULD alter performance.


Taking weight away from a flywheel will allow the engine to rev quicker. You might loose some pull during a holeshot, but it'll allow for the engine to wind up quicker.


Choose your favorite track, make a few hot laps, and pay attention to your gear selection and RPM.

Are you pulling maximum RPMs down that long/flat straight? If not, it needs to be changed.

If you're pulling the fastest section in 4th/5th? ......you can gear down (smaller front or bigger rear).

Custom cylinder work will get expensive quick. Other little tweeks are cheap and can reep big benefits.

Just make sure that you make one change at a time and verify each result.

For instance......

I had a CR250 w/every mod imaginable (pipe, bigger carb, porting, reeds, etc...).

I clay checked the piston, milled the head accordingly for maximum compression....the thing went from so-so to ballistic. The w/revised jetting it became even more crisp, and faster.

Hope this helps,


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