Want to add a bigger bore to my YZ125, any advice?

beasty6

beasty6

beasty6

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Hey guys,

I have got a devent amount of cash for Xmas and want to put a fresh top end in my 2001 YZ125, while I am at it, I'm thinking about getting the cylinder bored and adding a bigger high compression piston, will I have to make any modifications? Or canot I simply slap a bigger piston in there and re-jet to suite?

Any info will be great, thanks
 
The YZ has a nikasil lined bore, so it will either need to be machined and have a liner fitted or will need to be replated after reboring. If you are increasing the bore by any significant amount, the head will probably need machining to fix the squish band and compression ratio.
 
you're better off just leaving it as is to be honest
its a moto-x bike, its in a high state of tune, trying to up the bore will just make it unreliable in the long run.
if you did do it your going to need to change the exhaust to get the real benefit as its tuned to it being a 125
if you want more power get a 250
 
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I'd agree, were it my money, I'd save it, sell the 125 and get something more powerful, in truth id not get a 250, not much of a jump and you'd be bored within a week.

My advice would be look at 600cc-650-675cc bikes, depending on what the bike will be used for.
 
You'd have to source a suitable oversized piston to go in there first, before even thinking about any re-boring. Plus adding on the cost of a re-plate on the cylinder.

Did know of a 250 Kawa (I think) which had a liner put into a previously plated cylinder but it wasn't very robust - the liner never sat flush with the deck of the barrel, and iirc the liner also moved within the cylinder once it had been run a few times.

I'd say you're better off looking to see if maybe a later model cylinder will fit yours, and gives any performance improvements.
 
ah ok guys.

i did look at getting a 250 2 stroke but they just tire me out to much on the track/trails, but the 125 is a lot more tame and fun to ride, just wanted to give it a little more grunt while keeping the reliability.

it has a full DEP pipe atm, so i might get a high compression piston for the standard bore, and get a vforce3 reed valve, what else can i do to get more power out my YZ?
 
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