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will i was riding my bike was running great for 5 hours then right after i did a light burn out and drove home one of the piston rings pop out of the grove and in between piston and cylinder so now i have to replate it and repair it but imma do a bore out 70mm how much more horse power will i get from it going up 2mm on the piston from 68mm 

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Posted (edited)

YZ250s had steel liners until about '90 or '91. You're increasing by less than 8ccs, a butt-dyno test will likely yield little to no gain. If anything the power delivery will change because you're going to an oversquare cylinder.

Edited by smirmeister
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I owned a 1986 yz250. It wasn't a plated bore and can be bored/honed for Yamaha oversize pistons, up to 3 sizes over, in .010" increments. You would be wasting your $ trying to make it any bigger, imo.

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Not much but will be fresh again so that will be good. I wouldn't go 2mm tho thats pushing it. Little as possible just go .5mm or .010

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I have new set of crank bearings and seals for that if need down the road

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Just now, Motox367 said:

I have new set of crank bearings and seals for that if need down the road

yea i found out y my piston rings got like tha 

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the dude who sold me the bike didnt tell me he was running a &%$#@!in 175 main jet when it was suppose to be a 350 like wtf man 

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1 minute ago, Savage wolf said:

yea i found out y my piston rings got like tha 

Alot of times you do the top end on old bottom its to much for old crank bearings and bigend bearing. Especially cause 86 has small crank bearings compared to 88+

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Just now, Motox367 said:

Alot of times you do the top end on old bottom its to much for old crank bearings and bigend bearing. Especially cause 86 has small crank bearings compared to 88+

the bike bottom end was fully rebuilt i have the paper work of it 

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1 minute ago, Savage wolf said:

the dude who sold me the bike didnt tell me he was running a &%$#@!in 175 main jet when it was suppose to be a 350 like wtf man 

Sure its got a mikuni? Not sure they make that lean of jet for it. Sure its not a keihn carb

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Just now, Savage wolf said:

the bike bottom end was fully rebuilt i have the paper work of it 

Well it should be split and cleaned then if cylinder got chewed up.  If want it to last anyway 

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2 minutes ago, Motox367 said:

Well it should be split and cleaned then if cylinder got chewed up.  If want it to last anyway 

the bike was runing great had smooth cylinder walls but the bike was running to lean because it had a 175 jet instead of a 350 

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Just now, Savage wolf said:

the bike was runing great had smooth cylinder walls but the bike was running to lean because it had a 175 jet instead of a 350 

Hmmm well lean is mean lol 

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Just now, Motox367 said:

Hmmm well lean is mean lol 

yea tell me bout it first time this has ever happen

Just now, Savage wolf said:

yea tell me bout it first time this has ever happen

im think of just gettin the cylinder replated std bore and the power valve cleaned to 

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No replating on it, they are a steel liner. (a few have said this already) Take it to your local engine rebuilder and they will tell you how much it needs boring, then get the appropriate sized piston and take it to them to get bored. Easy.

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Hmmm well lean is mean lol 
I've got an old piston in my shed with a hole burned through the top from a lean condition

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35 minutes ago, smirmeister said:

I've got an old piston in my shed with a hole burned through the top from a lean condition emoji38.png

I've got few of them here lol and every other condition you could think of ?

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