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  #1  
Old 06-20-2011, 02:03 PM
coryR's Avatar
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Default should i replace piston yet?

     

I've had my 08 yz 125 since march of this year and have burned 30 tanks of gas through it.. roughly 150 hours.. I'm also unaware of how many were on it before i bought it. i mostly ride the local beach and mx track as they are within minutes of each other. i occasionally ride trails as well. The bike has a wiseco piston in it and i am mixing it with Lucas semi synthetic oil at 40:1 with stock jetting. there is a race coming up on July 2nd and i am thinking about entering, this will be my first time in a race and i wonder if it would be a good idea to change it before the race? my compression is at 150 psi which is pretty much the same as it was the day i got it.
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Old 06-20-2011, 02:10 PM
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I would probably re-ring the bike at least.... I change the piston every other ring change.

The first place your going to notice power suffering is off the bottom end, and not that your compression looks bad, but if I recall correctly from many of the other posts yz 125's are only one ring.

Thanks
Scott
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Old 06-20-2011, 02:21 PM
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If you're wondering about it, replace it.
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Old 06-20-2011, 02:22 PM
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Dang thats alot of hours, I do mine every 10 but like roost stated I do a ring then a complete. And all 125s Ive ever rebuilt have 1 ring. Compression looks decent but i would do it as a preventive measure
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Old 06-20-2011, 04:30 PM
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Take your Yamaha's shop manual and find the ring gap dimension and go into the top end, since you need to anyway. Put a ring in the cylinder near the top and again approximately at the bore mid point, slide the piston in against the ring from underneath to square the ring properly. At each position measure the gap with a feeler guage. If she exceeds the tolerance you need to, at a minimum, re-ring the bike.

With that many hours you know of, and who knows how many additional before you bought the scoot, I'd get a top end kit and replace the piston, rings, circlips, wrist pin and top end gaskets. It will be well worth your effort to keep a super neat scoot running crisply.

Best of luck, let us know what you find out and decide to do.

Bill
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Old 06-23-2011, 08:03 PM
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how can I tell what size piston I need? not sure if it has a regular or overbore piston in it.
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Old 06-23-2011, 08:16 PM
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Quote:
Originally Posted by coryR View Post
how can I tell what size piston I need? not sure if it has a regular or overbore piston in it.
Chances are that it is the stock size the best way to tell is a micrometer or a machine shop....

Thanks
Scott
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Old 06-24-2011, 05:29 AM
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Like Scott said, a micrometer will tell you the current bore. The piston should have markings to go by too. Possibly on the top, you may need to clean off some carbon to see them. The shop manual tells you what the stock bore is and how to correctly measure it at several different points along the cylinder and at 90 degree angles to each first location in the cylinder.

Bill
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Old 06-26-2011, 08:27 AM
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I would Definitely change Piston
these bikes are known for breaking rings and destroying cylinders
Piston change is way cheaper and less headache than the alternative
OSF
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