YZ125 Top End - OEM?

yamahuh
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yamahuh
Edited Date/Time 12/3/2012 3:03am
What is the best for performance, longevity, etc. I've been planning on using a vertex kit; however after reading the manual it references matching the piston to the cylinder based on the stamp and ordering the corresponding piston. In the past I always just ordered a top end kit after market and that was it.

Is there any benefit to ordering an OEM piston? For reference this is a low hour stock 06 YZ125 with exception of carbon reeds, +1 tooth sprocket, and a 420 mj. I run C12.

Any inputs appreciated
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project racer 8/4/2012 6:28pm
8/4/2012 6:28pm
I am sure you will get many different opinions on this subject. Mine is, no matter what brand of piston you use, if you really want to know where you are at clearance wise on the old piston to bore, as well as the new one, get someone who has a dial bore guage and a set of micrometers to measure the cylinder bore and piston (s).

This will also tell you how much taper there is in the cylinder bore, if measurements are taken in various locations in the cylinder.

A couple of thousandths either way can be critical in overall longevity and perfomance.
Falcon
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Falcon 8/4/2012 8:15pm
8/4/2012 8:15pm
Yamaha pistons come in A, B, C, and D sizes. There should be a stamp on the rear of the cylinder that indicates what size your cylinder is. The letters represent the variance in the bore due to different tolerances in the machining equipment at the factory - A is smallest, D largest. There are only 3 ten-thousandths of an inch between all the sizes or something like that, so clearly it doesn't make a huge difference.
The Yamaha race team used to run D pistons in A cylinders to get the tightest seal, but I tried that once and felt like it took too long to break in. Since then I always match the letters. Definitely do not put a smaller piston in the bore.
slipdog
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slipdog 8/5/2012 8:57am
8/5/2012 8:57am
For the best performance you would want to install a 57.96mm piston kit.

Your welcome!
jarret.stephen 8/9/2012 12:39pm
8/9/2012 12:39pm
i know iv had some weird issues with wiseco. but im pretty sure vertex makes those same sizes with a low friction coating and obviously cast instead of forged. millenium bored/replated my yz 134 only 16hrs later it let go on the face of a jump. i did proper break in then switched to 100% synthetic oil, proper jetting, always ran 93 pump gas. some how i landed( front fender first)

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MOTODAD422
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MOTODAD422 8/10/2012 10:42pm
8/10/2012 10:42pm
Vertex would be my choice,never had any good results with seizeco pistons. Also i believe in cast pistons as opposed to forged. Plus the Teflon like coating was always a plus.
Falcon
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Falcon 8/12/2012 8:29am
8/12/2012 8:29am
I was also happy with Vertex in my 125s.
Flatliner
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Flatliner 12/2/2012 7:57am
12/2/2012 7:57am
Just wondering about the piston sizes. When I went to the motosport website every kit only had the stock bore listed. No mention about a,b,c,d sizes.
Carlton_Fartman 12/2/2012 6:25pm
12/2/2012 6:25pm Edited Date/Time 12/2/2012 6:28pm
This is interesting... The microfiche for the individual pistons have the A B C D, AKA RED ORANGE GREEN PURPLE.

http://www.motosport.com/dirtbike/oem-parts/YAMAHA/2006/YZ125/CRANKSHAF…

I have used the complete kit from hondaeasttoledo, which comes with the B piston, and doesn't let you choose.
http://partsfinder.onlinemicrofiche.com/hondaeasttoledo/iframe.asp?make…
Jakes Dad
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Jakes Dad 12/3/2012 3:03am
12/3/2012 3:03am
project racer wrote:
I am sure you will get many different opinions on this subject. Mine is, no matter what brand of piston you use, if you really want...
I am sure you will get many different opinions on this subject. Mine is, no matter what brand of piston you use, if you really want to know where you are at clearance wise on the old piston to bore, as well as the new one, get someone who has a dial bore guage and a set of micrometers to measure the cylinder bore and piston (s).

This will also tell you how much taper there is in the cylinder bore, if measurements are taken in various locations in the cylinder.

A couple of thousandths either way can be critical in overall longevity and perfomance.
Read this again, it is a mystery until you follow this advice.

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