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YZ85 Top End Piston Question

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hey, im going to buy a Wiseco piston kit for my yz85, but how do i know which one to get. which size?, and sometimes i see a letter like A or B or top of the piston? which 1 do i need?

and whats the gap between the ring need to be?

thanks:ride:

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Wiseco is one size fits all, so you don't need to worry about sizing.

Your manual should have the ring end gap in it.

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Daniel96. Not sure I fully agree with ajd187 just based only on my recent experience with our YZ85.

I replaced the stock cast piston in my 2007 YZ85 with the Wiseco forged piston in March. Look to look on the back of your cylinder head (i.e. carb side not exhaust side) to see if it is ink stamped with an "A", "B", "C" or "D" size cylinder. My understanding is that Wiseco makes pistons for each of the different cylinder sizes as they can vary slightly from the factory and are stamped as such. My cylinder was marked with "c" in black ink.

Since I had no experience replacing a top end, no tools to measure bore size, ensuring proper piston to wall clearance, and/or evaluating cylinder condition, I sent my cylinder to Eric Gorr at Forward Motion, Inc. (www.ericgorr.com) to have them order the right top end parts (I also had some porting work done to give it more low-end power). The piston kit was $85.00. The wrist pin bearing was $10.00 and the Cometic Top End gasket kit was $20.00.

Here is what I received:

Wiseco piston kit number PK1202 with the following parts:

Piston 805M04750 "C". I assumed the "C" in the piston numbering reflected that I had the "c" cylinder size or perhaps that is the one size fits all that ajd187 is referring to above.

Circlips CW14

Piston Ring 1869CS

Piston Pin S517

The Wiseco Wrist Pin Bearing number is B1012 and it comes separate from the piston kit. For $10 I can't see not replacing the wrist pin bearings when you replace the rest of the top end.

I found Eric's prices to be as good as any internet retailer, but I really needed someone's expertise to measure and evaluate the cylinder condition and whatever is required since I had read lots of threads about cold seizing a forged piston etc.

Regarding your ring gap question, I believe this information is contained within the Wiseco instructions sent with the piston kit or in the Yamaha service manual. If I recall correctly it is .010-.012", but definitely double check me.

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The letters (A, B, C, D) are for OEM pistons. They are that way because the cylinders are sized differently from the factory due to manufacturing tolerances. These are literally thousands of an inch and it doesn't matter ocne the item starts wearing. Hence Wiseco sells one size for a standard bore:

http://wiseco.com/ProductDetail.aspx?ItemID=805M04750&ModelID=467&ModelYear=2010&AppID=2618

There are other sizes, but those are for oversized bores. It is unlikely that you'd need one of these since it's unlikely that your bike was bored over.

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ive never seen a Ink stamp with a letter on my cylinder head? but i will have a look soon.

and is 47.5mm standard size that i need?

but when you go to buy a wiseco piston it doesnt say anything to do with letters A,B,C,D ?

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ive never seen a Ink stamp with a letter on my cylinder head? but i will have a look soon.

and is 47.5mm standard size that i need?

but when you go to buy a wiseco piston it doesnt say anything to do with letters A,B,C,D ?

It should be on the back of the cylinder. Sometimes they wear off and you can't see them any more.

I would assume it's the standard bore as long as your cylinder hasn't been bored. I dobut that it is, if you aren't sure, measure it.

Wiseco only has one size per bore. The A,B,C,D stuff is for OEM pistons. And usually if you were going to buy an OEM piston you'd buy a D.

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