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New piston on a YZ

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On another post we were talking about stock and aftermarket pistons. someone brought up the markings on the cylinder that determined what piston you need to put into that particular cylinder. What if your cylinder has no markings? What piston would you order from Yamaha? I used a weisco the last time because I was in a hurry and didn't have time to research which piston to order and went that route. I bought a new cylinder and it has no markings either.

I see there is an optional piston kit from Yamaha that is listed separately from the other 4 sizes. Is that a fit-all piston that will work in any cylinder? It's about time to do another top end and I want to have all the parts I need before I start so I don't miss a race.

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It really doesnt matter too much. I mean the difference between the A piston and the D piston is so little that you can expect a D piston to measure about the same as a A piston in just a few good rides of break-in.

If it was me and I was unsure, I would just put in a D piston (larger piston=tighter clearance) just because it would make me feel a little more warm and fuzzy inside.

Just a tip to the 06 and 07 YZ owners... The paint they started using on the cylinders starting in 06 is smooth and shinny. Because of this the cylinder size marking (A-B-C- or D) doesnt stay around for long and fades away. So make note of what your cylinder is while the marking is still there.

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It really doesnt matter too much. I mean the difference between the A piston and the D piston is so little that you can expect a D piston to measure about the same as a A piston in just a few good rides of break-in.

If it was me and I was unsure, I would just put in a D piston (larger piston=tighter clearance) just because it would make me feel a little more warm and fuzzy inside.

Just a tip to the 06 and 07 YZ owners... The paint they started using on the cylinders starting in 06 is smooth and shinny. Because of this the cylinder size marking (A-B-C- or D) doesnt stay around for long and fades away. So make note of what your cylinder is while the marking is still there.

Ok then, what is the optional piston kit? Is that an even larger piston than the D? I am so :thumbsup:

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Double check with the shop, but I believe the replacement pistons are "B" sized.

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Double check with the shop, but I believe the replacement pistons are "B" sized.

I never had much luck dealing with the shops around here. There is a guy at the track that works on all kinds and seems fairly knowledgeable and I asked him that question before and he didn't know what the optional kit was. He told me about the same thing as UpAllKnight said. The pistons are all pretty close in size a few thousandths is all it is between them. If I can't find out what the deal is with the optional kit (which I have two of now) I'll probably go with the one in the middle and see what happens or possibly just get another Weisco.

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It does'nt take much dirt to cover the letter on the back of the barrel, sometimes a good clean with petrol or similar will remove the grime to enable the letter to become clearer.

What have you done with the old barrel Olddude?

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It does'nt take much dirt to cover the letter on the back of the barrel, sometimes a good clean with petrol or similar will remove the grime to enable the letter to become clearer.

What have you done with the old barrel Olddude?

the old cylinder is still on the bike. I just bought a extra in case of a emergency where something screws up at the track so I can get back in the race without having to be out of action for a week while a new one comes in.

And believe me, there is no mark on the old one at all. I have gone over that thing with a fine toothed comb. The weiscos seem to do ok but I would like to stay stock if at all possible. The shops around here wont tell you anything. All they want to tell you is how much the bill is when they are done.

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The only way you could check it properly would be to measure the bore and then match the piston as per the manual recomendations. You could do this on your spare barrel then order the piston for the spare and then replace both piston and barrel at the same time.

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The only way you could check it properly would be to measure the bore and then match the piston as per the manual recomendations. You could do this on your spare barrel then order the piston for the spare and then replace both piston and barrel at the same time.

What you said is totally correct and true. I guess what me and others are wondering is Why do they make 5 different pistons for the same bike engine and Weisco has just one size and it seems to work so well.

My main question has to do with what this optional piston piston is. Come on there has to be someone out there with Yamaha credentials that could answer this question for me.

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