220 piston - correct size

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99sr220
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220 piston - correct size

Post by 99sr220 »

I bought a Wiseco piston, standard bore 69mm, however it doesn't fit. The OEM one does. I see Wiseco sells 70mm pistons, however the mechanic feels this is too big. Why wouldn't the standard size fit when the OEM does? Is there anyway of looking at the old piston to see which one I should get?

As a short term measure, the mechanic installed a used OEM piston from a spare motor I'd bought. I'm living in Thailand, although I speak the language there can still be communication problems, it was only after I'd got the bike back and ridden for a day that I found out the OEM piston had been used.

Comparing the head on my bike, with the one on the spare motor, the mechanic says it's had a squish mod, it also has engraving on the cylinder, so I would guess it's been ported. Getting a new piston means importing, which is slow. I'd like to ride at least once before the new piston arrives, but am worried about the OEM piston breaking. Any advice?
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Julien D
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Re: 220 piston - correct size

Post by Julien D »

Are you certain the piston in it is stock, and OEM? Does it have any markings on top? What does it measure out to?
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99sr220
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Re: 220 piston - correct size

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The bikes at the mechanics, so I can't get to it for a week. However the piston is from an SR motor imported from Japan, I'd be extremely surprised if they'd installed anything other than stock. The piston fits the spare motor and also fits my bike.
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220 piston - correct size

Post by rbates9 »

Might be Wiseco just sent the wrong piston?
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Julien D
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220 piston - correct size

Post by Julien D »

Yep. Check the part number on top of the Wiseco and see what it is.
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99sr220
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220 piston - correct size

Post by 99sr220 »

Thing is, when I bought the bike it was a heap, ran terribly. I had a few things fixed, including putting in a Wiseco piston, but this was done by a very rough mechanic, who either never noticed or didn't bother to tell me the piston was too small.

The number on the Wiseco installed by the old mechanic matches the number on the new piston, so it would be unlikely that the same mistake was made twice by different ebay sellers.

Neither of the Wisecos are with me, but I still have the old OEM from when I bought the bike.
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220 piston - correct size

Post by 99sr220 »

I just googled and found a thread on TT (can I post it?), where they are talking about the Wiseco piston being too small, making the engine noisy and piston slap. Anybody heard of this before?
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Re: 220 piston - correct size

Post by Julien D »

Not really. Forged pistons expands somewhat slower than the stock cast pistons, so cold clearance is there for a longer period. That is why proper warmup is critical when using any forged piston.
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220 piston - correct size

Post by 99sr220 »

Although i hadn't ride it for a while, the engine sounds much quiter than i remembered. It is definitely running better, especially on idle, before it was difficult to get a decent idle, now it's great and never stalls, which was a problem before. Starting could also be tricky before the change. Although this could be due to switching carbs, which I also had the mechanic do (change tto 35mm).

Thanks for your help, any more suggestions would be much appreciated. The mechanic could be missing something, he is very good with 2t, but cannot read English, so maybe it was something in the installation in instructions he missed?
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220 piston - correct size

Post by 99sr220 »

Do the OEM pistons come in different sizes? I am reading that they come in A, B, C etc. and this is for different stages of cylinder wear and that there are also A and B cylinders - however I think this is for 200s/E models?

My bike is a 97 model and I'm guessing has been worked fairly hard, maybe this explains why the Wiseco looks too small.

The spare engine is in much better condition, so I'm wondering if swapping the cylinders is likely to solve the problem?
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Re: 220 piston - correct size

Post by Julien D »

The difference in A, B, C, and D pistons is very minute. For example, even if your cylinder was on the tightest end of spec, the D piston would still fit. Wiseco pistons are not sized incrementally like the stock/pro-x pistons are.

You are saying the wiseco seems small. By "will not fit", I assumed it was too large. So, it does fit, but you are concerned that the clearance is too great? Do you by chance have, or can you get, the measurement from the piston and the current measurement of your bore size?
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Re: 220 piston - correct size

Post by 99sr220 »

Julien D wrote:The difference in A, B, C, and D pistons is very minute. For example, even if your cylinder was on the tightest end of spec, the D piston would still fit. Wiseco pistons are not sized incrementally like the stock/pro-x pistons are.
This is what I thought.
Julien D wrote: You are saying the wiseco seems small. By "will not fit", I assumed it was too large. So, it does fit, but you are concerned that the clearance is too great?
Correct, except it's actually the mechanic who's saying it's too small, all work is being done by him.
Julien D wrote: Do you by chance have, or can you get, the measurement from the piston and the current measurement of your bore size?
The piston is 69mm, I don't know the bore size and doubt the mechanic has a bore gauge.

A big part of the problem is he doesn't believe there could be anything wrong with the OEM pistons, as 220s are rare here. Considering the mods to the engine, I'm not keen to ride it with an OEM, which he thinks is crazy.
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Re: 220 piston - correct size

Post by Julien D »

There is definitely something wrong with the OEM piston. You are right to be wary of using it. I am finding it difficult to imagine that a standard OEM piston fits, but a standard wiseco has too much clearance. Just something I haven't seen before.
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Re: 220 piston - correct size

Post by 99sr220 »

Googling a found a small reference to the Wisecos being a tad small and being noisy, but nothing firm.

I'm thinking the only solution is to print pics of broken pistons and the damage they've caused, maybe that will convince the mechanic. He used to work for Kawa and 15 yrs ago won the national enduro championship on a KDX200, so you can understand why he's reluctant to believe the piston issue just because someone says the read it on the internet.

Is it possible the cylinder has worn enough to make the Wiseco seem to big?

The bike is running much better since he worked on it, before it wouldn't idle well and was stalling, but this could be explained by the change of carb.
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Re: 220 piston - correct size

Post by Julien D »

If the plating is there and in tact, it is unlikely, but possible. Without a proper bore gauge it is impossible to say. FYI, the stock 200 piston does not present any issue, only the 220.
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