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Discussion Starter #1
Hi everybody,
I am going to fit new pistons in my bike
and because of that i have a question i hope you people can help me with.

Does anyone know how much the oversize is when the piston is marked with 1+.Is the piston1 mm larger in diameter than a original piston?
The bike is a 1982 550 gpz unitrack.

Thank's in advance
 

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I don't know. Are these Kawasaki pistons? Do you have new piston ring sets? And do you have the Kawasaki factory service manual? I would think you're going to need a good caliper & need to do some measuring, and I would also think you could/should measure the pistons.

I looked up the KZ550-H1 GPz pistons on Partzilla & eebbaaay.
The 13027-1058 OS pistons have 100 stamped into the top of them.
The 13029-1062 OS pistons have 50 stamped into the top & one seller says it's the 1st oversize.
The 550 manual says there are 2 oversize pistons available, 0.50mm OS & 1.00 mm OS.
Regarding the numbers stamped into the tops: I would assume 50 = 0.50 mm OS & 100 = 1.00 mm OS.

For comparison, the KZ550-A3 pistons are stamped with 0.50 & 1.00, and the parts diagrams more clearly differentiate between the two.

The 13029-1055 piston has 0.50 stamped into the top & listed as PISTON-ENGINE L,O/S 0.50.
The 13027-1053 piston has 100 stamped into the top & listed as PISTON-ENGINE LL,O/S 1.00.
I assume L = 1st oversize (0.50 mm) & LL = 2nd oversive (1.0 mm).
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Discussion Starter #3
I don't know. Are these Kawasaki pistons? Do you have new piston ring sets? And do you have the Kawasaki factory service manual? I would think you're going to need a good caliper & need to do some measuring, and I would also think you could/should measure the pistons.

I looked up the KZ550-H1 GPz pistons on Partzilla & eebbaaay.
The 13027-1058 OS pistons have 100 stamped into the top of them.
The 13029-1062 OS pistons have 50 stamped into the top & one seller says it's the 1st oversize.
The 550 manual says there are 2 oversize pistons available, 0.50mm OS & 1.00 mm OS.
Regarding the numbers stamped into the tops: I would assume 50 = 0.50 mm OS & 100 = 1.00 mm OS.

For comparison, the KZ550-A3 pistons are stamped with 0.50 & 1.00, and the parts diagrams more clearly differentiate between the two.

The 13029-1055 piston has 0.50 stamped into the top & listed as PISTON-ENGINE L,O/S 0.50.
The 13027-1053 piston has 100 stamped into the top & listed as PISTON-ENGINE LL,O/S 1.00.
I assume L = 1st oversize (0.50 mm) & LL = 2nd oversive (1.0 mm).
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Thanks for the quick reply martin.
Yes it's Kawasaki pistons.
I'm almost sure that i have to buy new pistons & rings because bad compression.The cylinder head and valves seems to be good.

I come to the same conclusion. I will measure the cylinders and pistons so I can order the right overzise pistons "L 0.50mm or LL1.0mm"

Once again thanks for the help martin!
 

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Engine work is beyond my abilities, but another possibility for the low compression could be the valve stem seals as they can get hard over time & may not seal properly.




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