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CT2 Piston ID

23 Oct 2020 15:32
bradocross
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CT2 Piston ID #1
I'm starting an engine rebuild on my CT2 (lots of gasket and seal leaks). I pulled the top end and found a lot of blow by (needs new rings), some scoring on the piston (needs a new piston) and some scoring in the cylinder (needs a bore). I don't have a micrometer to mic the cylinder but the intake side of the top of the piston is marked 96 "space" 8 at the exhaust side of the top of the piston is a big "6" below the arrow with a small "8" next to the "6". I believe that a "96" is a standard size Yamaha piston so is this a 96 "point" 8? Does that indicate that it's 8th over (I thought there were only 4)? I thought Yamaha pistons increased by .25mm and not 10mm? If I'm maxed out then what are the odds of finding a serviceable, standard bore cylinder out there?
Thanks in advance for your help and advice.....

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23 Oct 2020 16:04
Swoop56
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CT2 Piston ID #2
That's a Std size piston ( 65.96mm ) . From what you say , it is in need of a bore .
You should probably send it to a reputable engineer to check if a first oversize ( .25mm )
piston will clean up the bore , or whether you need to go to second oversize ( .50 mm ) .
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23 Oct 2020 16:28
bradocross
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CT2 Piston ID #3
Here are the pictures.
The 96 is standard but this is a 96.8....

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23 Oct 2020 17:54
MarkT
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CT2 Piston ID #4
Standard piston. Nominal standard bore size is 66mm. Marks on standard piston says it was 65.968mm when new, slightly smaller than bore.

On the assembly line the pistons were graded by exact size carefully and so were the cylinders. Sometimes you can see the ink mark sizing the cylinder where the sleeve sticks out of the bottom. Then the assemblers would select a piston that would provide the correct clearance for that cylinder.

Oversize pistons can still vary slightly in size from one to the next. But they are only marked with the nominal oversize... example "25" for 0.25mm oversize. It is expected that the machinist have the piston in hand so piston can be measured and then the machinist sizes the new bore to match that specific piston and provide the correct, very precise clearance.

I have a friend that had a twin bored a few years ago. Machine shop returned cylinders with pistons in bores. During clean up and assembly, pistons got mixed up... both were the same oversize, no problem, right? Wrong.

One side seized. Other side was too loose. Machine shop said there was about 0.001" difference in the size of the two new pistons (Both OEM, same size) so mixing them up was a problem.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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23 Oct 2020 19:47
bradocross
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CT2 Piston ID #5
So any piston that starts with "96" is a standard piston. I see, after perusing ebay, that some oversized Yamaha pistons have just the oversize number stamped on them (.25, .50) and some have 66.25 or 66.50. Confusing to the 1st timer though. Thank you for the good news, that I'm still on the standard bore and have some room to grow with my CT.

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23 Oct 2020 20:03
MarkT
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CT2 Piston ID #6
It's amazing how many CT cylinders are still on standard bore... I think I have three of them and two of them are still standard and within allowed specs still. Great bikes.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

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24 Oct 2020 13:39
bradocross
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CT2 Piston ID #7
Thank you Swoop56 and MarkT for your help! I'm knee deep in the rebuild now............

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