Piston size - Pocket Bike Forum - Mini Bikes
 
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post #1 of 11 (permalink) Old 2 Weeks Ago Thread Starter
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Piston size

So it looks like one of the clips that retain the piston on has failed and knackered the piston. Fortunately the barrel is ok. The piston that is already on there is stamped "c" but on the barrel is stamped "b" which should I replace with?
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post #2 of 11 (permalink) Old 2 Weeks Ago
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Usually, a b piston belongs in a b cylinder.
But it may be that the cylinder has already been reworked and therefore a c piston is installed. If so far there is no problem with the c piston, you can install one again.

Get your kick with 2 stroke mix
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post #3 of 11 (permalink) Old 2 Weeks Ago Thread Starter
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Well I've not had the bike long and I've not really had it running as I've been sorting out other problems with it
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post #4 of 11 (permalink) Old 2 Weeks Ago
Re: Piston size

You might need to mic the cyld and piston to see how much clearance you have. Is this a Blata or Polini engine??

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post #5 of 11 (permalink) Old 2 Weeks Ago Thread Starter
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Polini S2 air cooled
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post #6 of 11 (permalink) Old 2 Weeks Ago
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Definitely do what EPR says mic the cylinder and the piston and check the ring gap when you check the cylinder make sure you mic both the top and the bottom of the cylinder to make sure that there's no taper post the clearances here and the Euro bike guys will definitely help you out
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post #7 of 11 (permalink) Old 2 Weeks Ago
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Zocchi says 0,05mm clearance

Get your kick with 2 stroke mix
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post #8 of 11 (permalink) Old 2 Weeks Ago
Re: Piston size

That's almost .002", .001" all around, I believe. Pretty tight, lol.

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post #9 of 11 (permalink) Old 2 Weeks Ago
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Quote:
Originally Posted by EPR View Post
That's almost .002", .001" all around, I believe. Pretty tight, lol.
I'll measured 3 BZM barrels with the correct Piston and it looks like 0,05mm on radius.
That means 0,1mm at the bored diameter.

Polini has 4 piston selections
A = 35,95mm
B = 35,96mm
C = 35,97mm
D = 35,98mm

Get your kick with 2 stroke mix

Last edited by Chrisi; 2 Weeks Ago at 06:59 AM.
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post #10 of 11 (permalink) Old 2 Weeks Ago
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Re: Piston size

Quote:
Originally Posted by Chrisi View Post
I'll measured 3 BZM barrels with the correct Piston and it looks like 0,05mm on radius.
That means 0,1mm at the bored diameter.

Polini has 4 piston selections
A = 35,95mm
B = 35,96mm
C = 35,97mm
D = 35,98mm
How much of that is accounted for oil film thickness?

On my IAME engines there is a super tight bore to piston diameter but in order to keep it from killing engines we used to pre-warm the engines in the pits prior to a race...We also used to run the oil ratio at 32:1 which can be factored like a max of .001-inch or a lil less than...

If oil isn't factored in to the final sizing when honed than your scraping more and gliding less.....

We also had a max bore wear spec of under .004-inches,,,,,,,

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Last edited by CAM2; 2 Weeks Ago at 01:44 PM.
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post #11 of 11 (permalink) Old 2 Weeks Ago
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I can't tell you exactly, but Polini, BZM and the like are mostly driven with 1:50 mixture, with fully synthetic oil.
I recently had the case that an AA piston was installed in a 50cc BZM engine with a 00 cylinder and it stuck within a few minutes. I measured the cylinder and etched out the residues, installed a suitable 00 piston and the engine has been running without problems since then.

Get your kick with 2 stroke mix
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