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cylinder to piston clearance

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have a B cylinder for a yz 85. probably upwards of 80 hours on it. Don't have the recommended tools to check cylinder to piston clearance. want to determine if i should go to C piston. Any recommendations to get clearances with out telescoping feeler gauges/bore gauge?

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I don't think you'd have any issues using a C sized piston in a 80 hour B sized cylinder.

 

As an example, no matter if you have a low hour A cylinder on your bike

when you order a complete YZ125 piston kit from Yamaha, it includes a B sized piston.

 

On my YZ125, I put a B sized piston in my A cylinder that only had about 15 hours of use.

Just made sure I warmed the engine up gradualy before putting a load on it during break-in.

It looked fine 20 hours later when I replaced it.

 

Make sure you check/adjust the ring end gap. (feeler gauge required)

Edited by mlatour

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I don't think you'd have any issues using a C sized piston in a 80 hour B sized cylinder.

 

As an example, no matter if you have a low hour A cylinder on your bike

when you order a complete YZ125 piston kit from Yamaha, it includes a B sized piston.

 

On my YZ125, I put a B sized piston in my A cylinder that only had about 15 hours of use.

Just made sure I warmed the engine up gradualy before putting a load on it during break-in.

It looked fine 20 hours later when I replaced it.

 

Make sure you check/adjust the ring end gap. (feeler gauge required)

 

perfect, thanks man

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According to your other post it sounds like you had decided to try the C since you already purchased it.

 

https://thumpertalk.com/topic/1167668-fng-dropped-brand-new-piston/

 

If you choose to use that dropped piston, piston sizing won't be your main concern.

It should be ok but myself wouldn't chance it.

Edited by mlatour

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The difference between an A and D YZ250 piston isn't more than .00059" (.015 mm).  Exactly the same as the 125 pistons and 3 times smaller than the tightest recommended clearance (YZ250).  Probably similar for an 85.  I believe a D piston is recommended in all used cylinders, A,B,C or D.

Edited by bg10459

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As as comparison, this is from my 125's manual:

 

'A' = 53.957 - 53.960
'B' = 53.961 - 53.964
'C' = 53.965 - 53.968
'D' = 53.969 - 53.972

 

It's sometimes been said, grasp an A piston in your hand for a few minutes

and your body heat alone will make it expand almost into the B size tolerances.

Edited by mlatour
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...and if you want to get specific,

an A piston might measure as a

D, ...have seen major variences

with some piston manufacturers.

If you want to be safe, take the

cyl to a shop to measure,

only takes a minute.

About as tight as you want to be

on an MX 125 is about .0016",

with a cast piston. Forged pistons

are a more dense aluminum, and

will expand more when heated,

so 0020" is about minimal.

The worst thing though, is not properly

warming up a bike. Lotsa business out

there cuz people start a cold motor, rev

it to the moon. The piston heats up and

expands instantly, the cyl doesnt. The

piston just got bigger than the cyl bore.

Either a "cold stick ", or worse, the piston

cracks in half, and destroys everything.

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Never fails, everytime I go riding someone pulls up in the parking area, un-loads their 2-stroke and revs it to the moon as soon as it fires up... :bonk:

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