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Does Piston clearance affects compression?

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Hey folks im.having a bit if difficulty with low compression on a 2005 rm 125. I checked compression and got about 90. I threw in a wiseco and its reading 120 warm. Only after reading lots if stuff online i see there are A b c pistons. The piston that came out was either a B or a C it measured 53.97 at the skirts (at thr pin ,using a vernier).it was also a cast stamped ART.

There is possibly 150 or more hours on this bike. Would buying the C piston increase my compression to close where it is supposed to be or does that have no effect on compression?

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Just a thought....

Does the compression tester you're using have a shrader (one way) valve directly at the bottom of the thread adapter, where it threads into spark plug hole?

Many testers don't. If that's the case, then the hose itself becomes part of the combustion chamber and you'll see much lower numbers than what is considered normal. A 125 engine probably has around a 10cc combustion chamber. A 1 foot extension hose could easily hold 10cc. itself, which would approximately double the measured volume at TDC, so you would only see about half of the actual compression reading at the gauge. The smaller the engine, the greater this extra 10cc~ of hose makes on your reading. 

It's also important to use a thread adapter that is the same length as your plug threads for this same reason. Anything that adds volume to the chamber will lower the readings. 

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Will have no significant effect as long as the combustion chamber leakage is not excessive.

On a small bore motor like that you won't get very accurate compression test results with an automotive tester.  I bet 120 is actually closer to 150 and the bike probably runs fine.

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Piston clearance doesn't have anything directly to do with combustion chamber sealing. In the seriously extreme, it could, but it would have to be so loose that it allows the piston to tip significantly out of square with the bore, and since two-strokes have very long piston skirts out of necessity, it's essentially impossible to get to that stage with one.

What can happen is that since the rings are pinned to prevent them from rotating, they will wear to match an out of round condition in the cylinder.  Then,when the rings are replaced in that cylinder without restoring the bore to a round condition, the new rings will take longer than they should to wear into a good seal.  The bore would need to be at least .002" out of round for this to be a noticeable thing, I would guess, and that's beyond the limit for most plated cylinders.

 

Piston clearance doesn't have anything directly to do with combustion chamber sealing. In the seriously extreme, it could, but it would have to be so loose that it allows the piston to tip significantly out of square with the bore, and since two-strokes have very long piston skirts out of necessity, it's essentially impossible to get to that stage with one.

What can happen is that since the rings are pinned to prevent them from rotating, they will wear to match an out of round condition in the cylinder.  Then,when the rings are replaced in that cylinder without restoring the bore to a round condition, the new rings will take longer than they should to wear into a good seal.  The bore would need to be at least .002" out of round for this to be a noticeable thing, I would guess, and that's beyond the limit for most plated cylinders.

 

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The A,B, and C sized pistons allow for manufacturing tolerances of the pistons and the barrel. There's a specific range of clearance between the piston and barrel that the MFR is looking for.

As mentioned above, measure your barrel, and the manual usually has a table from where you select the appropriate piston.

You could probably get a "larger" piston to fit, but that doesn't mean it won't seize.

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On 4/5/2017 at 2:08 AM, rmkillasan said:

 

Hey folks im.having a bit if difficulty with low compression on a 2005 rm 125. I checked compression and got about 90. I threw in a wiseco and its reading 120 warm. Only after reading lots if stuff online i see there are A b c pistons. The piston that came out was either a B or a C it measured 53.97 at the skirts (at thr pin ,using a vernier).it was also a cast stamped ART.

There is possibly 150 or more hours on this bike. Would buying the C piston increase my compression to close where it is supposed to be or does that have no effect on compression?

 

I am always more concerned about the ring gap.  If I try a couple of new rings in a cylinder and they are near the "edge" of having too wide of a gap,  that cylinder gets relined.  

Edited by ossagp

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Bought a snap on compression tester and my compression now reads 180 as required! Dont use cheap testers. However i bored out to 144 and had it ported and replated. My psi now stands at 205!!

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