Difference in scoring and scratches...

[Griffbm3]

So I have taken a look at enough pistons by now to tell the bad ones, but it's the mediocre ones that throw me off, just by sight...

After talking with Bill G last year, he made a very good point. Sometimes compression tests don't tell you anything. They can produce compression just fine and be scored to heck, and other times compression can be bad and have a cleaner looking piston.

I can never tell ones that are "getting bad" or have had a bad day or two, but continue to run fine... The larger saws seem a little easier to see this on, but I suppose that is from the larger piston and thereby a bigger visual field.

I have an older 036, the piston is dark, the saw has gobs of hours on it, but I don't see any vertical scratches on the piston. I have re-ringed the saw once, but I can't see the machine marks on the piston at all...

I have a much newer 088 with machine marks visible and vertical marks on the piston. Granted the wall feels untouched, and the rings are fine, I was just interested in why some saws look like they have vertical scratches and others don't show it at all.

Old Olympyk that I sent to an AS member. That saw had great compression and not a mark on the piston, and that saw was over 20 years old. And yet, an old 041 I had showed plenty of marks, but the rings again were still flexible, and it ran fine.

So I guess I just wonder what is normal use and acceptable, as I have seen really bad I have a pretty good idea what that looks like...

Jason

 

[Justsaws]

Judge pistons by how well they function. A piston can look good but have thin worn out skirts, worn out or damaged rings, or damage to the top. Compression testing and peeking through ports will give you information but not always a complete picture. There are a lot of different pistons finished by the manufacturers in different ways so there are lots of variables.

Scratches across the rings = bad. Pull the P/C.

Light scratches in piston body but not across rings = lots of possibilities. If you want to be sure pull the P/C.

Deep scratches = bad. Pull the P/C.

Smearing = bad. Pull the P/C.

Low compression without visible damage = end gap/ ring wear/ring or piston damage/gasket issues/etc..

Lots of "techniques to cheat" but only one way to know, take the saw apart and rebuild it with the appropriate parts for its task.

 

[Justsaws]

A dark piston could be "blued", "Opti-mixed", "carbon baked", etc., if the fit in the cylinder was tight and the skirts, ring grooves, etc. were in good enough shape than I might polish it up and put it back in. The reason for the discoloration would have to be found and remedied. If some was paying me for a rebuild they would most likely get a new piston.

A dark piston would have me looking at the crank and bearing extra carefully.