Thread: Piston discoloration around rings

The piston on a 084 parts saw i bought has some discoloration perhaps burn marks or carbon build up around one of the rings. Is this piston salvageable? Ill try to get a pic up as well to show what I mean.

Originally Posted by Alex D

The piston on a 084 parts saw i bought has some discoloration perhaps burn marks or carbon build up around one of the rings. Is this piston salvageable? Ill try to get a pic up as well to show what I mean.

Compression test result?

What does the ring look like in that area? Any deposits in the ring groove that might prevent the ring's lower face from sealing against the lower edge of the groove?

Cylinder looks like?

Mix oil not rated for air-cooled 2-stroke can build deposits in ring groove(s) that make life difficult for ring(s), as, I suspect, can too much oil in mix.

Piston may be usable, if rings can be allowed to fully seal things, piston size is within specs, and no metal has changed hands.

Originally Posted by CTYank

Compression test result?

What does the ring look like in that area? Any deposits in the ring groove that might prevent the ring's lower face from sealing against the lower edge of the groove?

Cylinder looks like?

Mix oil not rated for air-cooled 2-stroke can build deposits in ring groove(s) that make life difficult for ring(s), as, I suspect, can too much oil in mix.

Piston may be usable, if rings can be allowed to fully seal things, piston size is within specs, and no metal has changed hands.

Compression is over 150 so its ok in that regard, there was heavy carbon build up on the exhaust side of the cylinder and in the muffler, anyway heres a pic its crappy but I probably won't be able to get a better one until i pull the cylinder

039.jpg picture by stihl1 - Photobucket

Originally Posted by Alex D

Compression is over 150 so its ok in that regard, there was heavy carbon build up on the exhaust side of the cylinder and in the muffler, anyway heres a pic its crappy but I probably won't be able to get a better one until i pull the cylinder

039.jpg picture by stihl1 - Photobucket

You're confusing me- not hard to do. Exactly where is the carbon deposit (piston/cylinder/port/muffler) and how heavy wherever? Your "heavy" might be my "normal, used".

Big possible problems are chunks breaking loose from piston crown and combustion chamber (they can score piston/cyl) and deposits in exh. port/screen/muffler throttling exhaust (noticeable power loss). Deposits in port and downstream can be removed carefully- not letting them into chamber. Maybe somebody has ideas on how to safely remove possible "break-aways" from chamber? No clue here, only prevention ideas.

Sounds like the engine was run slow and rich, and heavy on the oil. I'd suggest straightening out its "feed" and running it hot and hard in some wood, and then see what's what. Listen to what it tells you.

Of course, use ONLY mix oil rated for air-cooled engines, mix precisely to desired ratio, and be prepared to change plug as chamber deposits can glaze/whisker the plug. You'll KNOW when that happens.

It seems a waste of time, to me, to pull the cyl. of an engine with 150 psig compression, if it sounds and runs okay. Better to listen to it sing in wood.

I'm assuming that you know how to adjust both H & L on the carb. Brad Snelling has VG video on YouTube about adjusting H mixture by ear. (IOW, all the tach stuff is BS.)

Ok some better pics I hope these are clearer so that you can see what I mean:

019-1.jpg picture by stihl1 - Photobucket

014.jpg picture by stihl1 - Photobucket

Originally Posted by Alex D

Ok some better pics I hope these are clearer so that you can see what I mean:

019-1.jpg picture by stihl1 - Photobucket

014.jpg picture by stihl1 - Photobucket

From what I can see in the pictures, it looks just fine, for an engine that's been working. Can we assume that there's no indication of any scoring, or metal transfer? If there is no such indication, I'd just button it up and run it.

No idea if it's worth more to you as a runner or a box of parts. But I wouldn't lay any blame on the piston.