So i got bored tonight...(pics)

[brveagle]

Due to the rather rainy weather here in Missouri, I decided to pull my engine apart. I haven't logged onto here in a couple months, and Haven't touched the mighty gs550 (740cc) in over a year. Too many track days and repairs going on with my other bikes.

Anyways, I was amazed at some of the colors found on the #3 piston.

The head had the same colors but only around the #3 valves. Any clue what could have caused this? (Bike was running very rich)

[Nessism]

Heat damage?

[tkent02]

Could it have been so lean that the oil washed away from the wrist pin bearing?
Perhaps that piston was too loose in the bore so not cooling itself correctly?
It surely did get hot.
How did that bearing look when it came out?
The end of the rod look OK?

[GavinJuice]

How was compression on that cylinder? Could it just be blowby? What does that piston look like on the other side?

[brveagle]

Compression on all cylinders was low, but that was my fault. Bore/rings/pistons were all new.

Bike was running too rich the whole time just because it wasn't burning all the fuel.

The head is the same color around the valves and cam journals. but NO scoring on wrist pin, cam, or cam journals.

None of the other pistons have any coloration.

This is the only piston with any coloration, but no damage. The coloration is 360 degrees around the rings, but does not extend down the skirt of the piston on either side. It obviously is all around the wrist pins and on inside of the piston.

[psyguy]

some thoughts...

i dont think it would have to do with blowby as there is no discoloration or any damage to the skirt
i dont think it has to do with oil delivery to the piston wrist pin as you say that the discoloration is on the head as well

so, i would conclude it is a case of heat-discoloration

the 2 portions of the skirt that still look ok have better cooling properties than the rest of the piston (through the oil contact between the bore and the piston skirt), so this would be in line with the heat-damage theory

if the compression was low the plugs may have read "rich" from oil deposits and that cylinder could still have been running lean

[Don-lo]

It's a very pretty color. lol

Were you running a special fuel? av gas?

It looks like heat, but the distribution makes me think of solvent color. Did you try spraying gumout on the color to see if it came off? The blued piston pin certainly looks like heat.

That one edge definitely heated up to distort the ring groves. I pulled some Yosh pistons from a motor, and two of the teflon buttons were missing. The heat from the rubbing piston pin melted the aluminum across the rings, but there was no heat color.

[mcloud]

Found it!!

It's a very pretty color. lol

PPG #287-9987... lol