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Mystery Piston Damage / Marks

londonboards

londonboards

Forum Sage
Past Site Supporter
Pistons from a GS1150 / GSX1100. Look to be in good shape all round.

After de-carbonising them I found these weird marks:

P1120706.jpg


P1120707.jpg


P1120708.jpg


P1120703.jpg


I'm pretty sure I can still use them but I'm just curious to know how this kind of damage can occur.

Greetings
 
What do the corresponding head surfaces and valves look like?
 
hampshire - wow thanks for that link. Pretty comprehensive review of piston damage. Yes I would guess from that it was caused by knocking, pre-ignition, pinking which actually erodes the piston like that.

Thank you.

Greetings
 
Hey, I was going to say detonation, and I didn't even read that link. The question is why was it knocking? Best make sure the mixture is correct, and retard the timing if necessary.
 
Years ago a friend and I rebuilt a hot rod SL100 Honda for another friend. Reason for the rebuild was a hole in the piston caused by detonation. Running lean and flogged mercilessly.
 
It seems an odd place to be getting a hotspot for that erosion to occur, all the pistons that I have detonated have been directly under the spark plug it, possibly it was caused by a carbon build up of carbon getting hot and causing the pinking. Was there carbon on the head at his place? I'm guessing that it comes from sustained heating.
Are they all like this or similar and was this from one'v your tuned, modified engines?
 
Last time I saw something like that a spark plug nose insulator had broken up and rattled around the cylinder for a few seconds before being spat out.
 
tatu - it was from the motor you helped me pull apart. Yes the pistons were dirty as were the valves and combustion chamber and yes at some stage this motor had been breathed on with hot cams. But the pistons and liners had been replaced after the clutch explosion so they were more recently installed. Carbon in that area could have caused a hot spot and pre-detonation. This was the only one like it and to be fair the piston and the others are in good condition apart from this.

Greetings
 
coming from a turbo motor background, i was going to say detonation as well.

it is on the exhaust valve side so if the valves are hanging open a touch (aka not cooling by transferring against the seats) they could be the hotspot and if that cyl is a bit rich, there could be a pocket of unburnt fuel around that area to explode when ignited by the exhaust valves.

Brian
 
Brendan W said:
Last time I saw something like that a spark plug nose insulator had broken up and rattled around the cylinder for a few seconds before being spat out.
Click to expand...

I know Jack.

But when I saw the picture my first tought was that was an object bouncing around on the piston.
 
When I first saw it I thought someone left the base gasket out to make a little extra compression and did not check the piston to head clearance so the piston was tapping the head like what they show on page 48 of your link. After looking at page 48 of your link it still looks like it to me. If that is the case I would not use the piston.was there a base gasket in there? Did it have proper piston to head clearance?
 
Last edited:
Base gasket was in and the other pistons are all good. I think I'm going with pre-detonation which might have been caused by a bad valve?

Greetings
 
londonboards said:
Base gasket was in and the other pistons are all good. I think I'm going with pre-detonation which might have been caused by a bad valve?

Greetings
Click to expand...
You go with that.
But did you even check the piston to head clearance or look on page 48&49? If it is the only piston that did this and it's the only cylinder with bad valves. Your rod and/or crank bearing is going bad. If it only did it on one cylinder you really should check your rod and crank bearings.
" To measure is to know" I heard that somewhere here and always liked that saying. That piston is kissing the head I have seen it to many times. Yea I can be a smart ass sometimes but you really should look into what I am telling you.
 
Could also be the result of a lean mixture on that cylinder from an issue with the carb that's causing detonation. That could also cause burnt valves as well. What did the plug from that cylinder look like? How much carbon buildup on that cylinder?
 
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