Piston Autopsy

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Fast Eddie

Fast Eddie

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I have decades of piston breakage experience to fall back on :rolleyes: but even I have never done THIS to a piston before.

It’s out of a mates 1007 race bike. It has not done much work until this happened.

So, any (sensible) theories as to WTF caused this...


Piston Autopsy
 
Does it have 1" valve lift ?

Were the pockets cut deeper by someone ?
Could be the photo but is looks like a fracture at the base radius (<if it has one) into the ring land.
 
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Looks like the piston grabbed the bore between the rings (ooh! Matron!) in a seizure kind of way?
The fact that the rings have bent must mean it was bl**dy hot when it happened, so overheating perhaps?
LH or RH piston?
I seem to recall one of our US contributors saying the best Stateside tuners ran the LH cylinder with more clearance than the RH, but can't remember any more detail than that.

EDIT - the pocket breaking could well have caused hot gasses to bleed into the rings...
 
It was the timing side piston.

Interesting hypothesis Andy, I’ll put it to the patient...
 
The rings didnt break after all. You can break a ring just installing it. But bending...plasma like the Space Shuttle!
 
I agree with Time Warp re: fracture at base of ring land. Then escaping combustion gasses heated the zone very hot causing the bending, or more accurately, "flowing".

Slick
 
Was it a sudden stop, the carbon build up ring is untouched, and you would have some debris at least to score the carbon and for the rings to break if the piston continued movement.

Valve cutaway failed, heat and pressure bent the rings possibly. Is ring gap opposite the bent section.

Very strange.
 
Detonation and/or pre-ignition. At this stage in the game, it’s hard to know what happened first.

The unusual bent lands and rings may be a sign that it’s a forged piston and some sort of steel (not cast iron) rings.
 
Looks like a piston out of one of my blown up turbo Dodges. I’d say Detonation cracked ring land(s) then remainder of malleable forged piston’s aluminum bent along with malleable rings. Forged piston’s ring lands often are paired with rings more malleable than those used with cast pistons. Chrome plating ring faces and other surface treatments allow a more bendable ring to work as a system with malleable lands. Next piston might be redesigned with more meat between pin and land
 
Using samples of all three rings, see if you can snap off a piece cold...
 
The valve pocket giving away indicates overheating from lean mix or ignition advance detonation or both. From there everything got worse with the ring exposed directly to the combustion flame.

I want to know the distance between the ring and the nearest point of the valve pocket (measure it for me). This distance is critical.
 
jseng1 said:
The valve pocket giving away indicates overheating from lean mix or ignition advance detonation or both. From there everything got worse with the ring exposed directly to the combustion flame.

I want to know the distance between the ring and the nearest point of the valve pocket (measure it for me). This distance is critical.
Click to expand...

I'll see if I can get you the measurement later today. When we designed the piston, we lowered the top ring groove to get more clearance between the valve pockets and the back of the ring groove, just to avoid burning through the pocket into the ring groove. That was a problem we had both experienced in the past with big valve heads, race cams, and high compression. When pushing the pistons up higher in the bore for more compression, it was necessary to deepen the valve pockets, which put them too close to the back of the top ring groove. I would be surprised if that had anything to do with this problem. The large 83 mm bore pistons have more room between valve pocket and ring groove than smaller bore pistons.

Your suggestion of overheating or detonation sounds likely to me, but it's sometimes very difficult to sort out the exact cause.

Ken
 
I make them in the 83mm size but they fit my longer rods - not stock length rods. Same JE brand and same material. Gary Thwaites won about 40 races with this setup.
I also have to pay close attention to the distance between the top ring and the valve pocket.
 
Heat build-up is probably one of the main things which stops twin cylinder four strokes from developing more power. It is the reason, I only ever race using methanol fuel. Even then, it is possible to get piston-cracking problems, if you don't warm the motor up enough prior to thrashing it. I only know of one air-cooled Ducati which was ever four valves per cylinder.
 
jseng1 said:
I make them in the 83mm size but they fit my longer rods - not stock length rods. Same JE brand and same material. Gary Thwaites won about 40 races with this setup.
I also have to pay close attention to the distance between the top ring and the valve pocket.
Click to expand...

Right. I knew you made them for the long rod kits, but it just slipped my mind when I was posting.

Ken
 
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