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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,222
I see signs of detonation on those pistons. It's showing up as fine pitting erosion in the aluminum mostly around the edges of the JE piston and in the edges of the valve pockets.
Look at the new JE forged pistons. They have a smooth shiny machined finish and the broken ones are not a smooth machined finish anymore, instead they show fine pitting and erosion mostly around the edges of the valve pocket the very outer circumfrence of the piston where it comes up close to the squish band in the combustion chamber. That is from the extreme heat of detonation melting the aluminum on the surface of the pistons where the detonation is origonating.

I've installed 10:1 forged JE pistons in a normally aspirated 3.5 liter BMW motor and seen them again thousands of miles later when pulling the head to replace a rocker arm and that motor detonated a little bit sometimes under full acceleration but not alot. After cleaning the carbon off the tops of those pitons while they were still in the block the crowns looked no different from when they were brand new out of the box. They were perfectly smoth and the fine machine marks were totally visable, and there was absolutley no pitting and erosion from extensive detonation.

Thats not what killed your motor though... the intake valves and pistons hit each other either from your friend over revving it when he shifted gears or maybe the adjustable cam sprockets came loose and the cam timing changed. I doubt that though.. If that happened the cams would have gone to the fully retarded or late cam timing postition those sprockets allowed and then the exhaust valve would be closing alot later and the piston would come up and hit the still open exhaust valve as it reaches TDC on the exhaust stroke.

But, your valves are lunched on both sides and it's really unlikely that both cam sprockets came loose at the same time - that is extremely unlikely.

So in the end.. I think your buddy that is used to a much newer design faster shifting shorter throw Getrag gearbox with fast light acting borg warner synchros screwed up his up shift, missed the gear, and overreved your motor destroying it.

..do you think he would admit to that?
Old 10-09-2011, 02:53 PM
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