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Old 10-09-2012, 11:45 AM
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Originally Posted by ForceFed86 View Post
Not as healthy as I thought... The rod bearings on these two looked great. Not sure what took the pistons out. Bought this as a mock-up engine a while back for $200 dollars. Seller said it was running fine when pulled.

Ah well... Suppose I'll be looking for some cheap piston/rod assys.

Detonation, maybe with a little pre-ignition thrown in. This is an injected engine so this is kind of a strange injury as there should be a Detonation Sensor that pulls the advance back or in the case of this engine cuts the spark to the offending cylinder. So this kind of damage is pretty rare with late model vehicles. It looks like the rings held together so the cylinder walls might have survived without damage.

This is a very typical sudden failure of a cast hyper-eutectic piston. They go a long way past the edge but when they bust it's quick and nasty. Another thing I don't like about these is the damn circular dish. This configuration surrenders a lot of squish/quench which is in a word "mechanical octane"; OK two words. If I was rebuilding this I'd use a forged high-silicon 4032/MS75/VMS75 material replacement piston with a D Dish to bring the compression in while retaining very good squish/quench by keeping a flat surface close to the combustion chamber step and the dish under the valve pocket.

You have to be careful about terminology here hyper-eutectic like FM244 and other similar alloys are used for a cast piston. High-silicon forgings like 4032 and the VMS family are not over the eutectic for dissolving silicon into aluminum, but they offer similar high wear resistance and good thermal stability that allows running closer clearances than you can get away with compared to the low silicon 2618 material which is ultimately a bit stronger but brings a lot of problems when used on street engines as the stuff really requires the engine be brought up to operating temp before using it to simply drive around. When the engine is cold the 2618 gives a lot of blow-by with resulting upper cylinder lube wash out and considerable skirt slap as the piston hunts through its generous thrust clearance.

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