Partially melted piston in 502 engine

Edraper

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Aug 19, 2017
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I had my 502 Mercruiser engine rebuilt over the winter. I also had new plugs, wires and cap put on. After about 4-5 hours of use this summer the motor was done. The original engine builder took it apart and found the #4 aluminum piston melted on the one side with aluminum stuck to the cylinder wall. This motor is fuel injected so we sent the injectors away to see if one or more were running lean but they came back good. Anyone experienced this or what else could it be?
 

scoflaw

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Jun 2, 2010
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The 502 mag has a forged rotating assembly, maybe they used cast pistons on the rebuild
 

Bondo

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This motor is fuel injected so we sent the injectors away to see if one or more were running lean but they came back good. Anyone experienced this or what else could it be?

Ayuh,.... Welcome Aboard,.... Move further Up-stream,.....

Inspect the Contents of the fuel filter,....
The dip-tube, 'n anti-siphon valve in the tank, the quality of the fuel in the tank, 'n the tank's vent has to be clear,....
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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In addition to Bondo's list, get a fuel pressure reading

Also, if the valves were not adjusted correctly on that cylinder, a hanging valve can run it lean too
 

Edraper

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Aug 19, 2017
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The 502 mag has a forged rotating assembly, maybe they used cast pistons on the rebuild

This motor was rebuilt with aluminum pistons and stainless valves. Block is bored out.060 over and the crank was balanced before assembly. The intake and fuel injection was not touched and I had no problems with old motor before rebuild.
 

Bondo

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This motor was rebuilt with aluminum pistons

Ayuh,..... All pistons are aluminum,....

There's Cast, the cheapest, least durable,....

There's Hyperutectic, which is mid-range priced, very durable, 'n the most common pistons out there,...

Then there's Forged,... Super durable, 'n extremely expensive,....

But,......
Detonation from a lean condition will quickly blow a hole in any one of 'em,..... even Forged,....
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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This motor was rebuilt with aluminum pistons and stainless valves. Block is bored out.060 over and the crank was balanced before assembly. The intake and fuel injection was not touched and I had no problems with old motor before rebuild.

Bondo covered the pistons

boring 60 over gives you more cubes. comes out to a 514. lets assume the cam was changed too. if you didnt adjust the fuel tables, you could have been lean from the get go.
 
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