I have hade two engine failures due to a overheated number seven cylinder. I see no telltale signs of water intrusion or any reason this one cylinder is melting pistons after rebuild. It is a bone stock 1994 502 EFI. Anybody heard of this.

what fails ? and what do the parts look like ?

The motor starts knocking. after teardown we find the piston is heavily scored and transferring metal to the cylinder wall.

Running lean, injector, vacuum leak.

Injector problem on that cylinder ?

Injectors were tested on the first failure. the top of the pistons are all the same (tanish in color). no signs of lean condition here.

your right. it really sounds like that cylinder is getting really hot and seizing... the block isn't full of sand, is it ? ive seen that happen in raw water cooling cases. a vacuum leak would kill the top of the piston... i assume you checked the piston to wall clearence early on...

I am the owner not the builder. but I questioned the builder on that subject. they assure me all clearances were correct. this is the second failure of the same cylinder with different pistons. there opinion is it was water intrusion and it is my fault.(pay up) by the way I am a X auto mechanic of 20 years and now learning more than I ever wanted to know about boat motors.

My guess is a vacuum leak in the exhaust manifold of intake.

well.. you never know and i suppose they could be right but it seems to me that eveytime i have seen water intrusion there are about a million other symptoms that occur.

i have a hard time reconcilling a skirt seizure with a vacuum leak. if the top ring land is doing it then maybe but a skirt seizure is usually always over heat related...

i suppose a badly bent rod could do it... but this is all guesswork. if you could post a pic of the piston and bore, that would help.