Yep! No other choice! Let us know what you find out!

Ok. The engines were pulled and everything pulled off of them. Dropped the engines off to the builder and he just told me the first engine that I was having problems with is toast. The head gasket blew and there is a very large valley worn between two cylinders and the head is burnt. I need at a minumum a block and a head. He also said there must be a computer or fuel program issue that caused this. Has anyone had the same issues and what did you do to fix it?

Here's the issue. Loss of cooling water means everything is very hot - obviously hot enough to burn the headgasket fire ring between the cylinders - and cause pre-ignition + detonation on it's own.

Once that fire ring is melted away, the adacent cylinders are open too each other and the combustion of each cylinder will 'trough' right thru to the other taking cyl head and/or block surface metal with it.

Could the fuel system cause the same headgasket and valleys in block and heads ? Sure ! But so will an overheated motor.

With the engine being rebuilt, even if this issue did not happen,it is always a smart idea to have your inectors cleaned andflowed...new filter baskets installed, and then when put back in boat, verify fuel psi from idle all the way to WOT and top rpm.

I've rebuilt/replaced many a motor for same issues you have that where ust overheated. Have rebuilt/replaced many engines for same issue you have that where from pre-ignition and/or detonation.

Oh, and also from too much nitrous and not enough fuel. LOL.

I can find replacement heads. But I can't find a block. Did they discontinue making the blocks? Would it be a good idea to rebuild the bad motor or start with a new one. Possibly sell the good motor and get two like replacements? Is replacing them with 525s an option because I don't see a whole lot of 500s out there for sale. To many questions with not alot of answers.

You need a Gen 6 GM block. Not a hard find. If you wanted to sell them as is, I would be interested.

I thought they were 7s. But no matter, I am concidering selling them both as is minus headers. Rather they are 6s or 7s will standard GM blocks work or is something like the water jackets or anything for that matter different?

Had a handful of the same failures this year. Loose a fuel injector and the motor gets toasted. For this reason I don't like fuel injection in a boat. There are no warning signs while it's running. At least with a carburetor it will snap crackle and pop through the intake. Efi doesn't let you know anything till you come back down to an idle and have a miss. Even then the fuel injection does a great job of covering up the miss in the motor.

Yeah, I've seen it on a 500 EFI. had to replace the block with a bare block, one head and 2 pistons, it ended up being a bad headgasket between 4&6, Torn down the other motor and the fire rings were distorted ready to go. had the injectors checked and flowed all ok. My point is that like BB said the injection system will compensate for this and mask the symptoms till the damage is so great it can't any longer. Crappy Merc headgaskets was the cause in this case.

was your oil temp overheating and would it start at all before you pulled them I have a very similar problem with 500 efi mine

Can the block deck be milled?