How do you know when to replace your head gasket? I wanted to do preventive maintenance but don't want to create any problems by tearing her down. She is an 88 420HP with 500 Hrs. After inspection, I saw a little seepage and I'm seeking your advise.

The first question I would have to ask is if there is salt water or fresh water in that engine? If it has salt in it, I would be surprised they lasted this long. If fresh, you most likely would not have to change them as a maintenance routine. They would only need replacing if you were to tear down the motor for some other reason. Assuming that this is a Chevy motor, they have great clamping force on their head gaskets. They almost never wear out unless the engine is overheated.

We ran into a strange situation where the water ports in the head did not line up with one gasket perfectly. We were having a little seepage. Had to replace the head gasket and put a little goo around that particular area. Never did figure out why it was machined slightly different.

It has been faultless since.

Yes, it is in brackish water. Also, how many hour job is it to replace the head gasket on a chevy 420 hp?
Thanks