Hey guys I need some info on 502's. I am looking to repower my boat and have found Gen IV carbed 502's and Gen V EFI 502's. I thought all EFI's were Gen VI. These motors have almost the same hours on them and i am wanting to do the rebuilding at a machine shop that I trust. They will need to be gone through and checked. I was planning on using some hi-po parts which is a whole nother topic. Same money for both which is the better buy? Or do Gen IV and V motors have a weakness and therefore should I be looking at Gen VI's?

OK, I will give this one a shot. I hope I don't leave too much out. From what I remember, Chevrolet introduced the Gen V in 1991. Some of the differences between Gen V and Mark IV are as follows:

The water passage holes on the deck of a Gen V and Mark IV are different. You can't swap heads from a Mark iV to Gen V.

The Gen V has a 1 piece rear main seal vs. a 2 piece in the Mark IV. This means the cranks are different.

Gen V has the main oil supply passage up by the camshaft where as a Mark IV has it down by the oil pan rail.

Gen V has no provision for mounting a mechanical fuel pump.

The main bearing caps are longer in a Gen V block.

In 1996 the Gen VI was introduced. These motors came with roller lifters, the lifter bores were redesigned. I believe they were made taller as well. Other than the lifter bores, the Gen V and Gen VI are pretty much the same blocks.

As far as 1 block being better than the other. I couldn't say that I prefer 1 over the other. It is a matter of what you want to build and what is available.

I hope I didn't forget to mention anything too important. good luck in your re-power.