Mercruiser 7.4L GM 454 Gen IV or Gen V ?

As you may know from some of my previous posts, I am doing a top end rebuild of my 1991 Mercruiser 7.4L. That year is kind of on the border line between the "Gen IV" and the "Gen V" 454 versions. As best I can tell, the engine is a "Gen IV". It has an engine code on the block of T04060XW which matches to a "Gen IV". What is throwing me off is that the head gaskets that I pulled off the engine look more like the "Gen V" than they do the "Gen IV". (i.e they have 1 less cut out for the water passage on both ends of the cylinder row.)

I am wondering if this engine might have had a valve job done by the previous owner and they put "Gen V" version of head gasket on instead of the "Gen IV"?

Engine serial number is: OD446420 Engine block code is: T04060XW

TIA for any insight.

Re: Mercruiser 7.4L GM 454 Gen IV or Gen V ?

The Gen V started with Mercruiser Serial Number 0D603118 in 1992.

Re: Mercruiser 7.4L GM 454 Gen IV or Gen V ?

Don S said:

The Gen V started with Mercruiser Serial Number 0D603118 in 1992.

Click to expand...

Yes, the serial number range is within the Mark IV/Gen IV series.

Having done some more research, it looks like most Gen IV/Mark IV 454 engines used what is called 'serial coolant flow', yet some used 'parallel coolant flow'. The difference being that the parallel flow engine blocks had water passages (holes) between the cylinders to alllow water to flow from the block into the head along its length. The serial flow configurations had the water flow through the block first and then onto the head, and then finally out through the intake manifold casting.

The head gaskets for serial flow have an extra cut-out at the end of the cylinder row to support the greater amount of coolant flow through that one area. The gaskets for parallel flow do not need as much flow in that area and have 1 less opening. So, the gaskets that I took off are the ones for parallel flow.

The engine is definitely a Mark IV not a Gen V, based on the ID numbers, etc., and the head gasket are the ones for parallel flow, which makes sense for a marine engine. I'm not sure that it will matter which "Mark IV" gasket I get, since the block is configured for parallel flow.