evaluex

I’m rebuilding a +- 1991 454 GEN 4 w/2 pieces main rear seal
Mercruiser-GM engine, but I don’t have the right heads.
Serial engine # OD442097 casting block # 10068286
I do not want to put some exotic aftermarket (alloy) heads on it, just trying to get it back as stock as possible. I found different partnummers but NLA from Merc.
7.4 Bravo heads # 938-9553T
Replaced by # 938-883491R 1 = NLA
454 Magnum Bravo heads # 938-5924
Replaced by 938-814512 = NLA
Can anyone help me out which OEM castings were used at the factory ? Specifications ?

Bondo

Ayuh,.... So what heads do ya have now,..??

dereknkathy

and are they oval or rectangle ports? and why do they need replaced?

Mbam

That serial comes up as a Magnum. It would have had rectangular port heads.

evaluex

(Water leaking) GEN V heads
I want to make this engine "stock" again so (re) reconvert back to GEN IV heads. Somebody knows the OEM castings??

Mr Maine

188s would have come on it.

990s are the same head as well

dereknkathy

If you have gen 5-6 rectangular heads, there is a slight diff in water passages and a very slight diff in the appearance of the front of heads. Gen 6 have bigger accessory bolt holes requiring a bit of modification to the pump brackets. There is essentially no diff in port or chamber design. No noticeable diff in performance. This is not a numbers matching 68 396 camaro. Nobody but you will know the diff.

SB

iTrader: (1)

He said he has a Mark IV engine. Issue is that it has GEN V heads that are leaking and he wants to go back to the Mark IV heads that would have come on it.

This is how I understand it. Maybe I'm wrong ? Kind of hard to completely follow.

Edit in: if this is correct, the 188 or 990's as stated above should work. (both rect port, open chamber, Mark IV heads)

dereknkathy

I thought the only leak issue was MK 4 heads on Gen 5 blocks. I am pretty sure late heads on early blocks are no issue. also, speaking of leaks, they stopped using rect heads in cars around 71 or 72. virtually every rect head after that was marine use. raw water for 30 years and the #2 and 7 exh valve guides are about to finish rotting thru.

evaluex

No, you're correct

Searching by google (and ebay) For mercruiser 454 Gen 4 always "360" castings comes up...