big block gmc heads
 

i am in the process of rebuilding my 502mpi mag,1994.in the process of r/r of the heads the machine shop has found a crack in one of the heads.it is in the crossover port.bad spot they say and cannot be repaired.so ,here is my ??s.i have a set of merlin,castiron,square port,320 runners.i was told not to use the merlin heads on the efi motor because they are not stock heads and the efi motors run on such a narrow perineter that any small change to the motor upsets the tune-up,and decreases the realiabltey.what do you guys think.my machine shop says that the head will only flow what the intake will let it.thanks.

If you change the exhaust also you need to get the ecm tweaked. Just changing the heads I would expect the exhaust to keep your flow down. Some one else will chime in soon.

sounds to me like your machine shop is uneducated in what can and can't be repaired these days. There is a place in Ohio that can weld your head up good as new. Probably cost more in freight that it would to do the repair. You'll probably have to replace the seats if you go that route. On teh hother hand, if you have a set of Darts, put 'em on and be done with it. The exhaust side will flow much better than the GM head and make more power. Not sure if you would need to reprogram the ECM or not. Others might chime in on that one.

Check the casting number on that head. If it ends in "990", I have a good one you can have cheap. The computer can adjust for the subtle change the aftermarket head flow will cause.

I have to agree with Griswald about the probability of a repair.

big block gmc heads
 

the casting # for the head is 14097088.1993-94?the boat is a 94.putting a head on the motor that flows more exhaust will that make the motor lean?i am hopping just to r/r the heads and do nothing to the ecm.

gen VI head....

I have 2 sets of 500 efi heads that will bolt right up, one set is fresh w/ brand new valves. the other set has about 60 hours or so on them.

If you don't change the cam, you don't need to recal the computer.

big block gmc heads
 

gen 5 heads.everything stock except breather.

BBC Heads
 

Gen V blocks/heads had some issues anyway. (Actually with the coolant port size/shape on the block deck) If you have not had any head/gasket problems with the Gen V engine, you have been fortunate. That is why Gen VI engines came out so quickly after the gen V's were introduced.
I recomend two things.#1 Put on the aftermarket heads (That have been designed to work on Gen IV, V & VI decks) and you will end up with a more reliable, better running engine. (The ECM will be fine. They run too fat from the factory anyway) or #2 Find a replacment casting and put it back to original.(Use the correct Gen V Fel-Pro, Marine head gaskets)

Good luck, Curtis

I was recently told the only difference between the 5 and 6 was the roller cam/longer cam bores and an insignificant change to the timing cover. The cooling passages did not change. Just a guess but I think people assume there were problems with the Gen V because it wasn't here long, yet no one really cites specific issues they've experienced. So far, the only really factual explanation I've unearthed is they made the change to accommodate the roller cam.

BBC Heads
 

There is a HUGE difference in the coolant passages on the deck of the Gen V block -vs- the Gen VI block. While they were "fixing" the head gasket/deck problems, GM changed the timing cover bolt patern (The oil pan/front seal location was already changed in Gen V) changed cam snout sizes (affecting timing chain sets) and thickened the lifter galley in the block requiring taller lifters or the use of Gen VI roller lifters. They also took away the mechanical fuel pump provisions.

There are also early and late Gen V's. (Some have roller lifter provisions and mechanical fuel pump mounts and others do not.)

To make things more confusing... Bow tie Gen V blocks do not have these changes and have conventional coolant passages.

Curtis @ Bilet Marine

bigblock gmc heads
 

i havent had any head gasket problems.......but,i think i would have.i say this because on the deck of the block ,on both sides at the bottom of the deck,towards the exhaust side there was erosion about 1-2 inches.it looks like the gasket was going away.

I've looked at the two side-by side and see no difference in the cooling passages. Obviously they're quite different than the Mk $ but the V and VI even share the same gasket.

What am I missing?

The Merlin heads wil work just fine on that engine. Their flow numbers are a little better than stock heads but not enough to cause any problems. Port locations are the same as stock heads so no problems getting the exhaust to line up. Plus if you have them, no expence and maybe a small increase in performance.

I managed to locate a copy of the HP Books Rebuilding the Gen V and VI big block- there are lots of subtle differences between the two to increase reliability and possibly cut manufacturing costs. PS- that book is out of print - I had to get mine from the U.K. to avoid paying $100+ from the rare book people.