Mark IV heads on a Gen V block?

[tiger9297]

I have heard that you can't do it b/c of cooling. However, I have also heard that you CAN do it with the right head gasket AND if there is enough deck material to properly seal the head. My questions is: If I want to buy a set of Mark IV heads how do I know if there is enough deck material there? Is anyone doing this?

 

[bayou blazer]

I have heard that there are conversion head gaskets available for this swap. I will try to find the part number for you. I have a gen VI block and they addressed the water passage issue as I am running 781 castings on my 502.

 

[hack500]

finally found it.

"Mark IV cylinder heads, made by chevrolet, will not interchange to a stock, production Gen V. chevy block. There will be a water leak into the lifter valley because of a mismatch of water jacket core holes. Likewise, stock production Gen. V cylinder heads made by chevrolet will not interchange onto Mark IV blocks for the same reason.

There are exceptions. you can put Mark IV heads onto a Gen V. "bowtie" block because Gen V. "bowtie" cases have smaller water jacket holes in the deck and will not leak when you use a Mark IV head.

Mark IV heads will also fit Gen VI blocks.
"Signature Series" bowtie heads will fit any Mark IV, Gen V or VI block. Heads and head gaskets will not interchange between Mark IV and Gen V blocks. " - Chevrolet Big Block parts interchange manual.

 

[tiger9297]

I found this on Chevy High Performance site today. To me it seems as though they may have contradicted themselves. In the second paragraph it looks like they are saying you can put a IV head on a V block as long as you use the V head gasket. BUT in the third paragraph they say don't do it.

Cylinder Heads and Coolant Holes


Mark IV and GEN V-VI engines have significant differences in the location and size of the core holes and coolant passages in their deck surfaces. Mark IV blocks have round core holes; these openings were changed in production GEN V blocks to irregularly shaped teardrops on the intake side and flattened triangles on the exhaust side. GEN V cylinder heads have similarly shaped coolant holes. GEN V Bow Tie blocks have round holes—although the diameter of the upper holes is smaller.

GEN V-VI blocks also have two additional core holes at the front of the cylinder case. If these holes are not plugged with the correct GEN V-VI head gaskets, water entering the block from the water pump can “short circuit” the cooling system, bypassing the block and cylinder heads and returning directly to the radiator.

Mark IV cylinder heads should not be installed on production GEN V blocks because the decks may not have sufficient material to seal the head gaskets properly on the upper edge of the block. As a result, coolant may leak into the lifter valley. The core holes in GEN VI blocks were reduced in size to accommodate early-model heads; however, this conversion is still a risky proposition because the size and location of the cast core holes can vary. Mark IV cylinder heads may be used on GEN V Bow Tie blocks, which have smaller holes in their decks.

Here's the link if you want it: http://www.chevyhiperformance.com/techarticles/4567/index5.html

 

[k20]

Yeh, its the difference between production & bowtie blocks. You cant run em on production blocks, but you can on a bowtie block.

 

[cliles]

I am sure that everybody has long forgotten about this topic, but I have a special Gen V block. It is a replacement engine block, has the aluminum 6 bolt timing cover, mechanical fuel pump boss, etc. So while this is a Gen V block, it has a lot of Gen VI corrections.

Does anybody know if these 781 heads will fit with the special gasket? Is there a way of knowing by removing the old head and viewing the coolant ports?

 

[nvrenuf]

Sounds like a straight up Gen VI block. What makes you think it's still a Gen V version?

 

[cliles]

It's casting number is 12550313, from what info I can find, it is a replacement block Gen V block. But it has Gen VI revisions. I guess there is not much online info bc it was a replacement block of a then soon to be discontinued engine.

 

[nvrenuf]

Granted this is just an assumption on my part but I'd say it's a true Gen VI since GM is not producing Gen V's. An engine block is no different than anything else, once a part is superceded and old stock depleted you get what is currently available.

 

[nvrenuf]

To your question, if you have adjustable valvetrain the 781's should work.

 

[cliles]

So the special gasket will not contain the coolant on Gen 5 blocks?