OK to use steel shim gasket w/ Aluminum heads?

[J. Chesher]

My '81 350 short block has .025 deck height. In order to get the "ideal" .040 quench without decking the block, I need a .015 gasket, or something very close to that. The only option I know of in that size is the Felpro steel shim gasket. However, I am running aluminum heads and just wanted to see if anyone had concerns with using a steel shim gasket with aluminum heads. I'm guessing the general used them w/ aluminum heads, but just want to be sure. The gasket surface of the block looks excellent. Also. maybe someone knows of a better option than the steel shim in the range of .015 thickness?
I appreciate any opinions you can provide Thanks!

[Joe L.]

Re: OK to use steel shim gasket w/ Aluminum heads?

J.------

I know of no case in which GM ever used a steel shim type gasket in conjunction with aluminum heads. I would never use a steel shim type gasket with aluminum heads and would not recommend it, either.

You can use a multi-layer steel (MLS) gasket with aluminum heads. However, MLS gaskets are going to be way thicker than steel shim type, so you won't achieve the "thinness" you're seeking. Also, you very likely won't have block and head deck surfaces of the required finish for MLS, anyway.

[Clem Z.]

Re: OK to use steel shim gasket w/ Aluminum heads?

people have done it BUT i would never use a steel shim with aluminum heads. GM # 10105117 will work and compressed thickness is .028

[Clem Z.]

remember you need .5 more CR to get the

same HP with aluminum heads vs cast iron heads so make sure you calculate your CR as you may want to cut the heads

[J. Chesher]

Related question..

If not steel shim, how about copper shim, such as that available from Mr. Gasket? I think they offer one in .020 thickness. Or, is MLS the only option? I assume the reason is the differing expansion rates of aluminum and the iron block? Sorry. Just trying to figure out a way to keep the original block deck surface while getting as close to .040 quench as possible. Since I am also replacing pistons, one option may be to order pistons with an extra .010 of compression height... Other thoughts?
Thanks!

[Clem Z.]

Re: Related question..

unless you are racing the car i would not worry about .010 more deck using the GM gasket i posted above. i would just make sure you get the CR right to have a good running engine by CCing all the parts

[Joe R.]

Re: Related question..

Milling .010" off the surface of the heads will allow you to achieve the desired quench height with a thicker gasket. That's a pretty small cut that should have no significant impact on the head/intake geometry. In addition, it will ensure that the head surfaces are clean and true.

[Patrick H.]

Re: Related question..

Agreed.

You don't say what heads you will be using.

Checking for variation between chamber volumes in the head will get you far more than worrying about exactly how thick your gasket is. I don't think that 3 hp will make any difference between .015 and .025 when comparing gasket thickness. You have to know when you can stop being anal about that last little bit. This would be that time.

If you chambers differ by several ccs, that will make a noticeable difference in power and smoothness. Proper head work and preparation can gain you far more usable power than your gaskets and quench.

Patrick

[Duke W.]

Re: Related question..

Quench clearance is the sum of deck clearance and gasket thickness. Milling the heads does not change quench clearance.

Duke

[Joe R.]

Re: Related question..

Hi Duke:

You are right, as usual. Brain fade on my part.

[Duke W.]

Re: Related question..

Internet bloggers have this compulsive obsession with quench clearance - similar to their compulsive obsession with "rod ratio".

Unless you are building a racing engine with a CR that is pushing the limits of available fuel octane, quench clearance is not particularly important. Most OE engines were built with quench clearance in the range of .050-.070", and since you don't want to deck a numbers matching block to achieve "ideal" quench clearance - don't worrry about it.

Select a target compression ratio range that is suitable for the installed camshaft, then juggle the piston volume, head gasket thickness, and head chamber volume to achieve the target range.

Most OE engines were assembled with half a point less CR than was advertised, and they can be put together with the same as-built CR.

Only use a head gasket with aluminum heads that the manufacturers says is suitable to cast iron blocks with aluminum heads. It will be a compositon gasket, but not all compostion gaskets are suitable for dissimilar block-head materials.

Duke