Originally Posted by chrismac
Which head gaskets fit aluminum heads to an iron block on a small block chevy? Even with 12:1 compression.
Basically what you can't run is an all metal shim gasket. These won't hold a long term seal between cast iron and aluminum.
You need a composition gasket that may have a stainless steel face on one side and Teflon on the other, or similar construction that isolates the iron from contact with the aluminum. Graphite coated materials are common to GM and others. Some may use a copper fire ring rather than steel. There are two or three problems that need to be worked by the gasket when aluminum is on one side and iron/steel on the other.
One is to stop an electrolytic path between the iron and aluminum. The presence of dissimilar metals with an electrolyte (of which water or water and antifreeze is one) causes a current to flow (i.e. a battery is made). The softer metal is transferred to the harder, which is to say, aluminum is eroded from your expensive heads and deposited all over the block's coolant passages. Running straight coolant is an idea whether ethylene or propylene glycol. Propylene seems more popular for what's called non-aqueous systems that most all production has gone to in recent years. This eliminates the aluminum corrosion problem but does require a slightly better radiator as this stuff is not as efficient at heat transfer as water. The factories have gone to this along with oil cooling and aluminum radiators. So one concept supports the other when both are used.
Second is isolating the different expansion and contraction rates of aluminum and iron. More modern composition gaskets feature Teflon or graphite on one side to allow the aluminum to move more than the iron without loosing the gasket seal. These are often advertised as not requiring head bolt re-torquing. Personally, I'm not that trusting so I continue to do a re-torque about a week after putting the engine in service, then I like to see it a month later, and yearly after that.
Third, gasket thickness and bore diameter has an influence on calculated compression ratio. So you need to look at available thicknesses when you do the arithmetic. For your application, for example, GM offers 10105117 at .028 inch thick or 12557236 at .053 inch thick this is the ZZ4 gasket, or 14076505 which is their graphite gasket that was used on the L-98 aluminum headed Corvette SBC. Also, if given a bore diameter choice try for as close to your bore size as possible. Large bore diameters reduce compression and retain exhaust gases in the dead space. However, at the same time you don't want any of the gasket hanging over the bore so don't put a 305 gasket on a 350.
Look in the catalogs and check that the gasket is for aluminum heads and cast iron blocks, be aware of those which say for a sleeved block and steer away from these. Some people in their blogs recommended non-aluminum compatible gaskets to you! So research this carefully. The major catalog stores have technical help lines don't be afraid to use them. Remember the only dumb question is the one that didn't get asked.