Why do they recommend using Permatex 3H Aviation form a gasket on the head bolts? I am putting on some Trick Flow aluminum heads on a 1968 ford 302. The holes in the block seem to be "blind" or they dead end. How could any water get in---if that's the reason for applying?

 

Who is "they"? Only reason I could imagine would be as a thread lubricant. However, be aware that if a sufficient amount of the stuff got down in the hole in the block, beneath the end of the bolt, a "hydrostatic lock" condition could occur, robbing the bolt of needed tension: torque would be applied trying to compress the sealant, detracting from needed bolt tension to compress the head gasket.

Most Ford shop manuals recommend lightly lubricated head bolt threads using motor oil. BTW, hydrostatic lock has caused numerous failures of beaded steel head gaskets when sealant was applied to them: the sealant was trapped in the little valleys of the gasket, preventing proper compression of the material, and subsequent sealing.

(I worked in a head-gasket development lab for many years, at Victor Gasket Co.). imp

 

Some head bolts go into the water jacket on some blocks of some engines. If "they" are recommending the Permatex then "they" must think your application needs it.

 

As for why Aviation type RTV vs automotive, most likely for two reasons. It's anaerobic and can handle higher temps/oil/antifreeze. There are a couple of Automotive grade RTV's that can also handle it but the aviation grade is usually higher specd.

 

 

Obviously the guy who knows what he is doing probes all the bolt holes in the block before using sealer everywhere. imp