An interesting topic this and this is an area that I spent a lot of time over during my engine rebuild - to answer your question after web searching, I didn't use any secondary sealant on my COMETIC head gasket as Quote: "
The Cometic MLS gasket is comprised of three to five layers of stainless steel. The outer layers of the gasket are embossed and coated on both sides with a Viton rubber material designed to meet the demands of a variety of harsh sealing environments - but i also have not started my engine yet.
![Laughing :FUNNY:](https://www.lotuseuropa.org/LotusForum/Smileys/default/av-943.gif)
Cometic gaskets, (in the UK anyway), for the twin cam are available in many thicknesses up to 0.120"
and differing bore sizes to compensate for the effects of over enthusiastic head skimming and rebore work. I went for 0.070" as I wanted to return to standard head thickness and compression ratio but be careful here.... if you do use a COMETIC gasket they have small rivets which hold the laminates together and when trial fitting I noticed that one of these rivets sat between the block and the head face; I could have very easily missed this...
![Sad :(](https://www.lotuseuropa.org/LotusForum/Smileys/default/sad.gif)
I carefully removed it.
Now to what I found the really interesting bit - the twin cam engine really has two head gaskets - the main head gasket and then the cork gasket between the head and timing/water pump cover - and I personally believe ("personal" I say so please you engine wizards on the forum please don't shoot me...
![Laugh :))](https://www.lotuseuropa.org/LotusForum/Smileys/default/laugh.gif)
) - that this area is commonly overlooked during engine rebuilds and is the cause of many oil leaks.
The effectiveness of a cork gasket to seal (I am told) is when it is compressed by just 1/3 of its original thickness and I was equally advised by one of those wonderful retired and 80 year old engine builders that are still around that overtightening a cork gasket can cause leaks, as can a cork gasket that is not compressed enough.
With this info in mind - I measured my cork gasket, plus the head to timing cover/water pump face gap and made up filler gaskets from gasket paper accordingly - note: I did use sealant on these.
I believe you can buy these cork gaskets in differing thicknesses - but I could never find out where.
Hope the above helps - please note and I stress "this is what I did....."
Good luck with your build.