I found that all four end bolt holes in my heads had not been tapped deep enough. With the bolts torqued to spec, I could slide 0.005 to 0.010 feeler gauge under the bolt heads. The bolt heads were not touching the manifold. I removed the helicoil, tapped them down and put new helicoils in.
I use Victor Rienz (SP?) gaskets. They are metal reinforced.
I am assuming you are not using the gasket that lays across the block and are just filling it with a layer of sealant. All surfaces should be cleaned with a good parts cleaner. I like brake cleaner. Then clean it with alcohol and white cotton. keep cleaning until the cotton stays white. It does not stick to
oil.
I think a product called the right stuff is best for this service.
When I torqued the manifold down I must have ran over the bolts in the proper pattern a dozen or more times until the toque wrench clicked on every bolt (nut & stud actually) without any of them moving. Let it set 24 hr. Then ran it normally up to temp. Let it cool over night. Put it through 3 heat cycles, and then re-torqued all bolt again until none moved.
I used all studs with locktight into the heads. Then after final torque, I put a set of acorn nuts over top of the nuts used to torque it, which provided a jam nut to assure they never move. I used stainless steel nuts for looks.
I'm not saying the studs and acorn jam nuts are necessary, but I wanted to over kill this problem once and forever.
I do suspect you may be building pressure in the block. Are you sure the PCV system is working.
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