Originally Posted by clinebarger
Originally Posted by Trav
If no spec is available then no I do not reuse them in any case. Does the Duramax have a Teflon coating on the HG?
Good question Trav, Wish I had a definitive answer? However.....The updated LBZ gaskets appear to have a selective Elastomer coating on the cylinder head side & a solid Elastomer coating on the block side, I guess the core could have a Teflon coating?. They are also riveted together in several places vs the crimped gaskets used on the LLY.
The updated gaskets require a lower head bolt torque for whatever that's worth.
What I can attest to is.....The LBZ gaskets do not seem to fail from scrub, I'm guessing it's a combination of better materials & lower head bolt torque?
Your thoughts on this subject are appreciated Trav......
There was I thread some time ago where someone posted about the benefits of studs vs bolts, this exact question came in my mind regarding dissimilar metals and coated gaskets and it appears tighter isn't necessarily better when it comes to this type of gasket.
Having been in this business 47 years now I was working when aluminum heads came into mainstream production and saw the early issues with gaskets and cracking of the heads in the water jacket area.
I seems the manufacturers were trying to control expansion of the aluminum instead of letting it expand and contract and creating a gasket that could cope with it.
MLS coated gaskets seem to be the answer, I have seen them coated on both sides or in the middle layer on both sides only, both seem to work equally well, lowering the torque will allow the parts to move easier without damaging the coating.
The worst are the open deck engines like Subaru and even worse iron block aluminum head open blocks like some early Peugeot, I never understood why Subaru went from a good solid closed deck that had few if any HG failures to an open deck with nothing but failures and stayed with it. Peugeot did go closed deck and eliminated the problem.
As far as the stretch bolt thing goes the one thing that is not being taken into account in this discussion is how far the manufacture takes the bolt when installed.
VW for example stretches some of them to the point of almost snapping when first installed, the dog bone mount is a good example. tighten it 50nm + 90 degrees twice and there is a good chance you will have half the bolt in your hand.
Tighten it 50nm and go 45 degrees and you can reuse it multiple times (been there done that) or just replace the 8.8 grade stretch with 10.9 and tighten to 50nm and put blue loctite on them (even better).
On some of the others like the head bolts and subframe to body they are nowhere near maxed out. MB has always been conservative from my experience and most can be reused multiple times but unless you have the spec or know from experience how would you know, that's the reason I toss most of them.