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When assembling the head on the block for the mock up measurements, I've used a new head gasket, then discarded it and used another new one for the final assembly (typically a MLS type gasket) Am I just being overly cautious about the compression of the head gasket, or could the one for mockup be reused?
NOTE, opinions will differ, so get as much feedback as possible - YOU will be the one paying ;-)
Usually, yes, especially as you don't need to torque it down, it hadn't gone through a heat cycle and they're supposedly re-useable. While it may be good practice to be SURE, usually I'd just nip it down as the difference in compressed thickness is barely a couple of thou", and if you're running the clearances that close... However, if you are playing it safe, keep the check gasket as you can re-use it for the checking on later builds.
Don't know if it applies in your case, but if you used torque to yield bolts (they will specify an angle), and you did tighten them down, they MUST be replaced - another reason to just nip the head down as the bolts are fine if not taken close to the yield point. Oh, if you have a used head gasket that is in otherwise good condition, it's common to use them for checking as they have already gone through heat cycles and will have compressed as much as they're going to.