Hi Tommy, Find your observations most interesting.
I just measured used '69 Bonnie head gasket. Known history, genuine Triumph copper gasket. The thickness varies between .0452 to .0463" The thinner is along outside edges. The center bolt area is quite even from front to back with no dips at all. I measured with a micrometer at several places. With a straight edge the gasket is on perfectly flat. It has a slight twist & bow. About .005". Of course this could skew micrometer reading slightly as well.
My straight edge is rated at .0002" per foot. On my lathe ways it rocks with a .0005 blade under it so it seems quite flat.
Block showed flat within .0005. Straight edge pinched .0005" feeler blade. Head .0005 blade was a snug slip fit. .001" blade was pinched full tight. I'll call the warp .0005" on head. It was low on the outer edges. I found no dips or humps. The .0005 was a slight bow.
Point is on this motor I see no dip or humps in old gasket. I used Mercedes sealant on head gasket during assembly of this motor. The gasket area has remained bone dry for about 3 k miles so far.
My TR7Rv head & block both tested within .0005". Pinching feeler tight. Also used Mercedes sealant. Has remained dry for 6k miles so far.
I have measured pinch or should I say how much gaskets squeeze down on a handful of copper gaskets now. Overall I find they squish by only about .0012". Some times thinner near edges.
I had a small stock pile of genuine copper head gaskets from 70s. I find they measure flat within about .0002", Measuring copper gaskets from Rabers they measure the same variation. So quite close to flat. I find the thickness overall has some variation from about .041" to .055". These are considered standard thickness. Oversize are available in .070 & .120" for 650. I've only seen .070" for 750 twin. I've used oversized to the exact same results as normal thickness. The compression of oversize seems very similar. Just over .001 so the extra thickness doesn't seem to change that. The oversize do not seem to migrate out or loose torque any different than normal.
My experience has been bow doesn't seem to make gasket leak, but dips between bolts has been at least prone to leaking what looks like oil from combustion area. There is no oil around the area so it has to be from combustion gasses. I find dips more than .0015 tended to result in actual blown gasket. This has been across cylinders, not to outside.
I have always skimmed heads I felt were not flat enough. I didn't know about straightening until 5 years ago. All the heads that I've worked on that blew gasket had low spot between cyl. I do not believe this could be straightened to flat enough tolerance.
Most of these heads had to have .007-.015" or so removed to get flat surface. Morgan Machine & Marine Walnut Creek, CA did work. Specifically Tim Morgan did it personally. The machine had a very large cutter head with multiple tool bits so entire gasket surface could be cut in one pass. Usually took 2-3 passes to clean up. Tim was most careful to mount head to keep from tilting rocker box surface & remove least metal as possible. All these had 100% success. Back in the 70s & 80s I used K&W copper coat on gasket. To be clear, every head that was marginal even with K&W seeped oil from head gasket. All the surfaced ones did not leak. The ones that needed surfacing, but didn't get it blew head gasket again within 3k miles or sooner.
I find in most interesting that many have good results with heads that measure much worse than mine.
Tommy, exactly how are you measuring thickness of head gasket?
Seems odd you gasket would measure this way with both sealing surfaces flat.