Author Topic: Shims To Bump Compression  (Read 7170 times)

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Offline earlbrown

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Re: Shims To Bump Compression
« Reply #30 on: July 27 2017, 01:43:23 PM »
I use Cometics on my build. The block and heads were decked by me using a 14" cutter head with a CBN carbide cutting disk. Head speed is fixed but I have control over cutting depth and feed speed.

I'm pretty practiced at getting a good finish when the machines not in a bad mood. No idea on the RA.   Since I have a 4.1 I counter sink the hell out of the head bolt holes, and do the same to the heads as well.  Then when I'm done there I hit the surfaces with a flat diamond honing stone with WD-40 as a lube.


With the Cometics I pop out the rivet and copper coat all 6 sides and my hand. On the hard parts I smear a 1 molecule thick layer or Yamabond around the water ports. Since I have a 4.1 I also drill a pair of steam holes on the exhaust side of the head surface.

What I just described as had a 100 percent success rate with sealing and no fluid leaks. It works so well that I can bust a hyper piston with less than one second of leaning out due to a faulty clogged fuel filter.


It also works with reusing head gaskets. My enigne right now has a pair of used Cometics since I didn't want to wait for some .027"s when I was swapping my 2nd set of busted hypers for my forged slugs.


I don't think the surface finish is the more important factor like a lot of people seem to believe. Flatness is the important thing.  If the head bolt holes are pulled up on the block a few thou, that's a deal breaker.  If the head has a .004" low area between cylinders or leading to a water port, that's a deal breaker.

MLSs are squishy like composites.  If you pull the head down on a raised area of the block, the fire ring does not have the proper clamp pressure, period.  Couple that with our 4 bolts per cylinder and it's a recipe for a bunch of ''Cometic gaskets suck'' threads.
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