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Discussion Starter · #1 ·
I suggested using the 'ol cometic .030 to my engine builder. He said that he wouldn't use a thinner gasket because of potential for gasket failure. He said the SE stock thickness gasket would last longer.

Has anyone heard of a problem long term?

Also, I assume I can use the stock pushrod with the .030 gasket?
 

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I recall seeing one post on one of the boards about someone having problems with the cometic .030, with leaking. I think he went through a couple of sets, and then switched a thicker gasket, and didn't have any more problems. But moreover, I was told that the optimum squish area was about .035 to .040, and, that over time there's some carbon build up in the squish area, such that the .030 really gets a little too thin. Other than that, everyone really seems to recommed the .030.
 

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AAL said:
I recall seeing one post on one of the boards about someone having problems with the cometic .030, with leaking.

I believe that the leaking problem can usually be traced back to improper torqueing of the head bolts. The .030's have a different torque procedure and if you use the procedure in the service manual, they are prone to leak. I've no experience with this myself, it is just what I read on another board.

Teu
 

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Cometic has a torque spec of 42 ft pound and the manual says 40 ft pounds. I went with the Cometic spec. You also have to slowly torque them down 7 14 24 32 42. That way the squish is properly formed.
 

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I've been using the Cometic .030 at 10.7:1 CR for 15,000 miles with no problems. I used the Cometic Torque specs, not Harleys.
 

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The guys above have it correct. The only time I have seen a cometic fail is because of incorrect installation procedure. They ARE different than regular gaskets. If the builder won't use them then mill the heads to get to the compression you want.
 

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I would definately would not use a builder who said he would not use Cometic .030 head gaskets. I would not trust his knowledge about the rest of the build.

It is industry standard. I have never heard of a well known builder that does not use them.
 

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Discussion Starter · #8 ·
He's a factory trained tech who has been servicing all the police bikes in this county for the last 10 years, so I trust him. He says he's rebuilt more H-D Evo's & Twin-Cams than he can remember. They probably just don't use cometics on the police bikes.

How 'bout my stock pushrods with that gasket?
 

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kslg said:
He's a factory trained tech who has been servicing all the police bikes in this county for the last 10 years, so I trust him. He says he's rebuilt more H-D Evo's & Twin-Cams than he can remember. They probably just don't use cometics on the police bikes.

How 'bout my stock pushrods with that gasket?
Stock pushrods will work fine.
 

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I bought a Cometic EST motor set that was short 3 pushrod cover o-rings. Also their "race style exhaust gaskets" were too big to fit into the recess smoothly & smashed into the port after the pipes were installed. SE exhaust gaskets fixed that.
Next time I'll buy Harley gaskets and get the Cometic head gasket seperately.
 

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Discussion Starter · #11 ·
Just to make sure I am coming up with the right part # C-9721 for a Cometic .030 on a 1550 twinkie?
 

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kslg said:
I suggested using the 'ol cometic .030 to my engine builder. He said that he wouldn't use a thinner gasket because of potential for gasket failure. He said the SE stock thickness gasket would last longer.

Has anyone heard of a problem long term?

Also, I assume I can use the stock pushrod with the .030 gasket?
Cometics are fine, unless you plan on running it for a few hundred thou before tearing down the top end... not likely. ;)
 

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Both the SE and cometic are coated metallic layers. One will last as long as the other. I suspect the SE is .060 to prevent liability claims from some bozo crashing his pistons into his valves because he bought parts out of a catalog with no real research or knowledge. A .060 gasket is WAY too thick. Talk about quench clearance. The cometics are fine if that's the thickness you want. Did you know the multi-layer metallics usually have maybe a .010 or .020 thick base metal wafer with identical .010 embossed layers on either side to achieve the desired thickness? If you have a .050 thick cometic and want a .040 just drill out the rivet and peel out a layer. You'll see what I mean. Just make sure to leave at least one embossed layer on either side. THis is not probably endorsed by cometic but I've done it and I could see absolutely no difference between the embossed metal layers.

Also, cometic specifies a level of surface finish for head and cyl surfaces. Don't recall what they recommend, but how many guys can recognize the difference between a 120 Rz finish and a 250 Rz finish? I doubt most guys know which is smoother, the higher or lower number. So follow the cometic instructions and you'll be OK I think. If you don't want to follow their directions stick with paper and plastic.

Chilly
 

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Chilly said:
Also, cometic specifies a level of surface finish for head and cyl surfaces. Don't recall what they recommend, but how many guys can recognize the difference between a 120 Rz finish and a 250 Rz finish? I doubt most guys know which is smoother, the higher or lower number. So follow the cometic instructions and you'll be OK I think. If you don't want to follow their directions stick with paper and plastic.

Chilly
You could always use a surface roughness gauge to check the finish.... but who has one of those in their toolbox?? I used one as a QA inspector, but have never used it outside of work.
 
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