Author Topic: Cometic .020 base gasket question  (Read 1373 times)

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Offline JW113

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Re: Cometic .020 base gasket question
« Reply #25 on: August 13, 2020, 06:55:36 AM »
I believe all of the Cometic steel base gaskets are micro rubber coated to promote sealing.  It is so thin it does not add to the thickness.

Yeah, that's what I recall. I have them in my '92, nary a hint of oil weep and did not do any machining to the cylinders either.

-JW
2004 FLHRS   1977 FLH Shovelhead  1992 FLSTC
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Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #26 on: August 13, 2020, 03:32:52 PM »
Thanks guys, all great advice.

One question about checking the deck height -- big boyz site shows a technique where the cylinder is bolted down using spacers and cyl studs to then check the piston height at tdc. But they say to use an old crushed base gasket (which I  don't have) and they don't say how much they torque down the cylinder to check deck height?. See below.

Adam, clean the deck and do it without a base gasket.  You can then do the math and calculate what the deck height would be with a gasket. If you can come up with a used base gasket, simply measure it with a micrometer and if not .020 do the calc. based on what you have. You can torque the barrel with a gasket but IMO on a bare fit simply good and snug approaching the torque value is fine.  Always use a torque wrench so you don't go over the spec and pull a stud. Be sure to use something under the head bolt as a spacer to prevent scarring the aluminum cylinder gasket surface.  I use a sleeve / spacer made from PVC pipe and that works well. 

There are several ways to get the measurement and a dial indicator seems to be the best.  You are more likely to find the piston "in the hole" rather than the way your pic shows unless you are not using a base gasket.  With no base gasket the piston is more likely to be out of the hole as the pic shows.  Searches on this site will bring up many discussions and ideas for you to consider.
Thanks kd, that's exactly the info I needed.  I'll check them with no gasket at all, then add .018" to the cylinder height (for a .020" gasket) when doing the math to factor in the base gasket thickness crushed?
Does that sound right? 

Cheers  :up:

Offline kd

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Re: Cometic .020 base gasket question
« Reply #27 on: August 13, 2020, 05:34:02 PM »
If you're using the MLS Cometic gaskets, when they say .020, it's .020 installed and torqued height.  They will tell you the same and it has been my experience when setting mine up.
KD

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #28 on: August 13, 2020, 10:10:50 PM »
If you're using the MLS Cometic gaskets, when they say .020, it's .020 installed and torqued height.  They will tell you the same and it has been my experience when setting mine up.
👍👍

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #29 on: August 14, 2020, 12:49:58 AM »
I believe all of the Cometic steel base gaskets are micro rubber coated to promote sealing.  It is so thin it does not add to the thickness.

Yeah, that's what I recall. I have them in my '92, nary a hint of oil weep and did not do any machining to the cylinders either.

-JW

Yeah,  viton micro rubber coating. 001" thick - I just found out the hard way 😂 
I stupidly used a cloth with a little brake cleaner on it to wipe the gasket surface to get some oil off and this is what happened....

Is it good for the trash now?


Offline rigidthumper

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Re: Cometic .020 base gasket question
« Reply #30 on: August 14, 2020, 06:13:45 AM »
Should be fine- I clean all gaskets with a lint free cloth and non-chlorinated brake cleaner before installing.
Ignorance is bliss, and accuracy expensive. How much of either can you afford?

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #31 on: August 14, 2020, 06:24:58 AM »
Should be fine- I clean all gaskets with a lint free cloth and non-chlorinated brake cleaner before installing.
Ok, cool.   I got all worried about the special .001" viton rubber coating  😅

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #32 on: August 14, 2020, 06:30:44 AM »
If you're using the MLS Cometic gaskets, when they say .020, it's .020 installed and torqued height.  They will tell you the same and it has been my experience when setting mine up.
I checked the piston height on the front cylinder installed with no base gasket at all, and it came out at .010" above deck height.  So my guess is with the .020" of the base gasket I just need to remove .010" material from the base of the cylinder and I'll be at zero deck.

I actually will probably have to take .005 from the base and .005 of the top of the cylinder as the top surface doesn't look smooth enough for a good seal?

Online 98fxstc

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Re: Cometic .020 base gasket question
« Reply #33 on: August 14, 2020, 05:11:02 PM »
Should be fine- I clean all gaskets with a lint free cloth and non-chlorinated brake cleaner before installing.

Robin
Do brake cleaners usually indicate chlorinated or non-chlorinated ?
I haven't looked for this in the past.
Thanks

Offline rigidthumper

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Re: Cometic .020 base gasket question
« Reply #34 on: August 14, 2020, 07:00:05 PM »
The ones we use have to be notated due to EPA regulations. 
Ignorance is bliss, and accuracy expensive. How much of either can you afford?

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #35 on: August 14, 2020, 09:24:39 PM »
The ones we use have to be notated due to EPA regulations.

Hey rigidthumper, thanks for the  input.

If I'm .010" down the hole would you recommend getting the cyl bases milled or just use a .010" MLS base gasket instead of the  .020" one I have?
Thanks

Offline kd

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Re: Cometic .020 base gasket question
« Reply #36 on: August 14, 2020, 09:27:36 PM »
 :doh:  I completely forgot the .010 MLS base gasket was available from Cometic.  I actually have a couple for 4.060 barrels.
KD

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #37 on: August 14, 2020, 09:30:23 PM »
:doh:  I completely forgot the .010 MLS base gasket was available from Cometic.  I actually have a couple for 4.060 barrels.
Thanks, I think I'll go the gasket route instead of more machining if it's a safe enough option. It's only a mild build at 9.5 comp with zero deck and .030" HG.

Offline wfolarry

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Re: Cometic .020 base gasket question
« Reply #38 on: August 15, 2020, 02:44:31 AM »
I always true up the cylinder bases on the lathe. Those Evo cylinders are rarely flat & that Cometic gasket likes a smooth flat surface.

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #39 on: August 15, 2020, 04:07:50 AM »
I always true up the cylinder bases on the lathe. Those Evo cylinders are rarely flat & that Cometic gasket likes a smooth flat surface.
Thanks Larry, input much appreciated. I don't want to take any chances that may cause me grief later.....

While I'm truing the bases on the lathe,   I have the opportunity to properly set the quench / squish.... , What's your recommended squish setting?   30 ?  32 ?  35 ? 

EV27 with 9.5 comp pistons
.010" off the heads and basic valve job.
With Cometic. 020" gasket my pistons are .010" in the hole. 

Thanks for any advice.
« Last Edit: August 15, 2020, 04:42:24 AM by Adam76 »

Offline rigidthumper

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Re: Cometic .020 base gasket question
« Reply #40 on: August 15, 2020, 06:40:41 AM »
The ones we use have to be notated due to EPA regulations.

Hey rigidthumper, thanks for the  input.

If I'm .010" down the hole would you recommend getting the cyl bases milled or just use a .010" MLS base gasket instead of the  .020" one I have?
Thanks

I'd take a little off the top, 3-5 thou, just to have a fresh, true-to-the-bore, head gasket surface, and the rest off of the base, partly to set deck height, partly to have a fresh, true-to-the-bore, gasket surface.  I like to set quench distance on street bikes .030"-.035".
Ignorance is bliss, and accuracy expensive. How much of either can you afford?

Offline wfolarry

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Re: Cometic .020 base gasket question
« Reply #41 on: August 15, 2020, 09:15:28 AM »
 :agree:

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #42 on: August 15, 2020, 05:07:14 PM »
The ones we use have to be notated due to EPA regulations.

Hey rigidthumper, thanks for the  input.

If I'm .010" down the hole would you recommend getting the cyl bases milled or just use a .010" MLS base gasket instead of the  .020" one I have?
Thanks

I'd take a little off the top, 3-5 thou, just to have a fresh, true-to-the-bore, head gasket surface, and the rest off of the base, partly to set deck height, partly to have a fresh, true-to-the-bore, gasket surface.  I like to set quench distance on street bikes .030"-.035".
Thank you, that's a plan.

Offline Adam76

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Re: Cometic .020 base gasket question
« Reply #43 on: August 15, 2020, 05:07:38 PM »