Lotus Elan

Cometic Head Gaskets

PostPost by: Dilkris » Sun Mar 08, 2020 3:46 pm

I have a big valve head that has had, historically, 39 thou removed from the head face. (I have a head depth measurement of 4.564") On engine disassembly it appears to have been assembled using a standard gasket. :shock:
Available machining data quotes 0.010" being the limit that can be removed from a big valve head, so I am looking at "making up" 0.029"
I am leaning toward Cometic head gaskets but I am having difficultly in understanding how they are sized.
For example is a 0.030" Cometic multi shim gasket 0.030" thick or does it act as a shim, (as the description suggests), and is therefore 0.030" thicker than a standard gasket?
.
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PostPost by: nmauduit » Sun Mar 08, 2020 5:00 pm

The Cometic gasket come in a wide range of thickness, and their MLS structure let them compress less than other kinds - but they also work best on very rigid parts, which the LTC head is not always...

What I would do is double check piston clearance (I assume it should be ok for a street application) and compression ratio in regard to intended use (could be a good time to balance the chambers up), then use an Ajusa gasket (compressed height about 1mm).
Last edited by nmauduit on Tue Mar 10, 2020 8:23 am, edited 1 time in total.
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PostPost by: el-saturn » Sun Mar 08, 2020 5:39 pm

the machining calls for much more accuracy (tolerances) and caution on both sides (block and head surfaces!!) AND you can't use 'em twice!....................so i'd advise you to use an ajusa! sandy
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PostPost by: elangtv2000 » Mon Mar 09, 2020 4:59 pm

Cometic also makes composite gaskets in different thicknesses. On the advice of my friend Dave Vegher (renowned race engine builder), I avoided MLS gaskets and instead used an 0.059" composite gasket on my Cosworth BD build he has been helping me with, because I'm using an ex-race head Dave supplied which was skimmed for high compression (and milling the pistons wasn't enough to drop CR).

You can't find the composite gaskets on the Cometic web site, so you must call them to get the available thicknesses, and to order one made up just for you. Mine took 5 days to make, another 5 days to receive. Dave has used many, many, many of these, which is the only recommendation I needed.
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PostPost by: Davidb » Mon Mar 09, 2020 10:25 pm

I have a thick MLS gasket on my Elan and I just refitted it following an engine rebuild-it looks perfect to me! I was assured by DBE that it was reusable.
I will let y'all know if it isn't...
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PostPost by: rgh0 » Tue Mar 10, 2020 3:31 am

A standard thickness MLS head gasket appears to be to stiff for the twincam head and i know a number of people who have tried them and had problems getting them to seal consistently.

A thicker than standard MLS gasket will be somewhat more flexible as it will have more layers of rubber and steel to make the thicker gasket so it probably has more chance of sealing against the twin cam head but surface finish on both head and block is critical

However if you want a thicker than standard gasket and Cometic make thick composite gaskets i would use that as recommended by Vegher (who truely knows his stuff) rather than a thicker than standard MLS gasket as I believe you will have less problems and surface finish is not so critical.

I have used thicker than standard composite gaskets on heads that have been machined down until the cast lettering on the bottom face has gone!! with good results, so you can go well beyond the Lotus recommended skimming :lol: You just need to check a bunch of things to make sure everything else works with the thinner head ( e.g. cam timing, piston clearances, valve seats etc) . I used to use a local supplier for these thicker composite gaskets (up to 2 mm thick uncompressed but he can no longer source the thicker composite material so its good know that Cometic can supply.

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PostPost by: Dilkris » Tue Mar 10, 2020 8:42 am

elangtv2000 wrote:Cometic also makes composite gaskets in different thicknesses. On the advice of my friend Dave Vegher (renowned race engine builder), I avoided MLS gaskets and instead used an 0.059" composite gasket on my Cosworth BD build he has been helping me with, because I'm using an ex-race head Dave supplied which was skimmed for high compression (and milling the pistons wasn't enough to drop CR).

You can't find the composite gaskets on the Cometic web site, so you must call them to get the available thicknesses, and to order one made up just for you. Mine took 5 days to make, another 5 days to receive. Dave has used many, many, many of these, which is the only recommendation I needed.


Understood. Why did you opt to use a 0.059" composite head gasket? "Yes" - I understand as you point out it was to reduce the CR as the head had been skimmed for high compression, but I am curious to know how you arrived at that figure.
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PostPost by: elangtv2000 » Tue Mar 10, 2020 2:06 pm

Dave calculated the need for a gasket 0.062-0.063", but the thickest composite Cometic makes is 0.059".
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PostPost by: Dilkris » Wed Mar 11, 2020 8:37 am

Thank you all for your replies.
For those members who may be interested in Cometic MLS gaskets, (MLS = Multi Layer Steel), they are manufactured in Concord Ohio, USA. There is a significant variation in opinion on them.
Burton Power (UK) offers these gaskets for the Lotus Twin Cam in 5 bore sizes, 83,84,85,86 and 87mm.
83 mm Dia appears to be the most catered for, (This equates to 3.267" so I assume these would be used on a standard bore running 3.250" pistons), with 16 thicknesses available ranging from 0.027" to 0.140". I am advised that gaskets should be between 0.5 and 1.0mm in excess of bore size.(This comment though will probably kick start another thread... :? )
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PostPost by: elangtv2000 » Thu Mar 12, 2020 4:57 pm

"I am advised that gaskets should be between 0.5 and 1.0mm in excess of bore size.(This comment though will probably kick start another thread... "

I think that is pretty standard to avoid flame edge degradation of the gasket, but it is also useful to select the ID to help adjust CR. I went with 2mm larger ID to help drop CR.
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