85mm Head Gaskets

[alan.barker]

Thanks Rohan,
Lots of good info.
Alan

[HCA]

Cometic 85mm CFM part number C14098 - 43 - sourced through Australian distributor Serco PTY LTD in Queensland. Note Bore size actually 85.4 mm. I dont know anyone who has used these but no reason for them not to be good

Cometic 85mm CFM part No. C14098-43.jpg

regards
Rohan[/quote]

Rohan - as discussed, this is the one I have.

I am going by the comments by Summit on the compressed thickness is the last digits of their part number of similar gaskets...

https://www.summitracing.com/search/par ... ial/cfm-20

[rgh0]

Hi Hal
The thickness of 0.043 inch ( 1.09 mm) is the quoted "compressed thickness" by Cometic on thier website ( and Summit)

1.15 mm is the uncompressed gasket thickness, as I measure it, with the sample I have of the cometic composite fibre CFM-20 gasket for the Twincam. The fire ring itself is thicker on the new gasket as the metal is formed around the uncompressed gasket

While I have not yet fitted one of these actual gaskets to an engine I find other composite gaskets typicallycompress around 20% to 30% when the head is torqued down in my experience and rebound to about 10% compression when removed. Thus I would expect its installed compressed thickness to be around 0.8mm and thickness of a used gasket of around 1.05 mm.

Maybe i am wrong with the Cometic CFM gasket and it compressess only around 5% when torqued down. That is the sort of compression i would expect from a MLS gasket and is low for a fibre gasket. You need your head and block to be very flat and stiff if thats the case to get even compression over the whole head face which is why MLS gaskets struggle to seal on a twin cam head as it is just to flexible.

An interesting writeup in the attachment below on various composite gasket manufacutring technologies and how it affects the amount of compression experienced. Based on this it appears possible to get very low compression percentage in a composite gasket with less rubber in the composite mix and with pre compression of the gasket.

AA002 Torque One Technology.pdf

(44.07 KiB) Downloaded 123 times

I guess I will find out more when i get around to installing the Cometic gasket on an engine.

cheers
Rohan

[ill_will]

Interesting thread.

One detail I have struggled to find in the Cometic literature is their recommendation for installing CFM-20 gaskets. They say the MLS ones should go on dry, is that the same for CFM?

With the standard copper gasket I've put a smear of aviation sealant around the oil feed, but unsure if that's needed.

I haven't offered it up yet but looks like it might need to be cut for the oil drain tube, which is disappointing.

Any pointers welcome.

[JonB]

Interesting thread.

With the standard copper gasket I've put a smear of aviation sealant around the oil feed, but unsure if that's needed.

When I did the head gasket on my +2's Big Valve engine, I followed the book and smeared good old Wellseal on both sides of the gasket prior to fitting. Had no problems, other than a bit of Wellseal dripping down the newly painted block.

Of course, the water pump housing gaskets didn't go as well, but that's a different story.

[DavidP]

Tony Ingram sells gaskets described below:

Gaskets are composite and right around 1mm compressed.
Bore size is 85.5 mm. I run them on all my engines now from 10-1 street motor to 13-1 competition engines.
Made in Italy and run US$82.50