85mm Head Gaskets

[rgh0]

There was some discussion a couple of months ago around the availability of 85mm head gaskets for use with 83.5 or 84 mm bored engine blocks.

I thought i would post up a set of photos of the different 85mm head gaskets for peoples reference having just received a Cometic one that I ordered to see what they look like.

For reference a standard 83.5mm Copper steel gasket stamped with the Lotus part number 26E0026 probably made by Payen

HJ head gaskets Melbourne 85mm CFM - unfortunately now out of business but I still have a few new spares. This has an Aluminium paint style sealant on the gasket. I have been using them in high compression race engines for many years successfully.

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

Gasket Solutions 85mm CFM - purchased direct from maker in Queensland. A friend has used them in his race engines for a number of years with no issues

You will note differences in the details of hole sizes in the various gaskets but none of these appear to be critical to engine gasket sealing performance or cooling performance. The one significant difference potentially is the large triangular hole top left on the standard and HJ gaskets version that goes close to the fire ring and is often a point of failure with corrosion on the block or head in this area is replaced with a smaller hole on the Cometic and Gasket solutions version


Cometic and Gasket solutions are the only people currently making 85mm CFM head gaskets for the twin cam that I am aware of.

regards
Rohan

[fatboyoz]

Hi Rohan,
Can the 85mm head gaskets be used from standard bore upwards?
If not, what size head gasket is used for standard and the oversize sizes?
Cheers,
Colin.

[nmauduit]

Thank you Rohan for raising attention on this topics : I then found out from the Cometic site that the CFM-20 head gaskets may come in various thicknesses, which can be handy...

CFM-20
Tanged and galvanized steel core. An asbestos-free soft material compound containing aramid fibers, inorganic fillers, and high-grade binder elastomers is applied to both sides of the core.
Thickness: .043, .051, .059

https://www.cometic.com/materials/fiber-metal-laminates

Please let us know if/when you have an opinion of actual use on a Lotus TC, esp. comparison with the Ajusa Composite fiber gaskets...

[rgh0]

Hi Rohan,
Can the 85mm head gaskets be used from standard bore upwards?
If not, what size head gasket is used for standard and the oversize sizes?
Cheers,
Colin.

I generally aim to use a gasket with 1.0 to 1.5 mm larger gasket fire ring ID than the block bore, as that maximises the compression ratio while minimising the risk of gasket failure. The standard Lotus gasket was 83.5 mm fire ring ID for a 82.5 mm block bore.

If you use a gasket with the ID the same or only a small amount larger ( say 0.5mm) than the block bore the fire ring is very exposed to combustion heat especially as normal tolerances on the block and gasket dimension will not have every gasket fire ring perfectly centred on every bore. This can result in the fire ring overlapping the block bore and it cracking and leaking . You can try a size for size fire ring ID and bore and they may work in a road engine if your lucky with the tolerances, I certainly would not try it in a race engine

Gaskets are available in various fire ring ID and normally I would use as follows

Block Bore Gasket ID
std 82.5 std 83.5
83.0 84.0
83.5 85.0
84.0 85.0
84.5 86.0
85.0 86.0

I would not try to go above 85mm block bore and even that can be done on only a few blocks

cheers
Rohan

[ceejay]

Rohan, when I built the 1700, I sourced the 85mm head gasket from Larner Engines Melbourne, I think it was a Cometic brand, no issues with it so far. Sad to learn that HJ Gaskets no longer in Business, I had previously used two or three from them, all extremely high quality custom made gaskets.
CJ

[2cams70]

Glad to hear you are giving Gasket Solutions a try Rohan. I posted here about this company a while ago now. Unfortunately Dale the owner of HJ gaskets passed away I believe.

As far as I'm aware Cometic currently only supply the 85mm CFM gasket in 0.043" thickness for the Lotus Twin Cam in which case there's no real advantage over the Adjusa CFM gasket. Gasket Solutions like HJ previously can supply custom made CFM gaskets in a variety of thicknesses unlike Cometic and Adjusa.

The key thing when fitting head gaskets is to accurately align the gasket cylinder hole openings as concentrically as possible with the cylinder bores. You can't just rely on head bolts with heads cut off and used as guide dowels to do this for you. There's still too much tolerance and clearance between the head bolts and the gasket. The gasket hole alignment is much better done by eye before you lower the head. Use two very small drops of glue between the gasket and the block to prevent it moving once you have found the optimal position and before you lower the head.

Other engines (modern ones) tend to have proper dowel pins between the head and block to accurately align the the head to block and gasket to block interfaces. Not so on these ancient Ford based engines.

[fatboyoz]

Thanks Rohan,
My Elan is still on its original bore size at 93,000km. Just thinking ahead.
Are you doing either the Tasmanian Tour, or the Elan’s 60th birthday in the vines at the Hunter Valley?
Cheers,
Colin.










quote="rgh0"]

Hi Rohan,
Can the 85mm head gaskets be used from standard bore upwards?
If not, what size head gasket is used for standard and the oversize sizes?
Cheers,
Colin.

I generally aim to use a gasket with 1.0 to 1.5 mm larger gasket fire ring ID than the block bore, as that maximises the compression ratio while minimising the risk of gasket failure. The standard Lotus gasket was 83.5 mm fire ring ID for a 82.5 mm block bore.

If you use a gasket with the ID the same or only a small amount larger ( say 0.5mm) than the block bore the fire ring is very exposed to combustion heat especially as normal tolerances on the block and gasket dimension will not have every gasket fire ring perfectly centred on every bore. This can result in the fire ring overlapping the block bore and it cracking and leaking . You can try a size for size fire ring ID and bore and they may work in a road engine if your lucky with the tolerances, I certainly would not try it in a race engine

Gaskets are available in various fire ring ID and normally I would use as follows

Block Bore Gasket ID
std 82.5 std 83.5
83.0 84.0
83.5 85.0
84.0 85.0
84.5 86.0
85.0 86.0

I would not try to go above 85mm block bore and even that can be done on only a few blocks

cheers
Rohan[/quote]

[HCA]

Hi Rohan

Do you know what head gaskets QED supply?

For their 85mm option that I will need, they just list it as 'steel', no manufacturer name or compress info, at a price of £89. Or do I buy elsewhere the tried and tested Cometec version?

[pharriso]

Do you know what head gaskets QED supply?

For their 85mm option that I will need, they just list it as 'steel', no manufacturer name or compress info, at a price of £89. Or do I buy elsewhere the tried and tested Cometec version?

They recommended & supplied me with a 84mm AJUSA composite gasket a couple of years ago for my Std bore engine :

I fitted it in March last year dry & have had no issues.

[rgh0]

Hi Rohan

Do you know what head gaskets QED supply?

For their 85mm option that I will need, they just list it as 'steel', no manufacturer name or compress info, at a price of £89. Or do I buy elsewhere the tried and tested Cometec version?

It is probably a MLS gasket, maybe Cometic maybe from someone else. I would not use a MLS gasket on a twin cam as I think the head is not stiff enough to seal them reliably and i know a number who have tried and its not worked. Maybe an extra thickness one with more layers that lowers compression ratio is capable of being reliably sealed with a twink head but I dont know anyone who has tried that. The new twink heads may also be stiffer and more tolerant of MLS head gaskets.

Lots of variables and until better data I stick with the CFM gaskets which are harder to find in 85mm but are sold by Cometic.

cheers
Rohan

[HCA]

I can see your theory in a single sheet being better than multiple layers, and if you have been racing with them, they are going to be good for a road car engine.

I do not know if the new heads are any stiffer, other than the manufacturer claims them to be 'superior in construction' to the originals.

Cometic do still make the 85mm CFM head gasket (using CFM20 material if thais means anything...) under the same part number as the one above, C14098-043. Six week delivery to the EU, $US100.

[rgh0]

The problem with MLS head gaskets is that they are made of thin stainless steel sheets separated by a very thin synthetic rubber layer. This construction results in a very stiff non compliant gasket. This needs a very flat and stiff head to compress properly to seal. The Twincam head is marginal in this respect.

The CFM gasket is less stiff and more compliant and seals very well with a twin cam head. The original 3 layer -steel / fibre / copper is also more compliant.

The problem with the Twincam head I believe is that it needs a relatively compliant gasket that compresses and recovers as the head heats up and expands while still being held by the bolts. A to stiff gasket results in the bolts sinking into the head and loosing gasket compression and the gasket failing.

regards
Rohan

[ceejay]

Fully agree with your comments Rohan.
To provide more head bolt support with the twin cam head, I machine new T washer inserts and carefully counter bore the head bolt hole to create a slight press fit, (Inc a dab of Loctite 680) for the T washer insert. This mod has so far shown that its beneficial with no gasket failures. The T washers are machined from 4140, and blackened in a chemical bath. Photo attached of a typical twink head bolt recession, and T washer insert. We did all of this work in house, but any good engine shop can provide the service.

[2cams70]

The root cause of recession around the bolts is that the head has gone soft or head bolts have been fitted without using the hardened washers underneath. I was fortunate with my head as when tested for hardness it was in the upper range. It has no bolt recession whatsoever in this area of the head.

I'm not sure that putting a sleeve in will be a 100% cure. If the head is soft it will still be soft and will still recede where the sleeve and head make contact - i.e recession will still occur but it will be on the lower surface rather than the upper one.

[rgh0]

The Twincam head is thick compared to many modern alloy heads with 120mm of aluminium under the bolts down to the gasket. The thermal expansion of the head as it heats up needs to be taken in bolt stretch and gasket compression. Ultimately it results also in bolt recession into the head, especially as the heads age and get softer and if you use a stiffer head gasket like a MLS.

Many modern heads are much thinner under the bolts. The Lotus 9xx heads for example have the cam carriers on top of the main head casting and the thickness under the head bolts is around 70mm about half of the Twincam. The castings are also much harder material

cheers
Rohan