Head Gaskets

[rgh0]

Cometic appear to make a 85mm cfm one, its listed as made to order
https://www.cometic.com/products/C14098-043
Some of the race engine builders may stock them in the UK
Never used it as I use locally made Australian ones.

A 84mm bore gasket in a 83.5mm bore block has just 0.25mm / 0.010 inch clearance if centred exactly on every bore between the edge of the bore and edge of the gasket.

I am sure it can work but that's to close for comfort for me given the possible variation in gasket fabrication, tolerance of gasket location on the head bolts and variation in bore locations versus bolt locations from the original block machining tolerances even if you don't deliberately or accidently offset the bores when reboring. I don't like pulling engines for just a leaking head gasket so I try to minimise the risks due to tolerance variations

cheers
Rohan

[2cams70]

Cometic appear to make a 85mm cfm one, its listed as made to order
https://www.cometic.com/products/C14098-043

Cometic do not supply this gasket to Joe public as a custom order despite what they say. I know because I enquired because my engine is 84mm bore.

83.5mm is the most common rebore size for a Twin Cam. If you needed an 85mm gasket for this bore size all the major suppliers would supply it off the shelf. They don't unless in MLS (which is more difficult to achieve a proper seal with).

I'm sure there's plenty of people on this forum running an 83.5mm bore size with even a standard gasket without a problem. There's an ex ACL guy here in Australia that can do an 85mm custom gasket if you like. The previous manufacturer of these custom gaskets HJ Gaskets no longer is able to supply because the owner Dale has passed away i believe. It's not necessary to do this however. 84mm gasket will be fine. Any minor overlap (less than 0.5mm) won't be a problem.

Key thing is to get the gasket properly centered. You can't necessarily just rely on sawn off head studs as guide dowels. Best to accurately centre the gasket by eye before you fit the head. It can then be temporarily secured in place by using a very small drop of superglue at opposite corners before you lower the head.

[Ronminch1]

Thanks for the responce, you are correct the marks on the block do look like gasket was much larger, I did measure and thought both were similar. I will check again.
Ron

[jono]

A few options here:

https://www.burtonpower.com/catalogsear ... us+twincam

[2cams70]

A few options here:

https://www.burtonpower.com/catalogsear ... us+twincam

When I said major suppliers that includes Burton's. You wont get an 85mm composite fibre gasket from them.

[Ronminch1]

Hi all, having had another look, i think that the shot showing a large gap between fire ring and bore is actually the head, see attached pic of block, gasket and cylinder are virtuall the same diameter, i suppose the best option would be the 84mm? Ron

[nmauduit]

I would go with the composite 84mm (Ajusa), making sure it is well centered as underlined above. You should clean up the parts to assess the amount of corrosion (e.g. block with a flexible razor blade, checking any residual gap with a straight edge lit from the rear).

Again, if it was mine, I would assess the amount of effort (cost ) to dedicate to the repair according to the condition of the bottom end - the block will likely clean up well enough for a street build (I bet most of the corrosion is within the head, applying wellseal on both side of the gasket would help sealing it under clamping), so if in ok condition a decent compromise would be a skim shave of the head to restore flatness (where corrosion took place), this is not expensive and leads to a quick and lasting repair (I've done it within 24h in the past, thank's to my understanding machinist and long hours to reassemble and retime the engine) - the cheapest would be to mock up a reface by rubbing the head on a surface plate and see from there (that does not restore parallelism if need be, only flatness and to certain extent, while a machine shop would by indicating the head prior to skimming it)

[2cams70]

If that previous picture was of the head surface rather than the block surface and the head gasket is the correct size (which it now appears to be) then that corrosion around the head water passage is quite close to the fire ring area of the gasket. Not easy to assess condition from pictures alone. Take it to your machinist and they'll probably recommend either a skim of the surface or more likely a weld repair followed by a skim. You only want to remove the minimum amount of metal during a skim and that's why if the depth of corrosion is bad it's better to weld first and then skim. The head should definitely be resurfaced either way. Probably worth having the head tested for hardness too whilst you are at it. If it's gone soft the fire rings will dig into the surface (as has happened to yours in this area), the head bolts will loose tension and you'll never get a proper seal. Check the surfaces on the head where the head bolts make contact. If the head has gone soft these will probably be worn and recessed.

[rgh0]

So original gasket used looks to be 83.3mm and close to the same as the bore. 83.3 is a standard gasket bore size for the standard 82.5 mm bore. You can see in the first gasket photo where it has overlapped the bore and been burned on the RH side by about 1 mm where it has not failed which shows the degree of misalignment possible . I would bet if you cleaned up the inner edge of the fire ring where the gasket failed that you will probably find a crack in it and similar cracks in other areas where it overlapped the bore and been burned but that had not failed yet.

As observed 84mm may work but I would not be as certain as some that it "Will" work without a lot more detailed measuring and component and assembly checking. Personally I would have the head fixed in the corroded area ( and check its hardness) and then do the research to find a 85mm CFM gasket from somewhere. Titan Motorsports in the USA list them and as it says ships direct from manufacturer so they are probably made to order for them by Cometic ?
https://titanmotorsports.com/products/l ... -85mm-bore


I dont know the details of the ex ACL guy that 2cams70 refers to perhaps he can post them for future reference. I still have a stock of about 5 of the 85mm gaskets from HJ so do not need to order new ones for a while fortunately and have not done the research fully on where to source them now in the absence of supply from HJ

cheers
Rohan

[2cams70]

I dont know the details of the ex ACL guy that 2cams70 refers to perhaps he can post them for future reference. I still have a stock of about 5 of the 85mm gaskets from HJ so do not need to order new ones for a while fortunately and have not done the research fully on where to source them now in the absence of supply from HJ

Hmm...interesting. I did post previously details about this supplier in October last year. I checked my personal email account for the record. It seems to have been deleted from lotuselan.net since however as the topic no longer exists when you click on the topic link.

Here are the details of the supplier again - note it down quickly in case this post disappears!! John is the guy running the business. He is ex-ACL so really knows his stuff.

https://www.gasketsolutions.com.au/

Note whether the fire ring on a gasket fails due to combustion temperature is not just a function of how far or if it protrudes into the combustion space. It is also a function of how well the heat from the fire ring is conducted through the head and block surfaces, the sustained temperature in the combustion chamber, etc. Of course it is ideal for it to not protrude into the combustion space.

[rgh0]

Thanks for that it looks like the same place I have heard about in Brisbane that other historic twin cam racers have been using but I have never had the time to follow-up on. I will order a gasket to see what they look like and try it in my spare race engine I am rebuilding now

cheers
Rohan

[nmauduit]

Hi Rohan, Thanks for the advice, Would anyone have a prefered supplier in the UK for 85mm composite gaskets?
Ron.

I'm afraid there are no 85mm composite gasket supplier in Europe that I know of, only 84 and 87.5 Ajusa (which arguably would fit most applications) : I've looked a while back, also for the consideration of compressed thickness options, and contacted Ajusa to that end with no success (no reply from the Australian manufacturer then). If someone on the forum knows of an alternative I would be grateful to be informed.

[Ronminch1]

Thank you to everyone who has contributed. I have now gone down the QED route, 84mm composite, plus all the other gaskets, cleaned head and block which dont look great but certinly a lot better than they were. I have checked the head as best i can with straigh edge and feeler gauge and was presently supperised. Going to reassemble for now and plan to recondition the head in the future. Will let you know how it goes. Ron

[2cams70]

If you have grooving in your head from the fire ring of the gasket you should have it resurfaced. You need to do more than just replace the gasket. Even if the head is flat the gasket will blow again. That groove in the head in the critical fire ring area will result in a localised spot where the gasket clamping is insufficient and hence it will blow again eventually. You are best doing the job properly first time around.

Well worth checking the head for hardness. It's not uncommon for the heads to go soft. If it has gone soft don't bother spending any more money on it. Replace it.

In your case these aspects are far more important than whether you choose to use an 84mm or 85mm gasket. Head gaskets usually fail for good reason and not just because of age.

[Ronminch1]

Hi 2cams, Thanks for the advise, the mark from the fire ring is not what i would describe as a grove, i was not aware that Lotus heads suffered from going soft, unlike Stags. I may have to rethink the way forwards.
Regards Ron