Which head gasket set?

[JonB]

Thanks John, but do you know what size it is? I ask because there are various size options (83, 84, 85mm) and I don't really know which is best. Assume 83mm but best to be sure. I will go for the Ajusa one, though.

[elanman999]

Jon,
As far as I can remember it's a standard size.
I don't know where you are looking but most suppliers (in the UK) only seem to offer a standard size for a copper head gasket, for up to +.020 bore. Ajusa seem come in 84 and 87.5mm. Many sizes in a Cometic MLS gasket.
Perhaps if you ask the race prep people (JS, Wilcox etc) they might have more options on copper gaskets.
I only went to an Ajusa for my Elan as it's 83.5 bore, which is larger than I could find a copper gasket for.
Cheers
John

[JonB]

Some more info.

Burton sells a copper laminated gasket with stock number FL282 as part of their top end head gasket set (FL724K). This head gasket is 83.3mm. They sell 84 and 87.5mm Ajusa only.

One of the problem with supplier web sites is they offer no information as to suitability of the parts. So I can't tell what a "standard" head gasket really is, in terms of fire ring diameter, but the Burton kit seems to be suggesting 83.3mm.

[rgh0]

So if I have a 20 thou overbore, what diameter fire rings do I need?

This was asked before viewtopic.php?f=39&t=15834&p=78925&hilit=+85mm#p78925 but there was no reply.

I would use a gasket with a bore of at least 84 mm with your 83 mm cylinder bores. i would also check that the gasket is made accurately enough and the bolt locations and bore locations accurate so that the gasket does not overlap the cylinder bore at any point. Two of the head bolt holes in a the gasket normally left front and right rear should be a tight fit on the bolts so they positively locate the gasket when fitting it

cheers
Rohan

[rgh0]

Thanks for posting. Useful information. My bores are at 84mm too and I was wondering which size gasket to use. The one I had made up which I specified as being for an 84mm bore also has the fire ring lined up with the edge of the bore. I had read that 0.5 - 1.0mm oversize of the fire ring relative to the bore size was desirable and was previously concerned that the fire rings in my gasket were too small in diameter.

What critical is that the fire ring does not overlap the cylinder bore. To achieve that you need a gasket fire ring bore about 1 mm larger than the cylinder bore given the variation in blocks and gaskets

regards
Rohan

[rgh0]

I have used Ajusa gaskets and I still have the data sheet provided with the gasket (about 10 years ago) and it says to retorque the gasket. It also says to use 69lbs/ft. However for the Ajusa gasket that I have just fitted to my Elan there was no mention of retorqueing.
Cheers
John

All gaskets take a set with heat cycles and retorque is done to compensate for this set. The head cycles compress the gasket further than when cold due to the expansion of the alloy head compared to the bolts and the gasket needs to be compressed to a point where it fully recovers from hot to cold after a heat cycle.

If you dont retorque it your head gasket will fail sooner rather than never. How critical it is depends on the gasket type. The composite fibre style gasket takes the greatest set, the original copper / set the next and an MLS gasket the least. ( which is why they may be more reusable than the other types but also why they are hardest to seal in the first place) Thus most critical on the composite fibre, less on the original and less again on the MLS

I normally retorque 3 times when using a composite gasket.
1. after the intial startup and running head cycle while checking for leaks and setting the carbs
2. After the initial run in or around 100km
3. After the first 500km of normal driving.

When I have checked a 4th time after around 1000km I don't find the head bolts move on a road engine and maybe move a fraction on a race engine

cheers
Rohan

[2cams70]

I have used Ajusa gaskets and I still have the data sheet provided with the gasket (about 10 years ago) and it says to retorque the gasket. It also says to use 69lbs/ft.

Be careful about that torque figure. That's the standard figure for the pushrod crossflow engine which uses a very similar gasket. The LTC standard figure is less. Ajusa may have just assumed them to be the same.

What critical is that the fire ring does not overlap the cylinder bore. To achieve that you need a gasket fire ring bore about 1 mm larger than the cylinder bore given the variation in blocks and gaskets

Thanks Rohan. I had my gasket made up by the same guy you probably use and was surprised when it arrived that the edge of the fire ring was just level with the edge of the bore. I specified the cylinder bore size as 84mm. I would have expected the fire ring to be slightly larger in diameter. Whilst it does not seem to overlap the bore I'm not sure how it will be once compressed. Knowledgeable guy though and I'm sure he knows what he is doing.

That fact that Jeremy seemed to be in the same situation and experienced no problems helps (not completely however!) to eliminate my concern.

[rgh0]

You can have the face of the fire ring crack if its to exposed to the full combustion temperatures if it overlaps the bore by to much. --- been there done that Probably more of a problem in a race engine than a road engine.

The top of the piston is normally a smaller diameter than the bore and does not go past the top of the block in a standard build so it rarely hits an overlapping gasket fire ring though it possible given the combination of parts and machining that may be done or been done in the past on an engine.

Thus I try to ensure no overlap at any point on any of the bores and to do that you need a larger gasket bore than cylinder bore of about 1 mm given all the tolerances unless your lucky and it all lines up perfectly.

cheers
Rohan

[2cams70]

The top of the piston is normally a smaller diameter than the bore and does not go past the top of the block in a standard build so it rarely hits an overlapping gasket fire ring though it possible given the combination of parts and machining that may be done or been done in the past on an engine.

Thanks Rohan!! - You've just given me some more stuff to worry about because the edges of my pistons are flush with or perhaps only a few thou below the top of the bore!

[elanman999]

All gaskets take a set with heat cycles and retorque is done to compensate for this set. The head cycles compress the gasket further than when cold due to the expansion of the alloy head compared to the bolts and the gasket needs to be compressed to a point where it fully recovers from hot to cold after a heat cycle.

cheers
Rohan

Rohan,
I did retorque the head, only once though. I was just pointing out that the new Ajusa instructions do not tell you to, even though they once did.

2cams70, Good point about how Ajusa get the 69lb/ft figure.
Cheers
John

[rgh0]

The top of the piston is normally a smaller diameter than the bore and does not go past the top of the block in a standard build so it rarely hits an overlapping gasket fire ring though it possible given the combination of parts and machining that may be done or been done in the past on an engine.

Thanks Rohan!! - You've just given me some more stuff to worry about because the edges of my pistons are flush with or perhaps only a few thou below the top of the bore!

The pistons go higher when running due to stretch of the components and closing up of crank and rod bearing clearances at TDC under the piston reversal inertial loads. This is why the pistons are normally set around 20 to 30 thou below the block deck on a standard build to allow for this stretch. When building a race engine and looking for maximum compression and minimum clearance you can go closer but you dont want the piston hitting the head itself. With a standard gasket of around 0.8mm/ 30 thou compressed you dont want the piston when statically measured going past the top of the block deck in a race engine as that 0.8mm closes up fast at 8000 rpm. In a road engine you can push it a little closer but you want to be careful and dont have the same need for close head to piston clearance as you're not looking for a 13:1 comp ratio

cheers
Rohan

[el-saturn]

does anybody have the flatness tolerances for head AND block tx Sandy ps: adding ajusa gasket to the equation

[2cams70]

With a standard gasket of around 0.8mm/ 30 thou compressed you dont want the piston when statically measured going past the top of the block deck in a race engine as that 0.8mm closes up fast at 8000 rpm.

At least I think I'm OK in that regard. I had a thicker than standard gasket made up (around 1.5mm compressed) to give piston clearance and to reduce the compression ratio to something reasonable for road use and QED 420 cams - around 10.5:1.

[el-saturn]

the last ajusa i used was 1,2 and compressed 1,0mm

i was asking for the flatness of block and head: lets say at the full length of the head (in the combustion area/above the block) clearance 0,01mm and across one cylinder (turned by 90?/ other axis) : 200mm clearance 0,1 just examples the problem is the two rear (triangular water passages) sandy

[elanman999]

The Ajusa data I have says it should be flat to better than 0.05mm.
Cheers
John