Head saver shims or gaskets.

[Geoff Kingston]

I have acquired a 401 where the cylinder head has been skimmed to the point that it is going to need a head saver plate or shim together with a head gasket or alternatively a thicker than standard head gasket, to restore the original compression ratio, the previous owner has calculated that 3 mm is needed.
I would be interested if anyone has been in this situation and how they overcame the problem, if a head saver shim was used does it have to be bonded to the head and what the best way to do this is so that the process is successful. I have seen a few firms in the UK who advertise custom gasket manufacture but have not used any so any recommendations would be most welcome.
Geoff.

[Geoff Dowdle]

Geoff ,
a bit of extra compression is often good for a Bristol engine .

A few items you should check
1. compression ratio of pistons to be used . STD 401 are 7.5 to 1
2. maximum depth of each combustion chamber and calculate accurately amount milled off head.
3. during assembly, check clearance between valves and tops of pistons

With a 85 head you can remove the shroud around the valves on the lower face and flycut the tops of the cylinder liners (as per all 100 series engines) , this will lower the compression slightly and improve gas flow.

I suggest you do a dummy assembly to calculate the compression ratio and if it is actually too high have a thick head gasket made .
I had one made 30 years ago for a D2 engine and to the best of my knowledge is still fitted without a problem .

Others here in Australia have fitted a thick solid annealed copper gasket/plate instead of a head gasket but I have had no experience with this and can not recommend either way

Post a photo of the lower face of the head

Geoff D

[Max Tone]

If all else fails, try two head gaskets?
Rob

[Geoff Dowdle]

Two head gaskets is a recipe for them blowing .

I would not consider that under any circumstances
Geoff

[Geoff Kingston]

Geoff
Interesting comment about two head gaskets as I did that a few years ago on a Bristol and it is still going strong, Mike my friend, mechanic and welder said that they used to do this a lot on Hillman Imps without any problem. A local engineer advocated a shim being placed between two head gaskets, a tractor owning friend confirmed that is the way he would approach the problem. He agreed totally that two head gasket was risky so I may not try that again,
Forums regarding the Rover K series engine recommend that the head saver shim is glued to the head, I have come across an article by Spencer Lane Jones where he mentions sealing the decompression plate to the block with wellseal so it does seem all sorts of options present themselves with a fair degree of luck thrown into the equation.
I had a good look at the head today it is an early 85A which had suffered a lot of corrosion problems and despite obvious repairs it is actually quite true face wise with no distortion, as it is fully built up I am going to try and use it.
The previous owner of the car has worked out the CR at around 10.5:1 and to return it to 8.5:1 it needs gaskets and or shims of around 3mm.
I see one supplier offers a 2 mm gasket so what roughly would the CR be with that option, you mention the engine run better with a higher CR , is 10.5 :1 too high for road use and what sort of adjustment would have to be made to timing.
I have not asked the previous owner what type of pistons are fitted but they are domed without flats on the side.
The block is an 85 series with 85c crank and 100A cam from what I can gather so a real bitsa.
Geoff K.

[Geoff Dowdle]

GeoffK,
You mentioned you used twin head gaskets a few years ago , how many miles that that car covered since and how many hot days has it run at 3000rpm plus at 80 - 90 deg C water temp ???
I have had no experience with Hillman imp or Rover K engines but would expect the gap between each cylinder a lot more than a 2 litre Bristol, which is where a Bristol gasket will blow.

If the pistons are domed without valve cutouts they are likely original 7.5 to 1 compression. THAT IS THE MAX DEPTH OF THE COMBUSTION CHAMBERS.

I ran a D2 engine in my 400 for about 15 years . It was fitted with 9.5 forged pistons and plenty skimmed off the head . I estimated it was running 10.5 or 11 to 1 compression . It always ran well with either Super leaded or 98 octane unleaded fuel that was or is available in Australia . With a decent distributor and accurate timing mark I never had a problem with it in about 70,000 miles of use.

What octane fuel is available in UK ? Subject to that ,I would have no hesitation running 9.5 - 10.0 to I .
A thick head gasket and removing some material in combustion chamber and tops of bores as described earlier will bring the compression down a little

GeoffD