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6 cyl Bristol cars Type 400 to Type 406

Head saver shims or gaskets.

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Old 10-10-17, 07:17 PM
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Default Head saver shims or gaskets.

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.
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Old 10-10-17, 10:27 PM
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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
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Old 10-10-17, 11:12 PM
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If all else fails, try two head gaskets?
Rob
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Old 11-10-17, 01:40 AM
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Two head gaskets is a recipe for them blowing .

I would not consider that under any circumstances
Geoff
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Old 17-10-17, 10:55 PM
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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.
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Old 18-10-17, 06:54 AM
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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
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Old 18-10-17, 08:18 PM
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GeoffD,
I probably did about 5/6000 miles on it before it was laid up for a long time, it was recommissioned for a new owner and has been back on the road now for several years, as far as I am aware the head has not been touched and it has not given problems, not sure what the total mileage is since the job was done, we must have just been lucky with the gaskets supplied by Paul Burd. Mike could not remember the distance between the cylinders on the imp engine but said they did tend to blow head gaskets but no worse with two, now of course for the Bristolthere are thicker gaskets available off the shelf and the option to have them custom made, indeed this afternoon Brian at Bristol Cars gave me the contact details of someone who makes thick copper gaskets which he said would do the job quite well.
Obviously if we use this head we will do all the other checks you suggest but looking at it a bit more carefully this afternoon I am not sure we will, it is an early head off an 85A engine which as well as being well skimmed has had a lot of welding done on it because of past corrosion problems and the finish in terms of the shape of the combustion chambers is not uniform.
I have a much better 85C head which looks as though it has never been skimmed but has slight erosion between two pairs of cylinders where a gasket has obviously blown, I am taking both heads to a local engineer tomorrow and I suspect we will end up going with the later head which will work fine with a standard gasket and I know will be an easy repair for him.
Will also see what he thinks about the early head and if he can improve the finish of the previous work done on it.
We have got the super unleaded over here so a higher CR should not be a problem based on your experience.
Thanks for the advice which is a great help.
GeoffK
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Old 19-10-17, 11:33 PM
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Brian confirmed what I suspected regarding the early head, it needs a fair bit of machining work on it to make it serviceable, he suggested in fact that the whole face could have been welded and then cut back, as it stands the differences in the shape and size of the combustion chambers plus the way a few of the valve seats had been cut would not have given a smooth running engine.
The later head by comparison needs minimal work so that is the one that will go on the rebuilt block and the early head will receive further work and be kept as a spare.
As an aside he also told me that the early BSA motorcycle heads suffered similar corrosion problems to the early Bristol cylinder heads and presented similar challenges in repair and machining, at least it is in the right hands and will be saved rather than scrapped.
GeoffK
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