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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