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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