First we need to separate the notion of compression ratio of the engine and the head depth, which at first appears daft, but makes sense when you understand how Vauxhall changed the compression ratio of cars.
On the 1057cc and 1159cc engines low compression was obtained via a thick copper gasket between the block and head, but the 1256cc engine did away with this gasket and had different pistons.
In essence the compression ratio is not linked to the head depth, it is actually changed by moving the head away form the block and or changing the pistons, not by skimming the head. That said one can identify the correct head by its head depth and in some cases modify the standard head to become a 90 spec head by skimming it. (this doesn't take into account porting and or valve sizes)
Below is a table of the minimum depths of heads for each of the various OHV models:
Compression ratios for 1057cc engines:
Standard engine 8.5:1 (HA20)
low compression engine 7.3:1 (HA20) Vans and optional on saloons
90 engine 9.0:1 (HA21)
HA20 3.175 inches (to 20HA / 253413)
HA20 3.168 inches (from 20HA / 253414)
HA21 3.140
Compression ratios for 1159cc engines:
HB22 8.5:1 & 7.3:1 (7.3:1 low compression optional)
HB23 9.0:1
HB22 3.185 inches 80.89mm
HB23 3.157 inches 80.19mm
Compression ratios for 1256cc engines:
Standard engine up to engine number 1770433 9.2:1
Standard engine form engine number 1770434 8.7:1
Low compression engines 7.3:1
Swedish engines 7.8:1
HC 3.193 inches 81.10mm (9.2:1 & 7.8:1 CR)
HC 3.221 inches 81.81mm (8.7:1)
HC 3.235 inches 82.17 mm (7.3:1) Low Compression
The picture below shows the basic design of the head, and its layout, on the 1057cc and 1159cc heads compression ratios were altered via a thicker head gasket, 1256 compression ratio via Low Compression pistons in the block.
