Series Land Rovers
Hank Rutherford, Steve Denis, Peter Ogilvie
7:1 head is 3.700 in thick, chamber volume is 91 cc.
8:1 head is 3.600 in. thick, chamber volume is 75.2 cc.
7:1 pushrod length overall, 7.300 in
8:1 pushrod length 7.175 in overall (some parts suppliers are not able to tell you the difference between pushrods, just that they are different)
Within the range of our interest, there is about 1.2 cc change in volume per .010 in. change in thickness
Milling .100 in. makes a 7:1 head 8:1. Almost exactly.
Head gasket volume and volume above the piston crown is 4.25 cc.
Swept volume of one cylinder is 571.5 cc. One person on the LRO list sawed a 7:1 head into slices to measure casting thickness. He discovered that there is little material left between the waterways and the head face after milling the required .100 in. IMHO, it would NOT be advisable going any further as there are doubts the head would hold up for long. Cracks may develop on the head face. If you want a 9:1 CR, use an 8:1 head. The castings are definitely different, a stock 8:1 head has the same metal thickness in critical areas as a stock 7:1 before machining.
I would advise obtaining a burette for measuring combustion chamber volume to be sure you don't go too far with CR.
If you modify a 7:1 to 8:1, use 8:1 pushrods. Otherwise the rocker geometry is off, and the possibility of fouling the rocker exists. By rocker geometry, I mean the angle that the rocker arm contacts the tip of the valve stem. If the longer pushrods are used in place of the shorter (correct) ones, the rocker arm depresses the valve with it's very tip, concentrating wear and forces at a place where the rocker was not designed to be operated. This can cause premature wear of the valve stem, unusual wear of the rocker arm, and failure (breakage) in extreme cases.
AFAIK, there is no difference in the 8-1 and 7-1 heads in the later (after '62 or so) castings. Rover just mills an extra .100 off to raise the compression. The later castings can be identified by an extended head bolt boss in the area of the carburetor. There is a non extended boss head when a 7-1 head is mentioned in the above. It is doubtful that many of the early heads are still around as they seemed to crack fairly easily even when milled to 8-1 specs. Finding any of the longer 7-1 pushrods may be difficult. It might be that the longer push rods were phased out with the Series two engines or maybe even earlier.
BTW, There may be several later type castings. Three different 7:1 casting have been seen here in Canada, told apart by differences in the thermostat housing.