[Malcolm Towrie]

An update: due to the low compression test pressures reported at the beginning of this thread, we just pulled the engine to find the cause. I've only got the heads and intake manifold off so far, but checking each valve when fully open, I can see damage to the exhaust valve seats of the 4 low pressure cylinders. That seems to be the culprit. The other exhaust valve seats look ok, so I'm not sure why these particular seats went bad. Tappet clearance too tight and holding the valves off the seat perhaps?
The cylinders are 0.015" over and a bore gauge check of the bores just above the pistons at BDC and at the TDC wear "ridge" shows wear in the 0.002 to 0.003" range for all cylinders, so acceptable, IMO. But even the 4 "good" cylinders only had 85 to 95 psi results instead of 110 psi, so I think the bores need honed and new rings installed.
The valley shows a depressing amount of sludge on all surfaces. And the oil pan has a good layer of black sludge on the bottom. We use a 15W40 diesel oil in most of our vehicles. This is a high detergent oil and I suspect it is bad news in this type of application as the detergent dissolves the stable sludge deposits and circulates it around the engine. We should be using a non-detergent oil on these old engines.
I also found a water pump with the bearings almost seized, front crank seal rope packing installed so tight, it had burnt the paint on the timing cover, causing a severely scored front pulley sleeve, and a crack in the block around the rear boss for the oil cooler supply, due to overtightening the fitting. I'm not sure how to repair that. All in all, a typical 75 year old engine.
Malcolm