Cylinder head aluminium v Iron

[Parazine]

I've always suspected the folklore about the tendency to oil plugs comes from the days when the standard of engine rebuilding was "it'll do"

It isn't folklore at all. I had an engine in my first, 1928 Chummy which went like stink but if held up, with a partially open throttle, the 14mm plugs on the HC head would oil instantaneously. 

I managed to get a set of Cord rings (the block was +0.060, gas flowed, big valves and cracked), fitted them and all was well for 10,000 miles. Then the oiling started again.

Eventually, I opened out the plug holes and re-tapped for 18mm plugs. Problem solved and with the 1A911 number removed the eligibility scrutineer would have been happy too!