Jim, and all -
GMA wrote in several places about the desirability of minimum clearance between the rim of the head in the cylinder. I recall an article in, think t'was, an Aeromodeller Annual, where he described his method of turning down said rim and press-fitting an iron band on it.
The band was precision-turned to make a very tight fit in the cylinder.
That was back in the days before ABC/ABN/AAC... the days of Meehanite, or ringed aluminum, pistons in steel or iron sleeves, occasionally chromed for a ringed piston.
He cited a considerable gain for high-performance engines, and had a reputation for prepping very fine engines for such use.
For the somewhat less demanding conditions we have in stunt, at least a very nice free-running fit could improve cooling. A coat of charred oil is not a good heat conductor. However, I suspect the main heat flow from the head to the casting is via the tight clamping to the top of the cylinder...