Dan:

As best I remember the heads were natural aluminum.

I had some of the Spitfires too, but I built a whole bunch of the P-40s.

I was using the Allyn twin as I said, the planes I had it in eventually were replaced just because of the hard starting. So much flipping would eventually break the engine mounts, and the constant re gluing would turn the brick into a lead block. And this was using the beam mount adapter with hardwood rails inletted in the hollow log.

My main alternate power plant was an OK Cub 0.075 diesel, The planes it was on got replaced when the diesel fuel turned the balsa into mush. But I got a lot more flying with the Cub diesel than with the Allyn twin.

A long time ago.

Bill.