Hello John,
I wondered about the insert too, unfortunately the previous owner is no longer with us, Davey was a mechanical wizard
but the engine looks barely run in, the piston shows no marks, the honing marks are still very visible on the cylinder and the exhaust port shows very little blackening, you can still see clean metal.
Plus the Biplane wings had a single 1/2 x1/4 spruce main spar with lightening holes!! and it used shearing elastic for rigging wires.
I doubt it had many flights with banana shaped wings!! Probably survived the maiden flight then was put away
What I'm getting to is I don't see Davey modifying the engine without reason.
But then perhaps the bigger plug didn't fit under the cowl!!!!!
Perhaps the real test should be bolt it onto a test stand and see what it'll turn
Thank you for your deliberation, I've taken more shots this morning with measuring tape in shot hopefully this helps.
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