RE: FOX ENGINES
Iron piston engines just plain vibrate more, because the pistons weigh more. That's all there is to it. After running ABC/AAC engines for a while it always surprises me how much my old iron piston Fox Stunt vibrates. Haven't run my Mark IVs in a while, either, maybe I should build something period-appropriate (maybe a Mongoose) just for them. The Mark VI and VII (still in pieces in a bag) are ABC motors.
Not all iron piston engines took a long time to break in. Fox Stunts did, the old OS baffle piston engines did. The likes of the Mark IV didn't - you ran them rich on the ground for a couple of bladders worth, pinching the fuel tubing now and then to let them lean out and rev briefly, then broke them in in the air with a slightly rich needle setting. After a half dozen runs or so they were good to go. Unlike the baffle piston engines, at no time would they refuse to run unless blubbering rich, even new out of the box. They'd last a season before they needed chroming, after which they'd last several seasons before yet another chroming. I think they learned to taper the bore and piston, like ABC motors, though they never were set up tight at the top like ABCs were. The old baffle piston motors had straight bores.
Iskandar