I remember reading somewhere that it is easier to build an engine with 7or9 cylinders than it is for 7 and smaller.Must be for the reason your stating.The more cyls in a circle would mean the degrees apart from one another would be closer or less.Plus if your making articulating rods and only 1 master rod,the master would be the hardest.The articulating rods would all be the same and would look like a normal rod.After the first one,you could use it as a pattern or like i was thinking just use weedie rods already made up.Im going to look in my WWI encyclpedia and read the descriptions on the early radial engines.I do remember they had a Rotham air pump on the wing strut with the little propeller to pressurize the fuel tank.There was also a hand pump on the right side of cockpit in case the outboard pump failed.Oops,where getting of topic,maybe we nead a thread started (scratch built multi-cylinder),we"ll see if it gets as long as the echo thread.