I recently obtained one of Larry Jenno's Ohlsson .12 engines, a "copy" of the one-of-a-kind original from 1935. Its plug is 1/4-32 and may not be like the original. Dan Calkin came up with that size to fit his "Corncob" of 1936. Can anyone tell me the actual size of the Ohlsson .12 spark plug and who made it? I suspect the smallest production plugs during that era were 3/8-24, pretty large for a .12. Maybe Irwin made his own plug for that engine.
Thanks.
Jerry