Lot of very talented and patient machinists! Where did they find the tiny distributors & rings? As you can see, a lot of the engines had their push rods and rockers exposed, as they were in this original Curtiss 1915 engine which flew in the replica of the America, the first sea plane:
Back in the sixties Honda raced a 5 cylinder 125cc GP bike. And Suzuki raced a three cylinder 50cc bike. Pretty damned impressive.
