Lee, once the air tank builds up to 140 psi, there is avalve between the tank and the top of the cylinder that lets the air push the piston down. The piston is cranked over to just past top dead center prior to this of course. You may see some holes in the outer rim of the flywheel for that purpose si a bar can be inserted to pull it over. There is also a valve in the top of the head to release the cylinder pressure while this is being done. The side of the flywheel is marked so you know where just past top dead center is in its rotation. I assume it was lathe turned back in the day and there is smazingly little runnout to it.
BIll