Since the seal surface on a stovebolt crankshaft is 3 7/8", any standard bore size 283 or 307 piston will work- - - -just stuff a piece of dowel rod into the wrist pin hole for a handle. I happened to have a Ford 302 piston lying around in the scrap heap, so I just chucked the piston ring lands into a lathe and turned the 4" skirt diameter down 1/8". Use the skirt area of the piston to roll and flatten the rope seal all the way down into the bottom of the groove. A few taps with a rubber mallet or a dead blow hammer helps.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!