Jayp
Dick Jones here, Yes U can use a car pipe for break in, I recommend, 3 to 4 tanks of fuel thru the engine, at low idle, do not let the engine go to high RPM"s, use a rod and a clevis to keep the RPM's low, when U hear the engine start to "rev", running out of fuel, close the throttle, and put your finger over the carb intake to shut the engine off. Since this procedure is a no load break in, U will need to put a good marine pipe and header on, when U put the engine in the boat, grease the shaft, make the shaft length long enough to leave, 3/8, to 1/2 inch of the drive dog past the strut, as the shaft will be to tight against the strut if U don't, and a good possibility, of losing the shaft and prop. I always cut the shaft length so there is enough of the shaft to go all the way in to the collet ,I use a low temp solder to, solder the the shaft /,1 inch down from the end of the shaft. After I shoulder, I then use some coarse sand paper on the soldered area, while the shaft end is in my portable drill, until the od of the soldered are is able to slide into the collett.
Hope this helps, if I can be of any more help, just email please
dick jones,cape coral,fl
Past IMPBA President, US 1 champion, IMPBA hall of fame.