Changed the subject title to more accurately reflect where this thread has lead us.
Greg and Yanche: Those are all good ideas. I have choices, from making a bigger gearbox pulley fit, to turning a smaller motor pulley, to using a different motor to drive the gearbox altogether.
One plus for using a different motor is that I could save electricity. The current motor that turns the gearbox and combusion fan takes over 5 amps at 115 VAC. The distribution fan pulls over 2 amps. When everything is running, I'm probably at about 8 amps. That adds up. Yes, it's still cheaper that natural gas or oil, but if the stove run's at 50%, that's about $30 per month. Maybe that means I'm too cheap if that amount of electricity bothers me.
I'm guessing I could about half those amps. Less than an amp for the combustion blower and maybe a couple amps for the stoker motor, and the two amps for the distribution blower. I have a 115 VDC, 5+ RPM gearmotor I could try, I just don't have a motor controller for it. If I could rig up a PWM controller for it, it could be variable speed. I could remove the Prill/Wil-burt gearbox altogether.
Greg - thanks for the offer on the smaller motor pulley. Yes it's a 5/8" bore with keyway. 1-3/4" is the smallest I can order. My dad has a couple lathes that I can use. But I'm thinking that with a 5/8" shaft and a keyway, I can't get much smaller that that. Measurements for pulleys are ODs, so with about 1/2" of belt x 2, plus the shaft and keyway, I don't think 1" is possible. I'm fairly certain I need to half the rpms of the auger, so I don't think the motor pulley will do it. If you find something smaller, let me know.
Gotta decide between notching the coal bin for a bigger gearbox pulley, making a jackshaft, or trying a different motor. Hmmm....I'm thinking jackshaft or gearbox pulley is the quickest and cheapest. But the DC gearmotor could save me electricity in the long run. Now you guys got a little insight into how my brain works. I'm a civil engineer, and just always analyze everything to death. Drives my wife nuts.