I apologize for resurrecting this old post, but I didn't motorize my mill 2 years ago. After combing the web for info, I have put together a concise list of all the Grainger parts you need to motorize (1725 or 1750 rpm motor) the 4" mill that MoreBeer offers. Quite frankly, I'm surprised they don't offer this as an upgrade kit. All this for under $50 (hey, if I could "borrow" a VFD and lovejoy coupling set-up from work, I'd be typing this email while a PLC was making beer in the basement):
2" sheave: 3X896,
12" sheave: 3X938,
44" belt: 4L440,
bushing reducer: 4X664
All that said, you might need a different 2" sheave based upon your motor shaft and a longer or shorter belt, but these part numbers will put you in the neighborhood to find those options without spending hours combing all the chat boards and catalogs. BTW, I didn't do an exact calculation since 290 rpm vs 295 rpm doesn't make much difference (I'm a retired EE who doesn't miss all the calculations, besides, we're ignoring wind resistance, surface tension and friction anyway), but this configuration will put you under 300 rpm unless you have a synchronous motor running at exactly 1800 rpm.