You won't believe me!
I use a washing machine motor with an adapter and flex-shaft with collett end!
I got the washing machine motor for free, and it got PLENTY of torque. You should see this thing with a 36 grit sanding drum. You can really bear down and not even phase the thing.
I got the adapter from a machinist buddy who just took a solid bar of 3/4" round stock, and on the same center bored a 5/8" hole on one end and a 1/4" hole on the other with two setscrews each to fit the shafts!
The flexshaft I got has a collett head, just like the end of a die grinder, and came with both a 1/4 and 1/8" collet insert.
I like it a lot. The lower speed lets me take my time and the torque lets me really have at it without getting into chatteer and jumpout problems. I do my final polishing with an electric die grinder, but 98% of the heavy work is done with that simple setup.
I have a selection of mandrels from 2 to 8" long. What I'd kill for is a flexible mandrel 8" long that I could attach to a flapper wheel to run up and down the runners like a bore polisher.
Either way, the simple motor works. I've had it now since 1987! I can't tell you how many heads or intakes or valvecovers I've attacked with this thing. I think someplace even sells the adapters for motors now, so all you have to do is find them. Eastwood may have them even. I can't even remember who I bought the flexible shaft extension from, but I do know it originally came with an adapter to hook up to my makita grinder, so the shaft is rated for 14,000rpm, and I used it like that for about a year until I got a free washing machine motor dropped in my lap . The washing machine motor was quiet enough that I could (get ready for a "white trash moment!") port the heads while watching TV in my living room. I like quiet. rmmmmmrmmmmmrmmmmbrzzzzbrzzzzzbrzzzzz! Not really obtrusive in the least. Just don't do it if you have rugs, we had a tile floor where I was living at the time!
Great thing is, it runs on 110V, is easily controllable, and if you get a foot switch, is easy to pop on and off when you need to (I was unplugging mine for years until I broke down and bought a foot switch!). With a washing machine motor, it has a lot of torque, and I've often wondered if I could add a flywheel to it to keep a bog from ocurring (I really bear down when I get impatient). From what you've read so far, I guess you see why I limit my use of the high-speed tools! ;-)