400 Series Crystalyte motors are no longer made, they were the golden standard in the early days, are still being used a by a whole lot of ebike fanatics, myself included..
These motors come in Front ( 100mm ) and rear ( 135mm )
The front motors are custom made with long axles, the threaded portionsare 8" long, they fit in chopper frames, can be converted to Rear with the proper motor covers ( also available ) and with or without disk brakes ( with threaded covers )
Good at 48v, great at 100v, these motors can easily handle 1000w without overheating, i've pushed them in 20" rims at 5kw+ for years. They are the most quiet motor available as of this writing.
These are first generation, before they stopped production, the 2nd gen had offset spoke flanges.
1 x Front 4011
2x Front 4012
1 x Rear 408
2x Front 4013 ( rare find, very low RPM / v )
140 $ each
Video showing how they work.