I typed this all once and lost it.
I have had Timkens on the drive side of my Rapide (The Great Woolly Mammoth) for more than 100,000 miles. Apart from the first 100 miles when a defective cage (Bower bearing) disintegrated and rolled up in the rollers with everything grinding to a halt, they have been fine.
I had to rebuild the crank at about 70,000 miles and so just installed new bearings as they are about $40 for a pair.
Preload is held on them at about .004" and is controlled by the width of the spacer between them. Nice bit of tool steel turned up too wide, hardened and then fitted. If there is .006" free, grind .010" off the width. Done.
The whole plot is in a top hat type housing shrunk into the cases and held there by 6 small countersunk screws in the outer flange. There is a seal installed as well outboard of the bearings. The seal is in a sleeve that is screwed (LH thread) into the I D of the housing and holds the cups in place.
Except for the defective bearing I have never really even had to think about it. When the bearing failed originally there was a huge discussion locally about the use of a pair of tapers and the crush (or not) required. After looking at several similer setups (Velo mains have .004" crush) and some industrial stuff, put them in at .004" and never had another problem.
Funny this comes up as just a couple of days ago whilst sorting thru my bearing inventory I found the other bearing from the original pair and lookd at it. The steel spacers between the rollers are almost crimped (cut) through at one end where they were stamped out! Reminded me of my first Vincent experience. Ride for 100 miles and tear it right to pieces!