This is a direct replacement bearing for the Revox A77, B77, and PR99 rear bearing located inside the capstan motor. They are easy to fit and will improve tape transport. 
There are 2 bearings inside the capstan motor, the front being phosphor bronze which will not need replacing. The bearing you are buying here will replace the rear bearing in the capstan motor.
You will need circlip pliers and will have to remove the motor.
 
Brand new.
Free postage.