First question: Without going into too much detail, the machining of the groove has some impact, as does the ring itself (size and material).
Second question: This will not be a field repair. It will have to be done by the authorized reman center for the injector. Cleanliness, as you noted, is crucial, and the injector will likely be retested after the fix. No one in the field will have the equipment required for this testing (dedicated capital equipment needed).