DownT,
It depends on the engine, what you are rebuilding, and how much it costs. I replaced a set of bearings on a Rossi 45 and and OS 50 FSR for about $35 each motor. I bought the better bearings and could have done it for less than $20 each if I had put in the stock bearings. Now they run like new. The OS 50 is a good engine which you can't get any more so the $35 was well worth it.

It was neat so see what they look like inside and how they go together. I really felt like I accomplished something like the motor guys at the field by doing it myself. How about you Tim? Of course you want to make sure you don't have any left over parts when it goes back together. I figured it was already broke so why not give it a shot and save a few bucks. Can't really hurt it anymore than what it was and if I did then I can buy a new one or send it away for service.