Motorcycle Repair: How many Hours/miles before top end rebuild, tolerance stack, piston rings

Question
Bill, I recently purchased a 1973 Honda SL 100 that I want to do a complete restoration on. The bike shows that it has 3950 miles on it.The bike appears to be in very good condition and there is nothing that I can see that would make me believe that the mileage is incorrect. I have 3 related questions . How many hours /miles can you put on a single cylinder 100cc Honda motor before you consider a new top end? What brand of aftermarket piston/rings have you had the best  success with in terms of reliability if I can't find an OEM Piston/rings, and I have read where you talk about problems with the cam getting wobbly in the cylinder head due to premature wear and causing an issue with the way the bike runs. How many hours / miles does this problem typically start to surface? Thanks in advance for answering my questions. I have read alot of your responses to other peoples questions and they all were very good info for me.

Answer
Randy, the wear issues on the one-piece heads are a combination of production tolerance stack-ups, frequency of oil changes and overall use/abuse of the engine. I would also keep an eye on the camchain tensioner adjustment. Kind of a funky design, but it works okay. You want to prevent the camshaft shifting back and forth due to loose chain tension.

I have seen these engines with 10-20k miles on them, but they are usually pretty worn out by then. I doubt you will have much difficulty finding OEM pistons and rings for these engines. I don't work on these bikes anymore, so don't have my finger on the pulse of who is doing what with them. The 100s have a smaller wrist pin than the 125s, so are a little harder to find more pistons for, I guess. There is a lot of this stuff on Ebay. Also, companies like POWROLL in Oregon make hop-up kits for these engines, as well. Depends on what you want to do with the bike, overall.
If you are just going to ride it and show it, probably just leave it alone. I imagine that the motor is just fine with less than 4k miles on it.

If you keep the oil changed and monitor the ignition timing, camchain adjustment and valve adjustment, you should get many years out of the bike, as it is. Make sure the spark advancer isn't gummed up, after all these years, too.

Bill Silver