I`m in the market for buying a used OS 1.20 Surpass III engine with pump and would like to know approx. how many galons of fuel can be run thru an engine untill it needs rebuilding-Taking in consideration that the engine has been run with good common sense and been maintained well.

indefinitely,a nice low nitro content fuel,regular maintenance and after run oil after each flying day plus no lean running should see it last for ever.

Back in the 30's it was unusual for an engine to last for 200 hrs. of actual run time. With the modern materials used to build them now days, and the ability to buy parts for rebuilding almost any engine, plus factory service, a decent engine should outlast many airplanes. I wouldn't suggest buying an engine on E-bay or from someone who isn't known to you or a friend. Your LHS might have used engines for sale. At least you have someone to go back to.

I had a Fox Schnuerle 15 that might have set a club record for most flights on a single plane. Over 3700 flights over an 8 year period, averaging about 15 minutes per flight. Something over 115 flights per year average. Never allowed a lean run, always used at least 18% castor oil fuel, Used 10 or 15% nitro, 8-4 prop.

A two stroke is a bit different than a four stroke. Fours do require periodic maintenance and will require rebuilding long before a two stroke, but primarily the valve train.

I think if the engine is treated properly, and the valves are kept adjusted, then the time between rebuilds will be lengthened substantially. Depending on the engine, it seems to me that resetting the valves should be done every 100 hours max. But rebuilding is so infrequent that not too many hold onto the engine long enough for it to be necessary.
Dennis-