I just acquired an OS .61 SF that has little or no run time. I am trying to decide the proper method for run in. I am not sure whether it has a ringed piston or is ABC. When I look in the exhaust port, I can see a line about 1/8" down from the top of the piston. I can't determine if that's a Dykes ring or just an oil groove associated with an ABC piston assembly. Also I have a later E-4020 long expansion style muffler that I was thinking of using in place of the short muffler that came with the engine. I'm wondering if this might be a good choice as I know the engine was designed with a tuned pipe set-up in mind for F3 pattern. My use will be for sport flying and I plan to prop it to run in the mid 11,000 rpm range. Can one of you old school gurus help me out with the correct combo/tune. I'm guessing 12-7 or 12-8 prop running on 15% nitro-18% oil with 2-3% Castor would be about right?

If you had the ringed version, you'd see the black metal ring. Just a line indicates the non-ringed version. Do not break it in by running it in four-cycle mode. As rich as possible, but in two-cycle mode is the initial running method...gradually lean from there.

You can use a fuel with 18% oil as long as it has a major amount of castor in the oil package. An all-synthetic oil at 18% and then adding 2%-3% castor adds a bit of oil, but not enoug castor. We'd like to see the entire oil package have 25% or more of the oil being castor when running non-ringed engines. More is better.

If the E4020 muffler will bolt on your .61SF engine, you can use it, but it may not offer much back pressure for the fuel tank. The O.S. Max .61SF wasn't "designed" for a tuned pipe, though it can use one. A regular muffler works very nicely. Select a 12 x 7 prop...maybe a 12 x 8. It depends upon the brand. If you prop it for the high-11,000's, then any prop that gives you that RPM and flies your model as you desire is the "correct" propeller.