The 61 RF is the rear exhaust version of the SF...All that is different is the Case (because the exhaust routes out the back not the side), Piston and Wrist Pin...Everything else is the same...Then there is also the RF-P which would be like the SF-P (pumped engine)...

The RF engines would be used in an airframe which required a rear exhaust configuration such as Atlanta, Aurora and so on...You are pretty much gonna be using a tuned pipe exhaust system on a RF engine...

I have several RF's because most of the airplanes I like are designed for rear exhaust engines...All mine are the pumped versions...

There is also the OS 61 RF-P "Hanno Special" variant which is a little bit different than the standard RF-P...It's designed to run in the 9500 RPM range and was brought about to help lower the noise BITD...The engine with the read cylander head is the Hanno...

Don't forget there are others out there who need engines too....

Chuck