Roary, These are to replace the sleeve/piston on the OS 61RF-P "Hanno Special" motors. They will work with the 61 RF, but they do differ by means of a higher exhaust timing in them (which the long stroke Hanno used). These are NOT the OS replacement parts but rather after-market parts made by Henry Nelson. They will be an Aluminum piston and a chrome plated aluminum cylinder liner (AAC) instead of the OS ABN set. And if you have every heard of Nelson engines, then you know that these will be pretty much the best sets you can buy. Or wait, we cant even buy These yet... Gotta persued Henry to help us. Which, lucky for us, isn't a hard thing to do.
I hope this is clear. If it is we'll add you onto our list.

DM