Hey all. The piston pins between the SF and RF are definitly different. As far as piston pins are concerned the difference occurs becauce the piston pin on Rear Exhausts is exposed at he exhaust port, during the intake stroke of the engine. Because of this all REAR exhaust (VF, RF and RX) piston pins are Not hollow, see pics. If you use a side exhaust piston pin it's likely you will be losing a significant portion of intake charge as the hollow type will vent straight though the pin to the exhaust port as the piston comes down.

Here's some useless info I compiled of current and past OS piston pins with OS part numbers.

Os Max 61 piston pins and OS PN's

Side Exhaust (Hollow)
61 FSR 26606008
61 SF 27706000 (not as long as FSR or SX)
61 SX 26606008

Rear Exhaust (Not Hollow)
61 VF 27106007
61 RF 27806000 (not as long as VF or RF)
61 RX 27106007

Buy the RX piston pin if you don't want drive your self crazy wondering why your brand new rebuilt 61 RF, with its new AAC piston and sleeve, doesn't run right.

First pic from left is a 61 FSR (can see right thru it). Second is from a 61 RF. Third pic is why the Rear Exhausts' need to have a plugged piston pin. The forth pic, just got one from Omni Models .

Almost forgot, you will still need to grind down the RX pin to fit he smaller bore of the Hanno's and RF's.

Bryan