I typically recommend using the OS-F in some of the Jett higher timed engines. It is a very hot plug, and can accomodate a setup where an engine tends to get a fuel surge when staging on the pipe. The 4c plug is designed to accomodate a very 'wet' incoming fuel charge, and to stay lit during an exhaust stroke.

The Jett SJ-90LX and the 60LX both run very well on the OS-F plug. For most of the other sport engines it works too, but typically a standard K&B1L or HP, the Merlin Hot (supplied with the engines), or Rossi 3,4 works great.

Something to consider - the heat range for a glow plug determines the ignition timing in a 2c glow engine. That is why a variety of heat ranges are available. The size of the wire and the plug contruction also lend each type of plug to a certain level of durability.

On Rossi .60 and .61 engines, YS61, and the OS 61FSR engines I ran years past, I used K&B 1L or Rossi 4 plugs most of the time.

The only real oddball I had was my CMB60. That engine really enjoy a Rossi 6 or 7 (very cold), and REALLY ran great with an OPS300 plug.