RE: OS engines users Club
Sorry I'm late into this thread. Tim, you're right - I got a significant improvement by fitting an aftermarket thin walled header, but to my .91 Surpass 2 pumper. The .91 exhaust port uses an 11mm thread.
I don't have first hand experience with the larger FS 1.20 S3 (14mm exh port thread) but it may be worth looking at exhaust mods as a peformance boost opportunity with that engine, because it steps down from 14mm thread to 11mm thread at the 'joint". Perhaps an owner of this engine may chime in.
In keeping with the thread, I have the following OS engines:
Pet .099
FGR .60 Blackhead
FSR .60
VF .46
VF .61 (two of these - one with 7D carb, the other with a 7DV carb)
Surpass 2 .91 pump
Surpass .52