RE: Will an OS 46FX piston/liner fit an OS 40FX engine?
Thanks for your advice. That's what I'd have thought. The 40FX and 46FX do have different crankcase part numbers, so that'd make sense.
So it's still a mystery engine. It may be an aftermarket piston/liner, but they look like OS parts to me so I don't think so.
If anyone out there takes their 40FX head off one day, measure the bore and let me know what you find, maybe they're all .46 cu in! Mine has a bore of 22.0mm (and a stroke of 19.6mm), so definitely a 7.45cc / .455 cu in capacity. A 40FX should have a bore of 20.5mm.