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Graham, I assume you would go for bearings without seals or remove the rubber seals if they have them to keep drag to a minimum.
Instead of the cam pushing against a roller and spring to operate the shutter CHUK 4 lifts the roller that is mounted to some 1mm by 15mm Flat Ground Stock. This has a 10 mm slot milled lengthwise ( Scotch yoke like ) and uses the crankshaft as a guide. The improvement lies in the fact that a compression spring isn't there to sap energy from the system.
P.S. Rumour has it that these Hun-Yuns have the potential to contribute to global warming
but none were a inverted engine Jo
The slot option sounds good Graham, I was wondering how the lifted shutter would close again if there was no spring to pull it back down onto the cam. Jo you can send me a drawing if you want a CNC grooveFor the scotch yoke option is the horizontal slot larger than the pin to give some time where the shutter is stationary much like you had on Nattie
Or even this