Shay points gap?

I have recently acquired an early 50s Shay cultivator with their own engine - does anyone have the correct points gap setting?. I've cleaned the points and guessed the gap at 20thou but as yet no spark so before I go back in and test the coil and condenser it would be nice to know what the gap should be. Guess that it's the same engine as used on the similar vintage Rotoscythe and Countryman. Definitely not a Villiers.