Thorp Air Command - T18.net

On mine, the wing leading edge is the datum.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

You can make the datum whatever you like. The plans have it at the crank flange.
Cubes

I haven’t seen that particular drawing. Mine was in front of the spinner when I got it. I like points that are fixed and easy to measure from like many factory built airplanes use so I moved the datum when I put it on the scales a few years back.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson