Bump Switch

Hi guys,

I was wondering if any of the Electrical Guru's on here have a schematic for a bump switch I can build to help in turning over the engine when I want to set timing. I would prefer this over using a bar on the crank bolt to turn the engine over and I'd like to know how it works.
I'm assuming it connects across the ignition side of the starter solenoid , replacing the key circuit but not 100% sure. Dell? AJ/FormS ??
I used to sell these. Not cheap though but nice and heavy duty.

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