DVM, one of the accomplished Bulleteers in the royalenfield yahoo group (who has since sold his Bullet), used this option on his 65... he tapped the quill bolt and mounted the pressure gauge there.
As Kevin already said, when warm, the gauge showed almost 0 psi at idle, and would bounce up to around 30 psi when running. I recall he didn't think it provided any additional info on the status of his engine, he did it mostly as an engineering exercise. Looks pretty nice, though... let's see if I can get the image to attach.
Mike and Stumpy in Michigan
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