The 18 mm thread will not fit into a Vincent spark plug hole. Also why do you think that trying to find the exact point of TDC will be easy when the piston is hardly moving for a few degree about TDC? It is possible that a dial gauge will give you enough accuracy and I can see the advantage of only having to make one measurement rather then two and then splitting the difference but whatever you are up to you need to fit a degree plate so why not use that, with a piston stop, to get TDC exact.
I posted the wrong pic...I know it's a 14 mm hole. I've been using the degree wheel and a piston stop for years,
but was enamored with the gage. My degree wheel needs a bit of modification, the 'spindle' I run it on, a
threaded rod ground to a taper doesn't turn precisely true, so my degree wheel moves up and down a bit. I've
used it so extensively lately that I thought I'd upgrade everything from my spoke, to my degree wheel and the
gage appealed to me. I feel like I need a bushing in the timing chest to hold the spindle for the degree wheel,
but lately I've been removing the timing chest and stuffing my spindle/rod into the hole without as much accuracy
as I'd like. I ground a threaded rod to a taper and that's my spindle I run the degree wheel on, and I sure want it
to turn more true than it does. And I'm always buying or making tools that I just think are cool, and make the job
more craftsman-like. When I read that setting the mag at 2-4 degrees BTDC (with ATD closed) I wanted to make positively sure I was on the mark, and thought the gage would help. Actually I probably don't need more gizmos
and just have to spend more time thinking. I almost made the 'balloon' TDC tool once too.
