Okay,
@Harryc and
@h13man some dimensions:
o Cobra's mount is 16.50 mm thick in most areas and 25.25mm thick over the handlebar/risers. (There is a trough for the handlebar but I'm not taking the mount off to measure it.) My guess is that the top clamps could be sanded flat on top to make up for the difference. I'd probably buy another pair and work with them so I still had top clamps on my bike until I was sure the new one would work.
o Cobra uses the stock rubber liner around the housing so the hole has to be about the same as the stock one. I measure the interior opening ranging from 101.70mm to 102.85 mm depending on which direction I go. Thought it was round. Maybe that's close enough?
o Cobra has 6.75 mm around the speedometer in the thin spots. (Can't call them the top and sides on a circle, right?)
o Putting a straight edge across the "bottom" of the Cobra mount to eliminate the cutout (It should be removed as one of the last operations. Or not. It's only for cosmetics?) I get 80 mm between to the closest part of the hole for the speedometer.
o Hole spacing for the handlebar clamp bolts = 100 mm between hole centers going side to side and 36 mm between centers going up and down. Spacing is the same top to bottom and left to right -- a perfect rectangle.
o I'm guessing the that holes should be 8.50 mm in diameter, maybe 8.75 mm. The bolts are listed on the parts fiche as 8x32; somewhere I've got a template/gauge for translating that but I haven't seen it in months. Better to go a little small then drill it out if it's too tight. The measurements I can take are the opened-up diameters for accommodating the bolt heads. Is I understand the process we're discussing, that's best left for installation time. It is not hard to open the holes up to 15.50 mm. Depth should maybe be 6.50 mm -- but that gets determined by the thickness of the plate used and how much is left of the top clamps. It isn't much of a problem to have the bolts (or even just the plastic caps) stand proud of the mount.
o I'll have to get a picture of the stock mount ring posted. It has a couple of "tabs" protruding into the hole for the speedometer. I don't know if those are for alignment or a concession to manufacturing. I don't remember anything like that in my Cobra mount and don't see it in the picture. What I do see is that the Cobra mount has a larger hole for the speedometer on the "A" (i.e. top) surface than it does on the "B" (i.e. back) surface. That is definitely because the speedometer fits into the Cobra's hole deeper than it does into the stock mount -- the stock "ring" is only 3.67 mm thick. I can go back and measure the Cobra "A" opening but I'm not sure that matters with h13man's process. Another point-of-installation adjustment?
o I did not bother measuring the indicator light spacing. It's not critical in any way. I don't know how where to get colored lenses to press into the openings. Those could determine diameter of the holes if they're pressed in from the back. Diameter of holes in the stock (plastic) part is 13.30 mm. That's what is needed to accommodate the lights.*
Let me know if you can think of anything else I should measure. Let me know if what I listed above is unclear and / or confusing. I've pulled down the picture with the tow Cobra mounts that Harryc posted last week so I can see how much of a mess I can make trying to "mark it up" with dimensions. I'll do the same with the stock mount after I get a photo.
As for material: I recommend going with aluminum. It eliminates the need for chrome (Aluminum can be polished to be more shiny. It reflects more light.) taking away an expensive (for a single piece) secondary process while reducing potential for unlike-metal corrosion with the aluminum risers. Any kind of plating makes point-of-installation adjustments tricky.
I'll check back tomorrow to see what more might be needed here.
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* I see that the A.C.E. instructions from Cobra don't say anything about installing the indicator lights. I do not remember what I did other than pushing them into place. Guess I'll have to look at the other side of the Cobra instruction sheet to see if it tells what to do on a Spirit.