I’m planning to replace my Wahoo Bolt v3 with the Roam v3 and would like to know whether the Roam will fit my current 3rd party (the party has since ceased to exist) mount. How much longer is the Roam toward the bottom edge, or in other words, how much extra space does it need when locking into the mount? Put differently, what is the distance from the exact center of the mount interface toward the rider? I haven’t been able to find this information anywhere.
I don’t have an exact answer for you, but there are various reviews floating around that compared them side-by-side, so you can get a rough idea. You could probably even take a still and estimate by measuring against e.g. the length of the button on the front of your Bolt. Des Fit’s video review has them mounted side-by-side on a handlebar.
I would also note that if you’re pushing it fine, remember that you need a bit of extra space to mount it, as the diagonal is longer.
That’s the thing - I am pushing it very fine, so I would appreciate exact number. 
You could try posting on the #electronic-gadgets channel on the discord. GPLama is quite active there, and I imagine if you ask nicely he would be able to measure them both (I’m assuming he has at least one of each!)
FWIW, I compared two side-on pics of the Roam v3 (top) to the Bolt v3 (bottom), and used a screen measurement app to scale the pics appropriately. Both pics came from the same source. By the numbers, the scaling is within 0.4mm of the dimensions specified by Wahoo. I don’t know what the tolerance range is on the measured dimensions of the two computers. It looks like the Roam will extend another 3.6mm–I’d call it 4mm just to be safe–past the bottom of the Bolt.
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How about @Ray_Maker or @GPLama right here?
Pretty please.
I’d prefer that so much over ordering a roam and eventually sending it back.
Wow, very helpful!
Thank you, Robin!
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I just measured my Roam V3 and it’s 45mm from the center of the mount interface to the bottom (closest to the rider) edge. This includes about 0.5mm for the USB port cover which stands a bit proud of the edge.
From the center of mount interface to the bottom corner is 53mm; I’m including this because, depending on where your mount is relative to the bar vertically, you may need to clear that distance when rotating the unit into the locked position in the mount.
Thanks a lot, now I can die in peace because the Roam won’t fit. 
@Robin ‘s measurements were spot on, but I didn’t realize the Roam is so much wider and therefore longer diagonally.