re: measure 'em...
...see how much space you need.
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As far as I know, there's not a specified distance. At least I hope not, I've started drilling my panel for switches & breakers. Although, Van's does send a template for the switches for the -10, and I'm using it for some of them. Well, actually all of them.
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Just a word of caution, measure a bunch of times, & drill once.
There was neat article in Sprot Aviation a while back. It showed a picture a really nice cut out panel, the only problem was that the holes were too close together & the instruments wouldn't fit in on the back side. I, personally, would rather err on the side of too far apart, yeah, it may waste some panel space, but not the entire panel.
Marshall Alexander
RV10/N781DM
fuse-panel & top