I have been searching here for the proper dimensions to mount a plate to the roof rails themselves and was having a hard time. Eventually I found this post on
Reddit. I am copying the contents below for posterity. Hopefully it is accurate. The poster seemed to put a decent amount of time into it.
Code:
|t|
_______ _
/ ___ | |
/ | d | | A |
/ | a|_| |
/ | C H
/ | b|-| |
/ |___| | B |
| _______| |
| | -
| | G __
_|_____|_______/ \_
|------W-----|
A mounting face height above channel opening 1/2"
a interior face height above channel opening 3/8"
B mounting face height below channel opening 5/8"
b interior face height below channel opening 7/16"
C channel opening height 3/8"
d channel interior depth 5/16"
t channel wall thickness 0.103"
H total rail height 1 1/2"
W total rail width 1 5/8"
G gap between channel rail and roof gutter extrusion 11/16"
Cross-rail max thickness 13/16"
Cross-rail max width 2 19/32"
Width between rail mounting faces across roof 40 1/2"
Usable rail length front-to-back 42 1/2"
Appropriate bolt to slide into rail: 3/8" carriage bolt, with 1/2" hole in mating surface.