Thought I'd put some measuring to our XM belts,which on mine is the 417 300 391. Belt is in excellent shape....no strings and no black rubber on the clutches.
The 417 300 391 spec is a top width of 38.3mm with 26 deg side angle
I'm not going to use the top width to measure because its likely the most variable and least reliable place to measure.
The belt has 1200 miles on it and sled has DJ clutch kit. It's the other side of great clutch kits... fantastic belt life. Compared against a new belt here's what I got.
Measured at the cord (actually just below as the cords protrude somewhat):
New Belt: 35 mm
Used Belt: 34.5 mm (1200 miles)
Measured at bottom of top cog:
New Belt: 36.4 mm
Used Belt: 35.8 mm (1200 miles)
So fairly consistent wear of 0.5-0.6 mm.
I then put that in CAD to see how much the worn belt should drop in the secondary clutch. CAD says 1.3 mm is what it "should" be, but I was measuring something closer to 3.0 mm so the belt was likely running too low in the secondary. The deflection was excessive as well. I adjusted back to "spec" and all now seems to be good.
So, lesson learned for me. I wouldn't go over 500 miles before adjusting and I'll be adjusting clutches much more often given there is both stretch and wear to contend with.
OTM