I've noticed on two occasions problems with slipping and noise with
non-OEM V belts driving AC comps in cars. Once with replacement
belts, from NAPA and once from PEP BOYS. Both times they were name
brand, ie. Dayco or Gates belts with typical US sizing nomenclature
printed on the belt. The problems have been rapid and repeated
slackening after tightening, and noise. Replacing the belts with the
OE type metric belts seems to fix the problem, but they are sometimes
hard to find. Really a ****er that most auto parts stores seem to
only stock fractional belts that really don't work right. Anyone else
have this occur? The below is from a Gates bulletin:

Words of caution
Before substituting a U.S. belt for its metric equivalent based on an
interchange chart, check for dimensional
differences in cross sections, length, and belt ride-out (distance a
belt extends beyond the sheave OD). For multibelt
drives with joined belts, check for compatibility between belt centers
and sheave groove spacing
A U.S. belt may have a different cross section or length than a
comparable metric belt, even though their
specification numbers indicate interchangeability. Further, it’s
common to find metric belts in nonstandard cross
sections and lengths that can’t be easily interchanged with U.S.
belts.
Some interchange catalogs list differences in length between
equivalent U.S. and metric belts, with these differences
typically ranging up to 1 in. for the longest belts. The difference in
length that can be tolerated depends on the
application and the amount of take-up. It is usually up to the
equipment operator or maintenance engineer to decide
if the difference in length is acceptable for a particular
application.
Differences in belt ride-out and included angle of the cross section
between equivalent U.S. and metric belts can
adversely affect the speed ratio of the drive, as well as power
capacity.
Because of differences in groove spacing between U.S. and metric
sheave, Figure 2, interchanging joined US belts
with joined metric belts causes the belts to seat improperly in the
sheaves. This leads to early belt failure and rapid
sheave wear.
If you’re unsure about the effect of any dimensional differences on
your application, consult the belt manufacturer.
Failure to use compatible replacement belts can lead to reduced belt
service life and lost production due to