fan belt size

[rmmchl]

250sl without air cond.  I have seen  where the main fan/ alternator belt shows the size 10x910mm . NAPA gave me the stock number 7365 which shows it is 10 by 942 mm--OC I guess that means outside diameter. Gates makes the one NAPA sells.  I want spare belts driving to PUB. Does anyone know which belt is correct?

[WRe]

Hi,
there is some tolerance: 9,5-10 x 925-940 will fit, try it.
...WRe

[jameshoward]

A related question, what's the logic on changing fan belts? Is it time, or simply when they show signs of wear? I have never changed mine.

[rmmchl]

kind of like the question" when to replace our tires question." The old fashioned belts look fragile compared to the serpentine belts today. They say to inspect them and look for cracks in the rubber