High output alternator belt size

kickass audio · May 23, 2020

Since I wasted $160 in belts today as nobody had posted this anywhere online, I figured I would hopefully stop someone from blowing money and help them get something that fits.

I just replaced the factory alternator on my 2014 silverado 1500 today with a 370A mechman elite alternator. Since that alternator is high output it has a smaller pulley installed than the factory pulley. The OE alternator has a 2.5" pulley and the new Mechman alternator has a 2" pulley (that's their standard size for nearly all alternators). With the .5" difference in size you must get a smaller drive belt for the alternator, water pump and harmonic balancer or you will make the belt slip. In my case, the 72" factory length belt was right at the outermost limit of the tensioner so it definitely wouldn't work out.

I found that you will need to get a 71-1/4" outer circumference belt for the engine. In my case, NAPA didn't have the belt length in a 5 rib belt combination, they only had it in a 7 rib belt. I just used a utility knife to cut off 2 of the ribs and it works fine (plenty of other people I know do this and it's totally fine). The part number for this belt is through Gates as their fleet runner line under 080708HD, napa has it listed on their inventory as NBH 25-080708HD.

I hope this helps save someone else the hassle of cutting belts just to find they are too short. I will warn you, for this belt when it still had the 2 extra ribs on them, it wouldn't fit due to the extra deflection of the belt hanging off the lips on the pulleys, that made me worry as my two other belts were the same and even when I trimmed them it wouldn't fit.

Another word of caution, since you are getting an alternator with a half inch smaller pulley you will spin the alternator faster than the OE alt. Because of this I would not recommend you rev the engine up higher than 4700RPM. Going faster than that will over rev the alternator and can make it die (I did this in the past on my 2004 Envoy when I would floor it to get up to speed on the thruway and it would spin up really fast). The factory pulley size of 2.5" can handle you revving the engine up to 6000 RPM before risk of blowing up the alternator. Also you can NOT take the pulley from the OE alternator and stick it on the mechman alternator. The factory alt uses a centrifugal pulley that has grease inside (hence the black cap on the face of the pulley), I tried to remove that cap thinking it was a dust cover for the nut but it popped a bit of grease out in my face when I pried it off.

Rockdigga · February 7, 2021

Should be a 6 rib 13/16" on your truck...I believe. I'm having same issue with my new high output alternator and finding a serpentine belt for a 14 silverado but I need one about 70 in 6 rib 13/16"

WashMan03 · February 18, 2021

On 2/6/2021 at 9:40 PM, Rockdigga said:

Should be a 6 rib 13/16" on your truck...I believe. I'm having same issue with my new high output alternator and finding a serpentine belt for a 14 silverado but I need one about 70 in 6 rib 13/16"

What belt did you end up going with?