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Determining what accessory belt to use

I have read alot of threads asking this question and thought a simple explanation might help.

You've done something different than stock ie alternator relocation, powersteering delete, custom pulleys, or whatever and your stock micro-v rib belts won't work anymore. What to do?

First lets talk about what those numbers mean that are on the belts.

Here's a car quest belt number. k040372
Here's a napa belt number. 25-040372
Here's a gates racing number. K040372RB

The first number/letters are manufacturer related. Not a big concern unless your picky about your belts. K, 25-

The next two numbers signify how many ribs. 04 4 ribs

The rest of the numbers signify the belt length in inches. 0372
37.2 inches

The RB on the gates belt is just signifying something special with that particular belt. Like some are HD = heavy duty, RB = racing belt

Now to determine what you need just set your pulleys up in a normal position and then run a string around them in the route the belt will take. Mark the string with a sharpie at the point it makes a complete loop. Then just simply measure the distance. Then all you have to do is convert the fraction you came up with into a decimal. Either get a scale or just divide the upper number by the lower number of the fraction and stick that with you inches.

Lets say you came up with 37 3/16" 3 divided by 16 = 0.1875 So total length would be 37.1875 which you can just round it to 37.2"
Done go to the auto store.... I need a 04037.2 belt. Like everything custom just because you measured doesn't mean they do make a belt that size. So you may have to alter or mod your plans to be able to find a belt that is actually made.

Here's from the Gates site saying what fits our dsm's.
•High Performance Automotive V-Ribbed Belt — K040274RB
•High Performance Automotive V-Ribbed Belt — K04035RB
•High Performance Automotive V-Ribbed Belt — K040365RB
•High Performance Automotive V-Ribbed Belt — K040372RB
•High Performance Automotive V-Ribbed Belt — K040390RB
•High Performance Automotive V-Ribbed Belt — K050507RB
Here's what fits a 95 nissan 240sx.
•High Performance Automotive V-Ribbed Belt — K040305RB
•High Performance Automotive V-Ribbed Belt — K040355RB
•High Performance Automotive V-Ribbed Belt — K040360RB
•High Performance Automotive V-Ribbed Belt — K040365RB

Some are the same for different models. Hope this helps out the next dsmer.
Last edited by a moderator:


Feb 18, 2006
South Gilbert, Arizona
Great info. I'd like to add that when you do get the belt you need you'll want to go one size smaller than what you measured with the string. Reason being, the belts going to have tension on it once you install it in the car but the string you used to measure did not.

Once again great info!

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