does anyone know how the belt companies measure thier belts. or more specifically do they measure from the inside of the belt or the outside diameter. i am having issues with getting the right belts for my car and i would measure the pullies with a piece of string or something but i wasn't sure if i should take measurement from the bottom or the top of the pullies.

incase it helps i don't have factory brackets i have the march v-belt pully system for the alt and the power steering. the engine is a 72 402 but the crank and water pump pullies came off of a 454 i think was a '74. i wouldn't think that a 74 454 and a 72 402 would have diffrent size pullies but i could be wrong? i went to the parts store today and got the belts for a 72 402 and the alt belt is too long and the pwr. str. belt was way too short. and no i didn't have them mixed up cuz i tried to put the belts on the opposit accesories of which they were intended and they still didn't work. any help would be greatly appreciated

thanx

Bryan,

Belts are first classified by the section geometry.

A= 1/2" x 5/16" height x width
B= 21/32" x 13/32"
C= 7/8" x 17/32"

Note, however, that as a belt wears, both the section height and width get smaller. So, you will have to eyeball as to which nominal you are shooting for.

Next is either the outside diameter (sometimes called O.C. for outside circumference) or the pitch diameter. The pitch diameter is about impossible to measure, so measure the outside.

Next, cross reference a listing, like this one in Manhatten Supply Company:

Page 3787 in catalog (http://www1.mscdirect.com/CGI/NNPDFF?PMPAGE=3787)

or page 940 in the McMaster-Carr catalog (mcmaster.com)

Note that neither the pitch diameter nor the outside diameter quite match the belt number. For example, a B36 belt measures 38.9" outside and pitch diameter = 37.8". Close, but not quite.

Once you figure out the belt number, those are fairly universal. Go to NAPA, for instance, and they should have a B36 belt.

so to get the correct belt size i'll have to first find on that fits in the v of the pulley correctly, and then i'll have to measure the outside diameter to get the right size?? i was thinking about getting a piece of string and running it around all the according pullies then tieing a knot or making a mark on the string then measure the length.

That will work except that the string will go way deep in the pulley groove - much deeper than even the inside diameter of the belt. Maybe take that measurement and do a little geometry to figure out what the length should be at the correct radius on each pulley.

Maybe take a belt that is too long, measure it (or look on the reference table), and then fold up the excess to see how much shorter you need. Or, guesstimate how much shorter it needs to be. (that's my preferred method http://www.nastyz28.com/ubb/wink.gif )

Then there is the old trial-and-error method. If you have one that is too long and one that is too short, get one that is in-between, and repeat. http://www.nastyz28.com/ubb/smile.gif