I am replacing the alternator belts on my 84 300TD OM617. I did not have time to get the belts from Fastlane and picked up some from NAPA. They seem too long though. I have the alternator all the way to the end of the slot and am turning the adjuster and can still get the belt off the wheel.

This is the belt I have:
NAPA AUTO PARTS

The one they sell on this site is listed at 10x1037(part# W0133-1639250). I'm assuming 1037 is mm. The NAPA one measures out 7mm shorter so it seems like I'd be ok. I'm not sure what's going on.

Are there different alternator wheel sizes?

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"Gerwalt" 1984 - 300TD

I dug the broken belt out of the trash and it's a NAPA with a different part number. So far I have not been able to get the part number to come up to check the size. I'll have to take it down to the store and see if they can find it.

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"Gerwalt" 1984 - 300TD

You may have a different size pulley, your best bet is to proceed as you're going to do, measure the old one.

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83 SD

84 CD