I finally found the belt I needed but now how do I replace the v belt on a singer 251.2 industrial sewing machine?

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https://www.youtube.com/watch?v=aQgVgeksE9w

You Are Using the Wrong Bobbin If they are too small, they can bounce around in your bobbin case and cause a knocking or clunking noise. If they are too big, they will not spin freely in your bobbin case.
https://sewingsociety.com/why-is-my-sewing-machine-making-a-knocking-noise/

When you thread the top thread, the thread will drop down on the right side of the tension mechanism and go around the bottom (clockwise) and up the left side. Grab the thread somewhere above the tension mechanism with your right hand and then pull the toward the right (as though you are going to close the circle around the tension disk. The thread should pull on the metal wire (spring) and your thread should pop into the correct position of the tension disk. When you release the thread, the wire spring should pop back to the left. Then you will continue threading through the remaining thread guides and the take-up lever.

Here's a service manual for your machine (in case you need one):

Singer 20U31 Service Manual

Threading video:
Threading Singer 20U 20 33 industrial sewing machine

Singer 20U Industrial Sewing Machine Tutorial

Singer 20U Industrial Sewing Machine Tutorial

Bobbin winding on the Singer 20 33 or 20U

this video shows threading the tension disk:

How to Thread Singer Industrial Sewing Machine

Is there an adjustment to tighten the new belt? Wrong size belt?

There is usually a lever on the front of a Singer that you push in to make it sew in reverse. You will have to pull the lever forward or push it in again, to get it to release and then the machine will sew forward.

cant you just loop the belt over the wheel, I never take mine off

Set to forward using the dial

It was a tough find but here is your manual:
http://www.bestneedles.com/catalog/singer/Singer%20Instruction%20Manuals/251-Class-Inst.pdf

It might be the way you're asking that's confusing the problem. It's simple to remove the hand wheel, slip the belt over the track, and then replace the hand wheel. It really depends on the model of your machine - is it by chance a 328K?
Martie Johnson (USA)