Well I am going to try to explain the belt problems associated with the 420a with or without a UDP. I have had my UDP going on 2 years now and have had belt problems ever since. I have taken car to dealership and 4 other shops to fix problems. I have gone though over 20 belts, ranging from gatorback, napa, bando etc...

Squealing Belts-
Belts squeal when they are not tight enough. If you can grab the belt and twist it by hand more than 1/4-1/2 inch then it is not tight. The squealing comes from the slow movement of the belt over the pulley.

Belt Walking Off-
When belts walk off (slowly slide to one side of the pulley and chew up the ribs) it is caused by a pulley being out of alignment with the others. Mostly likely it will be that new UDP you just put on. Take a flash light and eyeball it or even a flat edge and see if they are flush. The stock PS\AC tensioner pulley is also too narrow and there is an updated pulley available at the dealership ($150).

Throwing Belts-
Throwing a belt is when the belt just shreds up and\or flies off the car. This usually happens on a sudden rpm change like downshifting or shifting during a race. The sudden rpm change will cause the belt to stretch for a short period of time; this slackness gives the belt the chance to walk off.

The Fix-
First identify which belt is squealing. You can do this by spraying any oil based substance on the belt it self (DW40, tire shine, belt dressing, etc...). Do this while the car is running. If the belt is squealing the oil base liquid with make it stop (usually last for about 1-2days). Next tighten the belt. If the Power steering gets hard or the AC light starts blinking (AC compressor not turning) then the AC\PS belt needs work\replacing. The AC\PS steering tensioner is towards the front, under the thermostat. If the car starts squealing and when it stops the lights get brighter or the battery light comes on, then the Alt belt needs work\replacing. The Alt tensioner is behind the Alt and can be reached best from underneath the car.

If you have the AFX UDP and are using the recommended belt sizes and don’t have any belt problems you are very lucky. Them belt sizes are too big.
AFX says to use:
Power steering with AC K050500
Alt. K040315

I like to use:
Power steering with AC K050490 (one inch shorter)
Alt. K040310 (1/2 inch shorter)

Using the AFX sizes causes for the tensioner to max out (turned all the way to tighten). When they max out they usually don’t have enough tension, the tensioner bolt usually snaps, and no more tension can be applied when the belt stretches a little after initial break-in. K0 = Vribbed 50 = 5 ribs 490 = length in inches. The best belt on the market is BY FAR the Gatorback brand belts. They are very tough, don’t stretch, and if they do slip where as a normal cheap belt would squeal these belts just tend to hiss (barely can be heard).

Nice.

someone plz move this somewhere "permanent!!"

the only thing I suggest different is to not use WD40 on the belt, but use a dab of Dawn® dishwashing liquid, it wont last as long as squirtin WD40, so you can know you fixed the problem sooner... or something.

actually my dad told me to use dawn over WD40 ages ago, so I dont remember the real reason for using one over the other, but he made it sound important at the time.