Belts should be almost loose enough to slip. Any tighter than that and you risk bearing issues and even alignment issues. If it doesn't slip or squeel then it is tight enough.
Bearings and oil seals
are MUCH more expensive and difficult to source and repair than belts. Err on the too loose side. Stock extra belts, they are easy to change if they wear too fast from slipping.
If you want a better guide as to how tight a belt should be invest in a "Kricket" belt tension tester. Most any auto parts
store will have one if you ask. They are cheap
, well inder $20, and don't take up much room in the tool box or roll. They are only the size of a magic marker.
Many old-time mechanics laugh at them saying they just "know" how tight a belt should be by feel and that is all well and good for them. But how do you learn what the right tension feels like without some sort of guide? Is the old-timer going to come on down and check your belts for you every time you install one until you get to that point yourself?
Just get a Kricket and use it as a learning
guide. When you feel confident in your "feel" you can give it away, or put it in the back of the tool box -whatever.
A trick to get extra purchase
on a tricky installation
is to use a big screwdriver as a lever. Put hand tension on the bolt and dial a bit more tension into the alternator or pump
by carefully levering it around the pivot bolt. Be careful not to break anything, especially cast mounting flanges and be aware at the red wire that goes to the alternator is always HOT even with the key off on many engines so don't short it out to ground with the big stupid screwdriver and scare the living S out of yourself with a massive DC arc
Gates (Kricket) 91107 Belt Tension Tester https://www.amazon.com/dp/B000MUTAGS..._el..CbQPCB86Y