At about 3,500 rpms and above I am getting a squeal on my port 489 big block from the raw water pump belt (as best I can tell). I doubt they have 100 hours on the belts. Water pressure gauge stays steady and pressure good.
I watched the belt carefully while my wife rev'd the engine (at idle but in the water) and I believe I could see an intermittent vibration (associated with the intermittent squeal) where I put a red line on the attached picture.
I was going to try a little more tension on the pulley (but I do not want to over-tension and do more harm than good) or belt dressing.
Questions:
1) Can someone explain (or send me maintenance instructions) on how to increase tension on this belt?
2) Does belt dressing work (or just mask the issue)?
I already have NAPA replacement belts but refer to question 1 on tension/instalation.
THANKS!!!

it might be on an eccentric / cam lobe ? or a elongated oval hole, loosen the nut and pry up and retighten

I run into this more with aluminum pulleys vs the oe steel ones.

The tensioner (pulley) is under your red line.

If it is eccentric/cam...does one direction on the nut add tension and the opposite direction release tension?
Assuming clockwise will add tension?
In the othercase, Is there a preferred place to pry it up?

It looks like the one on my old 502MPI's if you can see the backside of it it has a 'gear" type mechanism that you can ratchet tighter with a wrench on the very end of the shaft to provide proper tension then lock it down with the silver nut.

is there a hex or flat side on the unthreaded shaft ? if so put a wrench on there then loosen the nut and turn the wrench on the shaft clockwise and see if it tightens the belt then lock it with the nut, Im guessing cuz we cant see the back side

Loosen the nut on the tensioner, put a 5/16" socket on the end of the shaft, tighten the belt whichever way that is, then while holding the 5/16 tighten the nut with a 5/8" open end wrench.

Perfect instructions...tension did not solve BUT a new (old spare I keep under the bench seat) did the trick...THANKS OSO!

The 5/8" bolt seems to be getting rounded (from previous cranking down, etc)...is there a more robust material/version?

Glad I worked out!! I would say you could pull it and get a stainless bolt from a good hardware store or marine store I would think...make sure its a nylock nut like the one on there