Glad to hear the OP solved the issue with the belt by replacing... FWIW, Just throwing out there for anyone in the future looking to solve a belt type of issue I have seen that type of idler / tension-er pulley have a specific rpm related squeal (rather more like a chirp-chirp-chirp) as the bearing was giving up.

Also see a V-belt with less than 20 hours that glazed the sides, left over winter in a frozen boat house and squealed like a loose belt on the corroded pulley groove.

Roger...
I am curious how you knew the exact size wrenches etc...It helps to carry the exact right sizes from the garage to the boat without making a bunch of trips. :-)

All good points and I will take them into consideration. It was more of a squeal than a (bearing) chirp...but all good data points. I took the boat out and ran her to 5,000 rpms with no noise.

Tech tip:
NAPA brand belts are Gates. I brought oroof. Take a good look at part #’s to see likeness (helps with psrt cross) and nspa description (says Gates)

note: this is not your belt. It is one i purchased a while ago for anothet merc motor. Just using it as example.

My dad has been working on boats since before I was born and I followed in his footsteps for quite awhile but have moved on career wise but he still is a boat mechanic. Just one of the weird things that stuck in my mind since I have done it hundreds of times and I hate getting in a boat with the wrong size tools hahahaha

Are you meaning the 5/8" nut or the 5/16" on the stud?
Confused me because you said bolt and 5/8".
I've seen the studs get rounded a lot because they're 5/16" but not really the nut.

If he meant for the nut yes.
I'm sure you know
This is meant more for F14A.
Hardware store fastener won't be a good solution for the stud for 2 reasons.
No pinion on the back nor the 5/16 hex milled on the front.
They are like 20 bucks new.