I got a new serpentine belt from merc part # 57-865615Q01 which superseded my original belt 57-863876-1 for my 496 Mag serial number 0M687121. I cannot get this belt no matter how hard I pull on it. It's like just a hair too small. The only adjustment I have to loosen the pulleys with with the alternator tension adjustment screw. I have it loosened all the way until the alternator hits the mounting bolt/boss for the idler pulley. Any suggestions or tips to get this belt on?

There are 2 belts, one for the adjustable alternator and one for the alternator that is spring loaded, maybe you got the opposite one?

I only know of the one "bolt" to adjust the alternator that adjusts the tension on the belt. I'll try to attach a pic.

You have the bolt fully retracted?

Yes. Iike I said in the original post. I have it loosened all the way until the alternator hits the mounting bolt/boss for the idler pulley bracket.

Don't know, spec for 865615Q01 list 14 at BAM (figure 14 feet), the later model used 865635Q01 and list at 15.25

Do you have the old one to measure?

Mark belt and on the floor. Roll across the floor until mark comes back around

I'm using the Quick Silver 57-865615Q01 on my 2002 496. Looks like the same setup as yours. I bought a belt before this(don't remember number) and it didn't fit.
The belt that came off the engine was 57-865615-001 Mercury.

The new belt has a 57-865615-001 on the belt and the Quick Sliver part# is 57-865615Q01. So it's the same belt you have. It's just too small. I don't know what to do.

If you know what the dimensions that are needed, it can be looked up

Need length and how many ribs

So if your current belt is X and it needs to be 3 inches longer it can be found

Are you sure the alt is loosened all the way? Is the belt sitting in the pulleys/grooves? Are you sliding around a shoulder less/ non ribbed pulley last? How short are you?