Prepping for an upcoming trip, and reading some of @Stodge
's trip log, I'm going to change out my serpentine belt and while I'm at it, I might as well do the idler and tensioner pulleys. I'll keep the old one's as spares.
The belt generally looks ok, no cracking, but showing signs of wear, but I know it's been around a while with a number stamped on it of 57-862087 which has been superceded quite a while back it seems by 57-865615Q03
I do see a Sierra cross reference for a 18-15100. I also found a cross reference here: http://crossbelts.com/serpentine/6ribkseries.html
that show's equivalent automotive belts from makes like Dayco, Gates, Goodyear etc.
These might be easier to source where I am. I've read everything from "go ahead, it's fine and save half the $$$" to "Don't you dare, your boat will immediately explode, take out every other boat in the marina and you'll burn in non-marine part ****!!!.
The Tensioner according to Mercruiser EZ-Parts lookup is 807757T or Sierra 18-6457. Also found a Dayco 89014. Not sure if there's any others or not.
The Idler is 27-818382. I can't seem to find a cross for this one, but it seems easier to find. I might just order it off Flea-bay.