Okay, so I know this has been asked a few times, but it seems like every thread I find either says something different, or does not have my exact setup. So hoping to post for some resolution to help both myself, and others in the future. I just went out and bought 2 belts - SC and the AC/WP belt. Supercharger belt was P/N 4060665. Stock 3.8 pulley. Belt has good tension, and had to move the tensioner arm to get the belt on. So I'm calling that one good. The AC/WP belt, however, does not fit. P/N K060810. It is a Series 3 engine, and I am running Fierorog's low mount alternator setup. Anybody running this identical setup and have a part number for this belt? This was the recommended one, however, it does not place any tension on the tensioner at all, and can be pulled off the water pump by hand. I am thinking about going to 5060805, which has effective length of 80.51" vs the K060810's 81.0625". Thoughts? I figure the reduced 0.5" should get that tensioner arm to be moved so it can tension the belt. Thanks!

I ended up, through a bit of trial and error, being able to figure this one out. I tried the 5060805 and even it ended up being too long. There was no take-up by the tensioner so any belt stretch and it was going to start slipping. Next options were 5060800 and 5060795. I decided to try the 5060795 belt and it fit like a glove. Tensioner is now about 1/3-1/2 the way into travel which I assume will reduce just a bit after initial belt stretch. And FYI - my setup is Fierorog low alternator mount, on a Series 3 engine, with a Dayco 89217 mounted to Fierorog's doggone bracket. Hope this helps!

My install with my DIY low mount Chevy Astro alternator uses the Goodyear Gatorback 4060795 serpentine belt (79.5" as well) but different alternator mounting systems will use different size belts. We do not use the Dayco belts as we have found that the multi-ribbed Goodyear belts run quieter, stretch less and grab better.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "