Please Help, I need to know what the diameter is of the Idler pulley GM part number 12568996. I'm trying to fit standard C5 drive belt on the front of an LY6 the only two remaining truck items are the main crank pulley and the idler pully, the truck idler pully is 3.5" GM Part Number 12580771.

 

this may not help you but here is a idler pulley off of a 2006 GTO the diameter is just shy of 3"

 

I used a dial caliper, its 3 inches.

 

That must be my problem, I guess I need either find a 3 inch idler or play with belt lenght.
Mesquitebean is the one you messured a C5 or part number 12568996?
Thanks for the help!

 

I have that idler, 12568996, and it is 3 inches as Mesquitebean posted.

 

On order from amazon $31 THANKS