tx_zstang

Anytime you get slack in the belt when shifting, the belt can jump. Then it's a matter of where it lands.
Idler pulleys are not that expensive, and you can pick up geed used ones for half price or so (from those doing 8-rib swaps, for example).

The first thing indeed to check is alignment.

Having a manual transmission is a factor as well (the dropping off from WOT/high rpms to lower rpms does a number of the belt and that's where a good tensioner kicks in to keep the tension proper).

GerRod

I read these stories so I installed a Thump RRR tensioner with my kit. I'm going to throw a small tool kit and a spare belt in the trunk before I go out of town.

danzcool

I got over one year out of the belt that got thrown this weekend, so not really bad, pulley alignment looks good, but trust me, order a belt before you need it, while the "Gates K061247" is a real part number and can be ordered it is not published in the catalog, so it's a special order item where your auto parts supplier has to call the Gates rep in order to order it. I'm ordering my current one from Whipple just because their on the west coast and It's cheaper to pay their exorbitant price ($89.99) with ground shipping that will be to me overnight, versus a $40 with $60 overnight shipping for the same thing from the suppliers on the east coast.
I'd been holding off on the tensioner and pulley swap, electric water pump, etc. since I expect go to forged internals at some point in the future (just a lot of dough to throw out while everything seems to be holding together just fine).

Just FYI, in my case, the belt got thrown while throwing a rev, right at the peak, so at the high RPM/drop off transition.

danzcool

Good idea, I was 30 miles from anything, it took about 4 hours from breakdown to finally getting the car dropped off at a dealer, tow bill was $335. I had it taken to a dealer because I was using the ford roadside assistance which the rep said would reimburse the towing bill as long as it went to a dealer within 35 miles, it seemed like a dealership would be a relatively secure location to leave it (as opposed to the side of a highway), and in the end it will be cheaper to pay the dealer to replace the belt than the tow bill would have been. But next time, I'll be prepared and just do the roadside repair.

Simon1

You have to actually check pulley alignment. You can't eye ball it.

I rip through Gates belts. I used the Goodyear Gatorbacks and get less slip, longer life and quieter running belts. I know KB recommends the Gates, but I shredded one the other day and it dented my hood. The Goodyear Gatorbacks work better for me.

yzf29

So is it a high speed engine condition and a poor OEM tensioner combination that causes this problem?

I ask because I just shredded one on my whipple equipped F-150...I realize not my Mustang but...

I had a spare belt and a 1/2 drive ratchet and a piece of thick PVC pipe to cheat on the rachet with. I suspected when I installed the SC that if that belt let go- I would be screwed (no pun intended!) - so I prepared ahead of time.

My belt shredded the two outer ribs but was still "holding togther" - but not a good situation- so it got swapped.

Occurred just ( less than 5 minutes) after a redline shift pass on a slower moving truck.

Belt had almost 12,000 miles on it- been on the truck since Feb '10

I would like to mitigate this problem best I can.

Thanks for the info sharing...

07VistaChicago

You guys throwing belts, do you have upgraded tensioners??? What kind if you do?

SamsBlueGT

I just noticed you said 1/2" drive, OP you may want to double check as I originally thought 3/8" drive. I can't check for myself just yet

yzf29

SamsB- 1/2" drive- it is a 5.4 in my F-150. It may be different than the 4.6 in the Mustang.

But I am running a Whipple SC and it uses the exact same belt as the 4.6 Mustang SC belt.

I am on the OEM tensioner.

yzf29

allright I am going to guess here- High speed engine operation into a slower speed engine operation- abruptly, as in a shift- caused the belt to swell and jump the pully(s).

Possibly I did not have the secondary (not the spring loaded) pully not tight enough- leaving overall slack in the belt - that under normal curcumstances (not red line shifting) was not a problem.

I just adjusted the (right side) "secondary pully" as low in the track as I could get it.

Maybe that will help with the issue.

I guess I can always save the redline shifting business for "close to home" and drive conservatively when I am in BFE

Steve