B8.5 S4 Idler Pulley Bolt hole Partially Stripped – Fix Recommendations?

Hey everyone,

I’m working on my Audi B8.5 S4 and ran into an issue with the idler pulley while replacing the thermostat. I thought turning the idler pulley bolt would release the supercharger belt, so I kept tightening it clockwise with an M10 star bit. Unfortunately, that ended up stripping some of the aluminum threads in the mounting hole. I eventually got the belt off by adjusting the tensioner pulley, but only after the damage was done to the idler pulley bolt hole. I didn’t realize I’d caused damage until reassembly.

Now, the bolt still catches and can be torqued to 20 lb-ft, but it has a bit of play when pushed or pulled and would likely keep turning if I tightened it more. I’m worried this looseness will cause issues with belt alignment or lead to the bolt backing out due to vibration.

I know I messed up here and learned my lesson, so no need to remind me! Just looking for advice on the best way to reinforce these stripped aluminum threads. I’m considering Helicoil or a threaded insert, but I want to make sure it’ll be secure for this setup. Any suggestions on what’s worked best for this kind of aluminum thread repair would be awesome, along with any tips to avoid misalignment during reassembly.

TL;DR: Tried to loosen the supercharger belt by turning the wrong bolt and ended up stripping threads on my B8.5 S4’s idler pulley. Bolt catches and torques to 20 lb-ft but has play and would likely keep turning if tightened further. I know I messed up – just looking for the best way to reinforce the aluminum threads and secure the idler pulley bolt.

Hey everyone,

I’m working on my Audi B8.5 S4 and ran into an issue with the idler pulley while replacing the thermostat. I thought turning the idler pulley bolt would release the supercharger belt, so I kept tightening it clockwise with an M10 star bit. Unfortunately, that ended up stripping some of the aluminum threads in the mounting hole. I eventually got the belt off by adjusting the tensioner pulley, but only after the damage was done to the idler pulley bolt hole. I didn’t realize I’d caused damage until reassembly.

Now, the bolt still catches and can be torqued to 20 lb-ft, but it has a bit of play when pushed or pulled and would likely keep turning if I tightened it more. I’m worried this looseness will cause issues with belt alignment or lead to the bolt backing out due to vibration.

I know I messed up here and learned my lesson, so no need to remind me! Just looking for advice on the best way to reinforce these stripped aluminum threads. I’m considering Helicoil or a threaded insert, but I want to make sure it’ll be secure for this setup. Any suggestions on what’s worked best for this kind of aluminum thread repair would be awesome, along with any tips to avoid misalignment during reassembly.

TL;DR: Tried to loosen the supercharger belt by turning the wrong bolt and ended up stripping threads on my B8.5 S4’s idler pulley. Bolt catches and torques to 20 lb-ft but has play and would likely keep turning if tightened further. I know I messed up – just looking for the best way to reinforce the aluminum threads and secure the idler pulley bolt.