1997 Eclipse Spyder GS 4G64 2.4L

I recently put in a new water pump, balance shaft belt and timing belt a well as changing the 3 drive belts. After I tensioned the AC compressor belt I backed off a bit on the bolt that pushes against the bracket to tighten the tension up. (I hope that was the only mistake I made -- all else seems fine) After going for a test drive, I double checked everything and went to retighten the belts and the bolt had vibrated out and was gone.

Does anyone know the size of the bolt? I picked up 2 at Lowes, but they weren't metric and neither (surprise!) has the correct threads. If it won't thread in by hand, I never force it.

Thanks for any replies!