Just installed the system and was trying to mount the rear speakers on my wall. The problem arose when I found out that none the standard screws I could get my hands on would fit. I'm using metric and bought a pack of standard machine screws in sizes M5 & M6, I also tried M7. M5 was to small, M6 did go in for a few rotations but then locked up, and I looked, it was plenty more to go on the speaker. The screw only went in a couple of millimeters before it got stuck. I suspect the thread or something is off, but idk. Please help! What type of screws is right for my speaker?