So back when I originally changed my wheels to what I have currently, I took them to the local Discount Tire. I waited for like 2 hours for them to mount and balance my existing still-good tires onto the new-to-me wheels. I knew they were probably taking longer because ... well, the wheels weren't new, and they probably had to stick a buncha weights on them to balance them. This was only partially true; What had *really* happened was that it looked like there was some sort of mismatch between what was on the wheel and what the tire needed, so they changed my valve stems out. In doing so, it made my valve caps not fit-- which is probably why they were all missing. I had a few extras at home, but they no longer went all the way down the thread, so I knew they were different. I only noticed later that they weren't metal anymore.