Tub/Shower Mixer/Diverter

[Chuck38]

I'm working on a 3 handle mixer diverter valve assembly. Right handle is the mixer, center handle is the diverter, and the left handle is the on/off flow regulator.

The mixer had a leak, (bad O-ring). So took it apart, dropped one of the five screws down inside the wall cavity. Went to the hardware store. It is not US or metric. It is about half way between a #6 and a #8 in diameter, and the threads are different.

Any ideas on where to get a replacement?

The hardware manager said, from my verbal description that it sounded like a Delta knockoff.

[Nailer341]

Call the manufacturer of the faucet.
Most reputable manufacturers will send you the small parts quite quickly, and often times at no charge.