Russ123, All 5 grip screws on Vaqueros, Blackhawks Super Blackhawks, and Single-Sixes use 6x40 screws. Just some advice …. it's never a good idea to chase a cross threaded hole with a tap because it is very easy to get it started crooked and ruin the threads. Using a new 6x40 screw is a much better idea. It will chase the threads but won't "cut" like a tap.
More suggestions: when you reassemble any Ruger SA, it's best to use a small jewelers screwdriver to start and seat the grip screws until they bottom out, then back the screws out about a turn. After all 5 screws have been installed, tighten them down with a proper hollow ground screwdriver. Jeweler's screw drivers can't apply enough torque to strip threads or cross thread screws. Here's what I use: https://www.bing.com/images/search?v...jaxhist=0&vt=0