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Sorry to hear about the broken screw. Peter is correct about most crown wheel screws being left handed and you are also correct about screws with 3 grooves being left handed. Problem is that most left handed screws don’t have these grooves. So when you see these grooves you don’t have to guess whether it’s left or right handed and if you don’t see them well…. One way to check when in doubt (if it’s not a blind hole) is to remove the bridge and take a look at the leading edge of the screw thread from the underside with high mag.
To avoid etching or blemishing I usually use a fairly diluted alum/water (about 6 pts. water to 1 pt. alum) solution to help avoid any etching or discoloring. Usually have great luck and bridges come out with no blemish at all but I still like to test a little on the underside of the plate or bridge before soaking. I usually submerge the entire bridge so that no line is left across the surface in case the solution runs. Just in case.
If it helps any…don’t feel too bad Tom. I’ve snapped my fair share of screw heads off over the years.
Edit: Hey Randy…I just read your post. You must have posted while I was writing this one. Yes I agree with what you said about alum not etching the finish. I haven’t tried the bars so can’t comment on that but the powder works great.