If you shift the standard pattern to the rear by 1/32 inch the forward hole will NOT interfere with the existing front sight hole, there is about 0.11 inch of "meat" between the two. I have a model 67-1 and 617-0 that I have added these holes to so I could mount J Point reflex sights on them when wanted.
Attached is a dimensioned drawing in PDF format of the current 3 hole pattern. BTW, the PDF shows up as a black box, click on it and you can open it as a PDF or save it to your computer. Since this drawing is a product of my own work I grant full license and permission for anyone needing this to copy and reproduce as needed. Also attached is a pic of the top strap on my 67-1 showing all 4 holes with the 3 hole pattern shifted towards the rear of the revolver by adding 0.03 to all 3 standard dimensions.
PS: when I did my revolvers I used a Bridgeport vertical knee mill with a digital readout and spot drilled, drilled, and then reamed the holes for the tap to the exact size required for tapping. Drill used was a Cobalt 0.116 diameter #32, ream was a Cobalt 3mm(0.118) diameter and the tap a HSS 3 flute #6-48. I then put the mill in neutral and used a LOT of back and forth motion to slowly tap the hole using a lot of tapping fluid. Because any Stainless Steel has a distinct tendency to gall on high speed steel and a #6-48 tap is VERY easy to snap. So, lots of tapping fluid, a good sharp tap plus a good degree of caution is required tap stainless steel. This is NOT a simple matter of using a center punch and using a hand drill, anyone who trys that will end up with a badly "bubba'd" top strap. If your current gunsmith doesn't have a vertical mill with a digital readout I would suggest finding a gunsmith who does.
Last edited by scooter123; 04-01-2018 at 05:57 PM.
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