I own the revolver in the title, 8-3/8 barrel. It is of course untapped except for the #3 screw that holds the iron sight. I use Weigand mounts and this mount follows the current hole pattern used by S&W. When I place this mount on, the forward hole is real close to the factory #3 hole, it looks like the new 6-48 tap will cut into the edge of the #3 hole. Has any one dealt with this situation? Completely biased opinions welcome. Weigand has told me that the current S&W pattern was designed to accommodate this #3 screw hole. My gunsmith refuses to tap that last hole.
I can't help with your problem but I hope some good advice pops up. It always does here. I have a red dot mounted on a 686-5 but of course it was drilled and tapped at factory. Weaver top mount matched up perfect. I also have another 686 that has a scope mounted in discontinued (I think) mount by Weaver that uses the rear sight mount slot and the front part of the mount actually is fitted to the gun frame between cylinder and barrel. Its a tight fit. When I first saw it, I didn't think it would be a good mount but has proven to be very secure. I am interested in drilling and tapping a couple more but now I'll wait for the experienced ones to give you the correct answer before I proceed.
The back 2 screws will hold it fine for target loads. If you want a 3rd screw, just put it further back from the one that's there. There is nothing sacred about the screw locations. You will not weaken the top strap by putting one in a different place on the center line.
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.
The scope mount is supposed to butt up against the step at the back of the top strap to act as a recoil lug. This takes all the shear load off the mounting screws so they don't shear off due to recoil.
If you need to move the hole pattern back, put an equal spacer between the mount and frame to retain this feature. Screws are a clamping element, they can't take much of a side load. If the screws shear off, the scope and mount will come off the gun, probably hitting you in the face.
As scooter123 says, it is important to do this job correctly. You only get one chance to do it right.
I always use a spring loaded tap guide in the drill chuck when tapping to insure that the tap handle stays perfectly straight so I don't put a side load on the tap. Side loading the tap is the #1 cause of breaking the small ones.
Last edited by Protocall_Design; 04-01-2018 at 07:01 PM.
Scooter and Toolguy, I really appreciate your replies. When we did my 617-0 we just drilled two holes. It works fine after at least a 1,000 rounds. This will be the 4th revolver he has tapped for me and he most definitely has a vertical mill with a digital readout. 32 years as a gun smith, so not a rookie. I will take my lap top when I visit him tomorrow and go over your suggestions and see what happens. Thank you all.
686-0 scope mount advice
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