After determining that I had a way to drill the necessary holes without any new tooling, the repair of the threaded holes on the Latch Abutment (58H1) is finished.
Even with the holes properly center-punched, I had a little trouble with the drill drifting out of position on the right-hand hole but I managed to get the hole back to the correct position with only a shallow out-of-round spot. A close look at the photo will show the out-of-round hole. You can also faintly see the circle of brazing that holds the repair plugs in place.
Tapping the holes turned out to need no special tool: the body of my tap wrench was small enough and I just had to put up with the tediousness of sliding the T-handle across every half-turn.
Now I just have to paint all the parts and re-assemble the latch. I have spares of the small screws for the repaired holes. They are original parts, and although they are not an exact match for the tap I used (both the diameter and thread angle differ a bit) their heads have the shape to fit the countersink in the actual latch plate. I don’t feel like taking a pair of modern #8-36 screws and having to re-shape their heads only to have two screws that won’t fit anywhere else on the machine. Another option would be to use a product like Loctite Form-A-Thread to fill the thread in the holes to their proper dimensions.
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