September 5, 2017 at 12:15 pm #46935
A Super Blackhawk was brought to me needing a new original rear sight installed. Unfortunately the last owner put a big honker scope mount on it that required the original 6-48 sight mounting hole to be drilled and tapped to 8-40 so it needs to be bushed back down to 6-48. I was able to drill an 8-40 plug screw for tapping to 6-48 on the lathe with the thought of fixing it in place with green loctite before tapping it. I have concerns that this will fail right away or perhaps down the road due to the thin wall remaining. One possible solution is to drill the hole and tap it to 10-32 and drill/tap the 10-32 to 6-48. Mechanically I have no reservations about this but I’m concerned about weakening the top strap.
All thoughts and opinions are welcome!
September 5, 2017 at 12:35 pm #46939
It will not significantly effect the strength of the top strap. Not seeing it myself, I do think that the 10-32 would be stronger, but if you were able to drill and tap the 8-40 screw, then I think you’re golden. I do not recommend green locktight though. If it were me, I would turn the OD and ID threads out of a piece of 01 tool steel drill rod (commonly available through McMaster Carr) and SOLDER it in with low heat solder paste, then chase the solder out of the threads with the tap. Locktight has it’s place, but on a little bitty part with no constant tension on it, coupled with enormous side load force is not the place to be using it IMHO.
September 5, 2017 at 2:07 pm #46940
I like that solution. I have drilled but not tapped the 8-40 screw. You got me thinking, though, about soldering in the 8-40 I already drilled. Once it’s soldered in I could tap it. Wouldn’t that be about the same as threading O-1 and soldering it in?
September 5, 2017 at 2:13 pm #46941
Is there a particular brand of solder paste you prefer?
September 5, 2017 at 2:43 pm #46942
The surface of the screw will probably not take the solder like freshly cut steel would Dave, and you need a tight fitting thread with bright steel. I don’t know what tooling you have at your disposal, but if it were me doing it, I would tap a test hole in a scrap piece of steel to get the fit right on the threads when I was cutting on the lathe. I’d want the threads to be tight screwing into the tapped hole produced by that drill and that tap. Once I had the rod threaded, I wouldn’t cut it off yet and use the excess material to drive it into the hole drilled and tapped in the gun. One trick you can use to get that tight fit is to run the lathe in reverse, and dress the threads with a sharp 30 degree Grobet needle file by holding it by the tip and dragging it off the end of your threaded stub. This will produce a slightly tapered thread and give you the control you need to get the fit just right in the test piece.
Next, I would set the revovler frame up in the mill, then drill and tap it. Then I would zero my DRO on that location, roll it out of the way, clean it thoroughly, dope it with solder paste and start heating it as I screw the stub in place. Once I was sure the stub was bottomed out in the hole, I’d let it rest till the solder hardened. Remember when heating it, that the frame will take roughly 600 degrees before you run any risk at all of damaging it with heat, so be careful when melting the solder. You want to play cat and mouse with the liquid/frozen state of the solder to make dam sure you don’t go too far.
At this point, I would use a hacksaw to cut the stub off a little proud of the surface, then roll it back under the mill and use an endmill to cut it perfectly flush with the surface. Then I would go back to XY zero and drill and tap the hole like it was fresh.
That process will produce nothing short of PERFECTION and somebody would have to look REALLY close to tell it was not factory, which is important to me here at MBT.
The solder I use for most jobs is no longer made and I’m nursing the jar along. However, when it finally bite’s the big burrito, I’m going with Brownells 70PA homogenized solder paste. That’s the stuff I recommend you use.
September 5, 2017 at 4:21 pm #46946
OK, the plan is coming together and I know your solution is a good one. Would you open the existing 8-40 hole out to 10-32?
September 5, 2017 at 4:50 pm #46947
Yes I would.
The minor diameter of the 8-32 is .1302 and the major diameter of the 6-48 is .138, which means one thread will cut through the other and you’ll be holding on by metal fibers. No bueno.
The minor diameter of the 10-32 is .1562 which means you’ll have some meat in there.
If there is a question of the 10-32 thread causing damage to the surrounding metal for some reason, then you may consider making a cylindrical plug and soldering it in place (ie: forget the threads) but if you do that, remember that your joint must be perfect.
Solder is not glue. There is no such thing as a bond line, and the less space you have, the stronger the joint is. On a microscopic level, studies have found that the solder material no longer exhibits it’s original metallurgic properties, because it is actually sharing molecules with the metal on either side (Ref: engineering at Kester Solder) thus, the tighter your joint, the stronger the marriage. This is key information and something you should keep in mind whenever soldering gun parts as joint quality is paramount, and low temperature solder is the weakest of the solders. That molecular sharing is it’s only ace in the hole. This is why I insisted you make the plug from scratch, and make sure it fits TIGHT.
Don’t over think it. Know what you know, function within that knowledge, and build your process accordingly. You will never fail if you follow this method, even when your margin of error is small.
That said, if you do not feel comfortable with this, consider sending it here to MBT and I’ll handle it personally.
September 5, 2017 at 6:28 pm #46949
Thanks for all the info Tim. I’m comfortable with machining small parts and soldering. There’s plenty of room for the 10-32 drill and tap without touching the ears of the sight’s saddle. I made an arbor for a drill chuck for my mill that measures .002″ TIR with my Starrett Test Indicator so this shouldn’t be too hard. I’ve been soldering steel parts for decades so that doesn’t concern me. I really like your solution. Thanks!
September 6, 2017 at 6:18 am #46956
Well, I type a lot, but I figure a few precision shots followed by a shotgun blast of information never did anybody any harm. LOL!
YOU GOT THIS!!!
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