I am attempting to have a barrel changed on a 1930s K frame 38. My gunsmith is very capable and, so far, has removed the barrel from the frame and tried hand turning another 4th Change barrel into the frame. It went in 4 turns and I could not hand tighten it any further. He put the original barrel on the frame and could hand turn it 8 turns before it would no longer. Obviously, the old barrel was removed without issue using a wrench.

I started taking measurements to find that the old barrel thread OD measured .535” at the rear and .539” next to the barrel swell at the front of the threads. I then measured the replacement barrel and the thread OD measured .535” at the rear and .542” at the front. The only measurements I could take on the frame was the diameter of the “hole”, which would be inside peak to peak measurement. It came up at .510 at the rear (cylinder) and .520” at the front face of the frame.

Threads were cleaned before trying both barrels and done with light oil. The replacement barrel appears new and probably never installed on a gun, so my first question is if anyone knows how the factory fit barrels during manufacture? Standard threads appear to be .540” X 36tpi so if used would generate equal sized straight threads, but both barrels and the frame appear to have a taper? Which way is standard?

Most important is what suggestions do you all have in order to modify the new barrel to screw on without endangering the very thin bottom frame web? Any comments or suggestions are welcome. Any tips on how to fit up a new barrel to the frame would also be appreciated. It is understood that .540 taps and dies are not available today, so options seem limited.

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Gary
SWCA 2515

I have a .540 - 36 tap and die from Brownell's. That is still the standard K frame barrel thread size. If you run the tap thru the frame and the die on the barrel, they will fit together.

The sku# for the tap is #080-598-540WB

The sku# for the die is #080-598-541WB

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protocalldesign.com

When they fit too tight upon assembly like that, I lap the threads in.
It doesn't take a lot of work and the process goes fairly quickly.

I've used the method on rifle bbl/action assembly threads as well as handguns.

Keep the frame and bbl wrapped in a protective covering to prevent the lapping compound grit from scratching the surfaces in the event that it gets on your fingers as you handle the parts.

Wash all the compound out of the threads when completed and before final assembly.

Use of the proper tap & die is much quicker and cleaner of course, but lapping the threads also works.

Thank you for those numbers. I had searched Brownells, but apparently could not come up with the right search terms to find the tap and die.

The old barrel measured under .540 and was tight. With an as new barrel that measures very close to .540", do you think that tapping the frame would all that is needed?

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Gary
SWCA 2515

Thank you for the advise. My gunsmith did mention this option and I will discuss further with him. I do think I might buy the tap, since the threads do not look perfect after the old barrel was taken out. Maybe lapping, if needed, will be easier with freshened threads in the frame.

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Gary
SWCA 2515

I would do a tap and die of both the barrel and the frame. That way there can be no question as to the proper fit. Or which item is not fit correctly.

Measuring the inner diameter of a tapped hole or outer diameter of a threaded shaft is almost completely meaningless from a machinist's point of view. You are most interested in the pitch diameter and secondarily the root diameter. An optical comparitor is a good choice, or you can use a specialty thread micrometer, but failing that you can use fine diameter pin gauges and a standard micrometer. If you don't have three small pin gauges the same diameter you can mostly get away with using a set of three drill bits of the same brand and lot. Actually, considering you want a wire that will touch in about the middle of the depth of the thread I suspect three ordinary sewing pins would work pretty well in this case. It will give you something for the sake of comparison at least that is far better than the major diameter.

All that said, assuming the frame wasn't damaged and the old barrel fit was good, If I were on a budget I'd just buy a die and run over the new barrel in the hopes that it will solve any problems since the frame did accommodate a barrel at least. If I weren't on a budget, then I'd buy both the tap and die and run them both. I'd much rather need a drop of locktite to keep them tight than to constrict the barrel and suffer poor accuracy as a result.

What is Pitch Diameter?

I just ordered the die as well. Already have a member who wants to buy the set from me when done if I do not want to hold on to them.

This approach takes all the variables out of other options, chase both frame and barrel to be safe and minimize the amount of stress placed on the lower frame where it is extremely thin. For those of you who have not seen a frame without the barrel attached, here it is. Scary thin.

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Gary
SWCA 2515

I am a big proponent of lapping threads is cases such as this. I would not use anything coarser than 2A grit on these fine threads.

Post #7 mentions finding the pitch diameter. Decades ago I bought the proper wire gauges to do this for 36 tpi. The wires are 0.01605 in diameter (and it's a PITA to do). You aren't going to find drills that size. Plus, since the diameter of the barrel shank is an odd size, the only standard available is from measuring a barrel that properly fits a frame. When you do custom, you do trial fits on the frame when you get close to the measured diameter.

The tap & die are the way to go. A slight taper on the barrel shank isn't unusual for production barrels. I expect the shank/tenon was machined to size back then by using a shape tool that fed in at a right angle to the barrel blank. If the tool wasn't set up/ground exactly correct, there would be a slight taper. The parts inspectors had the tolerances to verify if it would be within specification.

I would try lapping first.
Timesaver compound

Timesaver Lapping Compound Yellow Label and Green Label Compounds, non imbedding lapping compound, manufactured by Micro Surface - Newman Tools

I have not used this on a barrel, but have lapped many gears, bushings and pins with it.
Timesaver is fast and it is soft so over lapping is minimized.

Thanks all for your comments. I received the tap and die for K frame barrels post-1903 with the introduction of the Model 1902, 1st Change. The die was adjustable, so set it to cut the least as I could do. Could run it with my hand and a gripper pad. The tap is a straight die and fixed diameter at .540". Ran in smoothly and done in a couple minutes. I threaded the barrel on the frame and it was much better, but still did not turn in by hand, so set the die tighter and the second time was the charm.

What amazes me is that a near 100 year old gun can accept a random barrel without any further work required. First, the replacement barrel screwed in by hand smooth as glass until it was maybe 1/8 turn from top center. Wrenched it and it stopped exactly where is was supposed to. Measured the gap at .005" - perfect! The barrel aligned perfectly with the lug as well.

The replacement barrel already had a pin slot and I was not sure if it had been installed on a frame or not, but assumed that with the slot it must have been on another gun? I picked out a number drill bit smaller than the hole in the frame to check alignment and the bit dropped right through the hole - another perfect fit up. I am now of the opinion that the manufacture of barrels and frames were so precise that they milled the slot in the barrel prior to installation??

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Gary
SWCA 2515

I believe that is correct, all Ive seen were blued in the pin groove, and I believe Ive had some never used barrels and they were grooved already. I recall seeing pictures somewhere of many barrels on a rack ready to blue at the factory and they were already grooved.

They didn't have to be particularly precise in the pin slot, it needs to prevent rotation so the distance of the slot from the bore matters, but it could be almost any width and do it's job and they are cut a bit wide.
Usually you can rotate a barrel a degree or more without disturbing the pin, so it's not that precise there either.
I've actually seen a loose barrel where it could rotate several degrees until you hit the pin. It's a nice touch, but I can see why they discontinued the practice.

Just saw this. You should have gone right out and bought a lottery ticket too!

Im glad your barrel exchange worked out well
for you..It reminds me of the old adage..
The 6 P Principle...Proper Planning Prevents Piss
Poor Performance😁😁
It was drilled into folks by me and others at my
USMC work center building weapon stations(gun turrets) for our young Marines.
Always a joy when a plan comes together.
Best Randy..