class of fit for barrel thread tap

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class of fit for barrel thread tap

Post by SteveM » Thu Nov 28, 2019 9:32 pm

I need to make a tap to chase the barrel threads of a S&W .357 mag frame.

The threads on the barrel are 0.667 on the OD and 36tpi. I measured with my thread pitch micrometer that is for 32 to 40 tpi and it shows 0.650 thread pitch diameter, and based on calculations I found online, that looks right (calculations have 0.649).

My question is: what diameter should I make the tap?

That boils down to what class of fit is required for an application like this.

Note that the frame is already threaded, it just needs to be chased. I just don't want to make the threads any larger than they are and make a sloppy fit.


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Re: class of fit for barrel thread tap

Post by GlennW » Thu Nov 28, 2019 9:57 pm

I would make the tap match the barrel threads since that is what needs to screw in there.

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Re: class of fit for barrel thread tap

Post by Harold_V » Fri Nov 29, 2019 12:51 am

Class of fit, even for interference fits (which I think is a class 5), should be determined by the pitch diameter, NEVER the major or minor diameter. Thus, the major diameter should be in keeping with the formula for that particular thread and fit. The size is bastard, or non-standard, so S&W may not make that information known and it may not be found in thread specifications, although the formulas found in the H28 manual would allow for making a determination of the proper sizes required.

If the task was mine, I'd make a tap with the major diameter a couple thou larger than the barrel thread diameter, guaranteeing that the major diameter of the barrel did not interfere with the minor diameter tapped in the frame. Pitch diameter of the tap? Do as Glenn suggested. If the barrel fit previously, and there's no deformation of the threads on the barrel, it should fit as it should.

Armed with that thought, I'd highly recommend you pay close attention to the threads on the barrel, to ensure that they have the correct form and are not sporting deformation.

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Re: class of fit for barrel thread tap

Post by earlgo » Fri Nov 29, 2019 10:28 am

Many years ago I swapped out a barrel from a S&W M&P and found out the hard way that the frames are relatively soft. So beware when applying too much torque to the unsupported frame. I ended up casting a cerrosafe block that encompassed the entire barrel frame junction so it could be gripped in a vise.
cerrosafe s&W frame mold.jpg
cerrosafe block
(The mating half is missing so it must have been used for another project.)
Just a caveat if you need it.
Before you do anything, you must do something else first. - Washington's principle.

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