How to create a female taper thread?

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Ca-g

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Can anyone clue me in on how to use a tapered tap? The thread is BSBT 5/16”. I have not yet seen the tap, does one act in the same way as a parallel tap? Use the same drilling size? I need to make a blind nut with a hole in the blind end for the flared pipe to extend through.
Best
Chris
 

TonyM

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Tapping is as per a parallel tap. Depth of tapped thread being important though I am not sure why you want to use a taper thread with a flared pipe. It is possible for the thread to lock before the seal is made on the pipe flare. These joints usually use parallel threads. Also 5/16 is a non standard BSPT thread.
 
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Ca-g

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I am replacing the nut; the very complicated male fitting already exists and the setup probably worked because the device is 80 years old. I have read that tapered fittings seal better than parallel. I understand the timing issues with making the taper tighten at the right moment. My question is, does one use a taper tap in the same way as a parallel one, ie, tap into a parallel drilling.
PS.it is BSBT, not BSPT
 

BaronJ

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Hi Chris,

Would you care to unwind the meaning of the letters BSBT please.

All the 5/16" brass 26 tpi threads I can find are parallel. I've not been able to find a tapered one.
 
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Tim Wescott

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From somewhat casual US practice, fixing various plumbed bits:

Using a tapered tap is the same as using a straight tap in that you screw it in the same way, clear the threads the same way, and look up the proper drill size to use and use that, the same as for straight taps. You don't say "oh, a thus-and-so thread uses a this-here drill for a straight tap, so I'll use that same size for tapered". Because in general it'll be different, and there's all sorts of reference material on the web if it's not in your Machinery's Handbook.

It is easier to start the thread, and gets progressively harder as you screw it in, as you should expect. I suppose that you also need to pay attention to tapping too deeply or too shallow -- I'm not sure how deep is enough, see the word "casual" in my first sentence. For something critical and standard I'd look it up. For something that's critical, old, and specially engineered, I think I'd tap it undersized, test fit, and then progressively deepen (i.e. widen) the tapped hole until things fit right.
 

BaronJ

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From somewhat casual US practice, fixing various plumbed bits:

Using a tapered tap is the same as using a straight tap in that you screw it in the same way, clear the threads the same way, and look up the proper drill size to use and use that, the same as for straight taps. You don't say "oh, a thus-and-so thread uses a this-here drill for a straight tap, so I'll use that same size for tapered". Because in general it'll be different, and there's all sorts of reference material on the web if it's not in your Machinery's Handbook.

It is easier to start the thread, and gets progressively harder as you screw it in, as you should expect. I suppose that you also need to pay attention to tapping too deeply or too shallow -- I'm not sure how deep is enough, see the word "casual" in my first sentence. For something critical and standard I'd look it up. For something that's critical, old, and specially engineered, I think I'd tap it undersized, test fit, and then progressively deepen (i.e. widen) the tapped hole until things fit right.
Hi Tim, Guys,

I've spent some time trying to get to the bottom of this.
I cannot find a recommended drill size for a 5/16" inch 26 TPI brass thread ! I've found out that its a 55 degree whitworth thread and that it is easily confused with the so called "Bicycle" thread, which is also 26 TPI but a 60 degree angle. The OP mentions a taper tap ! I've not been able to find anything about a taper or tapered thread in that size. Actually it is not even a "British Standard" brass thread.
 

retailer

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My Newnes Engineers Reference book gives the following tapping drill sizes for 5/16" Brass 55deg thread -
Class A close fit 17/64",
Class B medium fit letter drill I
Class C free fit 7.00mm
 

TonyM

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Was the original female a taper thread? It is common practice to use a male taper into a female straight. It would make more sense if a flared pipe is being used. The Brass nut would partly take the taper form when tightened so it would be difficult to tell from the original part other than the outside of the nut would also be slightly tapered.
 

johwen

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Can anyone clue me in on how to use a tapered tap? The thread is BSBT 5/16”. I have not yet seen the tap, does one act in the same way as a parallel tap? Use the same drilling size? I need to make a blind nut with a hole in the blind end for the flared pipe to extend through.
Best
Chris
Johwen here,
The set of taps normally come in three a taper starter, intermediate and bottoming tap. For a male thread to seal as the thread tightens it does so as the starter or intermediate being tapered on the OD. It is the female thread minor diameter is left tapered, it is missing the full form of the thread and thus seals on the OD of the male thread. In other word a taper tap is a tap the has a parallel thread cut to length and the the OD is tapered to enter the tapping hole.Hope this helps stay well in this topsy turvy world John
 

ignator

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Can anyone clue me in on how to use a tapered tap? The thread is BSBT 5/16”. I have not yet seen the tap, does one act in the same way as a parallel tap? Use the same drilling size? I need to make a blind nut with a hole in the blind end for the flared pipe to extend through.
Best
Chris
I use a tapered reamer for pipe threads, as a straight drilled hole is a worry about tap breakage. But I don't see 5/16" as a currently available reamer size. The sellers catalog I just looked has 1/8, 1/4, 3/8, 1/2, 3/4, and 1 tapered reamers.
 

Richard Hed

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Can anyone clue me in on how to use a tapered tap? The thread is BSBT 5/16”. I have not yet seen the tap, does one act in the same way as a parallel tap? Use the same drilling size? I need to make a blind nut with a hole in the blind end for the flared pipe to extend through.
Best
Chris
Can you tell us what this is used for? Can the part be dis-assembled? If it can, why not just re-thread the whole thing, or re-fit the thing
 

dkwflight

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Hi
Select the proper drill and crank the NPTsp in until the plug for the same taped hole goes in to the right depth.
I do know the bigger pipe taps do better with a tapered hole drill.
Dennis
 

justintime

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Can anyone clue me in on how to use a tapered tap? The thread is BSBT 5/16”. I have not yet seen the tap, does one act in the same way as a parallel tap? Use the same drilling size? I need to make a blind nut with a hole in the blind end for the flared pipe to extend through.
Best
Chris
Please clarify the use / purpose of this nut in your assembly. Also photo(s) should be helpful. Thank you.
 

L98fiero

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Can anyone clue me in on how to use a tapered tap? The thread is BSBT 5/16”. I have not yet seen the tap, does one act in the same way as a parallel tap? Use the same drilling size? I need to make a blind nut with a hole in the blind end for the flared pipe to extend through.
Best
Chris
Without trying to figure out what you're doing or why, if the thread you want is BSPT, not BSBT which I couldn't find, the tap drill is 6.4mm or 1/4". Tapping is different though. Hand tight engagement, the depth you can screw in a male thread gauge or barring that a male fitting is about 6 turns. The charts I found don't list 5/16 so it's a non-preferred size, show hand tight engagement for 1/4 BSPT at 6 turns and 3/8 at 6.4 turns, you have to watch how deep you tap.
 

wazrus

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Taper tap in BSB? Good luck with that. Somebody else suggested using a taper tap's first lands and that seems as good a suggestion as any.
The Brtitish Standard Brass thread, for me, has been most useful when threading thin-walled tube and I've found it used in light fittings. I've also got 5/16"BSB taps and dies, which I've had for ages. BSB is hard to find, ordinarily and I've adopted a sort of 'de facto' replacement using 1mm pitch metric and these I have up to 40mm. The taps in the larger sizes are very considerable lumps of metal but they are available. They are a bit pricey, but not prohibitive. My sources give a 17/64" tap drill for 95% engagement in BSB
 

L98fiero

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The Brtitish Standard Brass thread, for me, has been most useful when threading thin-walled tube and I've found it used in light fittings.
So I learned something new today, British Standard Brass thread doesn't actually exist as it's not covered by the standards so it's really just British Brass Thread, all sizes use 26 tpi Whitworth 55° thread form and the tap drill size is 1/64(.016/0.4mm) under the nominal size. The idea behind the threads is that all brass tubing is about the same thickness so the same pitch is appropriate for all sizes.
 

RonW

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Taper tap in BSB? Good luck with that. Somebody else suggested using a taper tap's first lands and that seems as good a suggestion as any.
The Brtitish Standard Brass thread, for me, has been most useful when threading thin-walled tube and I've found it used in light fittings. I've also got 5/16"BSB taps and dies, which I've had for ages. BSB is hard to find, ordinarily and I've adopted a sort of 'de facto' replacement using 1mm pitch metric and these I have up to 40mm. The taps in the larger sizes are very considerable lumps of metal but they are available. They are a bit pricey, but not prohibitive. My sources give a 17/64" tap drill for 95% engagement in BSB
In reading GTs book he mentions 26tpi for Myford graduated dials and said he had a full set. I looked on line, eBay I think, and found an Indian manufacturer who sells a 1/8" to 3/4" x 26tpi in 1/16" steps for under $150 Canadian dollars. I had never seen reference to BSB Before. I've bought BSF from this same manufacturer before and they look well made and certainly work.
RonW
 

Ca-g

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Here are pictures! My apologies for not coming back to this thread before now. I was away from home with only a tablet and while I could read your replies I could not post. Thanks for all of the suggestions. No actual, "use x size drill" but clearly it is a rare circumstance. I think I may need to do it by trial and error so may step drill it first. Pictures. The first is of a kerosene lamp called a Gloria 400, dating back to 1940s. Its big. Yes, its off topic but the expertise needed is exactly what you are all so good at. The nut I want to make can be seen at the place where the fuel supply comes out from the doughnut ring tank. The other pics are of the male withdrawn from the tank.
 

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TonyM

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The fitting in pic 3 looks exactly like a brake line fitting. That would be 3/8 - 24 thread for the larger size that fits to the tank. The nuts should be readily available from your local motor factor as would be the flared tube made to your original shape.
What size is the tube O/D ? 3/16 would also indicate brake line fittings.
The other point I would reiterate is that you would not use a taper thread with that type of flared tube.

I just checked and refrigerant uses 5/16 x 24 for 1/8 flared tube. Same for hydraulic.
 
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