# A 5-40 and 8-56 tap



## Brento (Apr 7, 2021)

First off i need to order a 5-40 tap for a few screw holes for a little toolbox i am restoring. What else is a 5-40 thread used for. Seems very oddball. Also what would you use a 8-56 thread for?


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## Braeden P (Apr 7, 2021)

Brento said:


> Also what would you use a 8-56 thread for?


to show off other than that I guess it is for aircraft I have a 11/16 20 tap wacko size for sure!


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## jwmelvin (Apr 7, 2021)

5-40 is used in the end of the Bridgeport “reverse trip ball lever” as a way of extracting it for disassembly. Though, (1) my Millport clone uses metric fasteners in the head assembly (imperial fasteners for the main machine structure); and (2) I couldn’t get mine out. But that’s a recent use of 5-40 I have run across.


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## T Bredehoft (Apr 7, 2021)

I use 5-40  1/8" set screws to hold clutches on propeller shafts of Rubber Band flying models. Shafts from .047 dia to .094 dia. Also to secure internal parts in torque meters for winding them. Also use 4-40 flat head scsrews to assemble the torque meters.


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## macardoso (Apr 7, 2021)

5-40 on a model steam engine I'm building. Full size 1/4HP unit. Holds the steam valve covers to the cylinder. Never used one before that, had a bit of a tricky time finding quality ones to buy.

Never used an 8-56 yet.


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## Brento (Apr 7, 2021)

Ill take pictures tonight. But i cant find much on the 8-56


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## Asm109 (Apr 7, 2021)

#5  major diameter is 1/8 inch. Only number size that falls on a fractional size. #10 is close.  You can pick up a piece of 1/8 rod and put threads on with a die.  No trying to turn down a small diameter.  Best reason I can find for choosing it.


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## Bi11Hudson (Apr 7, 2021)

5-40 is the fine thread. A 5-32 would be course thread. Also known as a 1/8" "stove bolt" colloqually. I haven't seen 1/8" fasteners (5-32) for a very long time. But at one time it was a fairly common screw size. A 6-32 is known to be difficult to thread for the depth of threads cutting into the diameter of the stock. The only reason it has hung on so long is the use on electrical devices. The thread should have been 40TPI (6-40) at that size. (in my opinion~) As it happens, a Nr 6 screw is within a couple of tenths of 3.5mm. Although rare, I have seen 3.5mm fasteners. With the "standardization" of screw sizes from WW2, machine screw sizes were stepped by 2 as a rule. (0,2,4,6,8,10,12) Nr 7 sheet metal screws are (were last year) still available. Further, I have some recent can (servo) motors threaded for 3-48.

8-56 is a very fine pitch, probably considered to be "special" thread. I have never crossed that particular thread, but it certainly is plausable.

FWIW, in the seventies I worked for an "end user" of Leeds & Northrup, an instrument house. I did many (most) repairs in-house. Round recorders and valve positioners were the majority of my work. Optical pyrometers were also a part of my concern. L&N (not the railroad) was a very old instrument house that worked closely with the NBS(now NIST) in setting and maintaining "standards". Most of my "test stands" were "NBS traceable", being only 2 steps removed from NBS. There were many odd (to me) sized fasteners. Most of the odd sized fasteners had been in use for a long time, some predating the second world war. 5-32 was one of those. Of course, I was in my 20s so didn't have a lot of experience.

.


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## markba633csi (Apr 7, 2021)

I've seen 5-40 used on some of the smaller sizes of those black phenolic electrical barrier terminal strips made by Jones and T&B
-Mark


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## Mitch Alsup (Apr 7, 2021)

I used to tap 5-40 into 4-40 holes that had been worn out but the rest of the part was still serviceable. R/C Race cars mostly.

And some scope mounts on Remington receivers use 5-40 threads.


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## WobblyHand (Apr 7, 2021)

Not 5-40, but an odd-ball thread, nonetheless.  I have an old (Navy) electric motor that is held together with 4  4 inch long 8-36 screws (and nuts). Not a hardware store item!  For that matter, not found anywhere else either.  Yes, it is _relatively_ easy to find a 1 inch long 8-36, but 4 inch long, nope!


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## hman (Apr 7, 2021)

@Brento - I seem to recall that there were various 56 pitch taps (including 8-56?) in the pass-around box this year.  If you need one, you might want to post an inquiry at https://www.hobby-machinist.com/threads/pass-around-2020.86254/page-20


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## Brento (Apr 7, 2021)

@hman i believe that is where i got that tap from honestly.


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## hman (Apr 7, 2021)

Heeee-Yaaaa!  Go figure.  I looked over the selection of those taps when I got the box, decided to grab a couple of 2-56, but passed on the other "special" sizes.  I guess it'd be interesting to hear from whoever put them in the box, to find out why he had them in the first place.  Should be quite a story.


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## Ulma Doctor (Apr 7, 2021)

8-56 is undoubtedly a special purpose thread
for the life of me, i can't understand why someone would choose 56tpi


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## Old Norseman (Apr 7, 2021)

Brento said:


> First off i need to order a 5-40 tap for a few screw holes for a little toolbox i am restoring. What else is a 5-40 thread used for. Seems very oddball. Also what would you use a 8-56 thread for?


I had a few odd fine threads on my small lathe which needed repair or retapping. Local prices in Australia for such taps was crazy and deliveries were worse. I found a specialist Chinese company who supply all manner of odd taps and dies at good prices. I purchased 2 as I was worried about the quality not they sell mostly plug taps and just touched one on a diamond wheel to create a taper tap. They worked well and the job is now complete and delivery was 2 weeks to Perth in WA. Let me know if you want a name as not sure if posting suppliers is allowed.


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## Brento (Apr 7, 2021)

I take any tap if i dont have it so i have it. Id love to have some 2-56 taps. I ordered from shars my 5-40 taps so now i will be ready to finish my project.


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## kopcicle (Apr 7, 2021)

8-56? What, no gunsmiths here ???


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## MrWhoopee (Apr 8, 2021)

Bi11Hudson said:


> A 6-32 is known to be difficult to thread for the depth of threads cutting into the diameter of the stock. The only reason it has hung on so long is the use on electrical devices. The thread should have been 40TPI (6-40) at that size. (in my opinion~) As it happens, a Nr 6 screw is within a couple of tenths of 3.5mm. Although rare, I have seen 3.5mm fasteners.
> 
> .


I was standing in the hardware aisle at Orchard Supply (RIP) trying to find M3.5x.8 socket head cap screws to replace the phillips head screw that failed to retain the distributor rotor on my Honda Civic. It took me a couple of minutes of mental math to realize that it was, in fact, a 6-32 that Japanese had simply translated to metric. Why they would have chosen that particular thread, regardless of what you call it, is beyond me.


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## RJSakowski (Apr 8, 2021)

In my experience, it seems that the 5-40 srew was developed solely to confuse the heck out of assemblers. To the casual ey, the difference between the 4-40 and 5-40 threads is minimal resulting frustration in trying to run a 5-40 screw into a 4-40 tapped hole.  We had an engineering company design a piece of equipment for us and for some unknown reason, they elected to mix 4-40 and 5-40 fasteners.  We didn't use that company again.


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## Braeden P (Apr 8, 2021)

i have 5 4 40 taps but no 5 40 taps for some reason


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## wlburton (Apr 8, 2021)

5-40 threads are used for various things on woodwind and brass musical instruments, along with 1-72, 1-64, 2-56, 3-48, 3-56, 4-48, and many other small threads--not to mention all of the small metric threads.  A person needs a lot of small taps and dies in order to repair those instruments.  I've never needed an 8-56, though!


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## pontiac428 (Apr 9, 2021)

Bi11Hudson said:


> . Further, I I have never crossed that particular thread, but it certainly is plausable.
> 
> .



Bill, tell me you knew you were fully intending this great pun when you wrote it!


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## Bi11Hudson (Apr 9, 2021)

Actually, Sir, I wasn't intending a pun. Just the way I express myself, which varies depending on how much sleep I haven't had. Taken quite literally, I have never crossed paths with an 8-56 thread. I believe it is used in gunsmithing though I am not in that particular hobby. Can't see well enough, had to put my guns in storage back in the early 2000s. From which they were stolen, as much a problem with the ATF as anything. Political rant deleted. . .

As a model (re)builder, I make frequent use of smaller sizes. From 000(3/0)-120 to 1/4-40 is my normal span. I run into larger pitches as well, such as 0-48 as opposed to 0-80 machine thread. They are not "sheet metal screws" but rather for use in softer solids such as aluminium. When used as "self tapping" screws, they set much faster and are somewhat easier for my old eyes to handle. And hold well enough in their specific application.

The first thing that comes to mind for an 8-56 thread is the (lack of) length. It could well be so short that an 8-32, the "normal" pitch would only hold by a single pitch, maybe 1-1/2. By using a finer pitch, there would be a greater number of threads, hence a more solid grip. That may well be the reason for a number of "custom" threads. 56TPI is a well documented pitch in the sequence that steps by 8 TPI. (32, 40, 48, 56, 64, 72, 80, etc)

Another would be an adjustment screw, such as the case for 1/4-40. One rotation being 0.025 inch, similar to a common micrometer. For smaller sizes where such an exact movement per revolution is not necessary, odd sizes often show up in older electro-mechanical instruments. Having worked with Leeds & Northrup on older equipment (in the '70s, I frequently saw date codes from the 1930s and 1940s) such fasteners adjustment screws were commonplace. This, of course, was in the BC(before computers) days, when measurements were literally measured, rather than "interpolated" from silicon.

A simple example for larger sizes would be the gas hoses on torch fittings. One, I believe, is 5/8-36. Or was that hydraulic fittings? In any case, finer threads show up in a number of places.

Bill Hudson​
.


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## hman (Apr 9, 2021)

@Bi11Hudson -
Reading your post, I got a sudden wild hair ... if 56TPI were close to some "even" metric dimension, it could be used in some kind of a metric measurement tool.  But no joy.  1/56" is about 0.45357mm.  Not a useful number.  Alas!

PS I recall there was aso a 5/16-56 tap amongst the others in the pass-around.  If 8-56 is odd, 5/16-56 would qualify as even odder!


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## whitmore (Apr 9, 2021)

Brento said:


> First off i need to order a 5-40 tap for a few screw holes for a little toolbox i am restoring. What else is a 5-40 thread used for. Seems very oddball. Also what would you use a 8-56 thread for?


I've  used 5-40 lefthand on fine-bore hydraulic tubing; the "standard size" 1/8" OD tube 
is fitted with a threaded-on collar, and when the clamping nut is applied, either
the nut slips against the collar (and RH turning the nut tightens the tube-to-fitting contact
point), or the collar slips against the tube (and RH turning the nut tightens the tube-to-fitting
contact point).   The 316 SS tube  thus deforms where it enters its fitting, and that makes a gasket
substitute good for ~50 ksi fluid.
HIP fitting details

Weird system, but it works.   The "5" gage is exactly 0.125", sometimes the label will say 1/8 instead.


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