# What's slightly bigger than an 8-32 thread?



## Frank O (Dec 4, 2019)

Strictly speaking I suppose this isn't machining per se, but I'm sure there have to be people here who can answer my question.

I have a piece of gear made in the U.S. probably in the 1950s, which has several threaded posts for electrical connections. There's a corresponding nut on one of the posts, but the remaining posts lost theirs.

I took the remaining nut to the hardware store and tried it on their test threads. It seemed to be a good fit for 8-32, so I bought a packet of those and brought them home. However, when I tried to put one of the new nuts on the device's posts, they wouldn't fit.

So, is there a thread size slightly larger than 8-32 that's a good candidate to be the one I need? Close enough to 8-32 that I might have thought it was a fit? I seriously doubt this is metric, due to the vintage.


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## extropic (Dec 4, 2019)

Do you have a micrometer and a thread gauge?
What is the OD of the thread on the stud?
What is the thread pitch (TPI) on the stud?
Those are the questions you need to answer.
The thread in question may not be a current UNC or UNF. It might be an obsolete standard or even proprietary.

To answer the question you asked: there are many threads "slightly larger" than 8-32, depending on what one considers "slightly".
Slightly is not a well defined unit of measure.


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## Aaron_W (Dec 4, 2019)

Machinery's Handbook shows a "fine thread" 8-36, which uses the same #29 drill, so would probably be pretty hard to tell apart from an 8-32. The wrong thread would certainly make it hard to screw a nut on there.


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## higgite (Dec 4, 2019)

8-36 max diameters are "slightly larger" than 8-32 max diameters.

Tom


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## tq60 (Dec 4, 2019)

Meterc?

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## gr8legs (Dec 4, 2019)

Did you consider that it may actually be an 8-32 and the ends of the threaded studs you are trying to put the new nuts onto may have been peened to retain the original nuts?  That was (and still is) pretty common with electrical fittings and the stud ends being a bit buggered make putting new nuts on a bit of a challenge. But it certainly can be done -

Another shot in the dark - YMMV

Stu


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## RJSakowski (Dec 4, 2019)

It may be a 9-32 thread.  I have never seen one but I have had the same problem with 5-40 vs. 4-40 threads.  A 9-32 nut would fit an 8-32 screw (loosely).  Many of the electrical studs that I have run across are 10-32 but that would be a very loose fit on an 8-32 screw.  9-32 is definitely not a standard size.



			https://www.mscdirect.com/product/details/03931060
		

 edit: strike erroneous link


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## markba633csi (Dec 4, 2019)

I suspect 8-36 as well
Another bit of trivia: It's a very close match to metric 4mm also


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## RobertB (Dec 4, 2019)

RJSakowski said:


> 9-32 is definitely not a standard size.
> 
> 
> https://www.mscdirect.com/product/details/03931060



RJ that die is actually 9/32 - 32, not #9-32


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## RobertB (Dec 4, 2019)

gr8legs said:


> Did you consider that it may actually be an 8-32 and the ends of the threaded studs you are trying to put the new nuts onto may have been peened to retain the original nuts?  That was (and still is) pretty common with electrical fittings and the stud ends being a bit buggered make putting new nuts on a bit of a challenge. But it certainly can be done -
> 
> Another shot in the dark - YMMV
> 
> Stu


If the old nut felt snug as you were removing it this would be the most likely reason your new nuts feel like they don't fit don't fit. Best thing you can do is measure the diameter of your studs down close to their base, past where they would be peened.


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## pstemari (Dec 4, 2019)

I would definitely try an M4 and an 8-36. Have you mic'ed the stuff to get a good major diameter and/or measured the thread pitch?

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## kb58 (Dec 4, 2019)

extropic said:


> Slightly is not a well defined unit of measure.


Beat me to it.

The idea that the ends may be peened seems pretty likely. As recommended, measure the OD and thread away from that area to see what it really is. Hard to believe that a US mfg (even in the 1950s) would use a non-standard thread for that application.


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## Superburban (Dec 4, 2019)

The diameter difference between the 8-32, and the 10-32 screws are only 0.03"






						Bolt Depot - Machine Screw Diameter Table
					






					www.boltdepot.com


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## RobertB (Dec 4, 2019)

Superburban said:


> The diameter difference between the 8-32, and the 10-32 screws are only 0.03"



In context, that is a significant difference in size. A 10-32 nut will just about slide on to an 8-32 stud with no thread engagement.
Note that the OP stated the nut he had was a good fit on the store's thread checker.  This also pretty much rules out M4 and 8-36, neither of which will thread on to an 8-32 stud without force and thread damage.


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## ddickey (Dec 4, 2019)

FYI times by .013 and add .060. That how you figure out the diameter of numbered screws.


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## Tozguy (Dec 4, 2019)

Use a thread gauge to confirm that it is a 32 t.p.i. thread. Then run a 8-32 die on the posts that are too big. Your new nuts should work now.


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## RJSakowski (Dec 4, 2019)

RobertB said:


> RJ that die is actually 9/32 - 32, not #9-32


Thanks Robert  My mistake! I need new glasses or more coffee.  
MSC does sell a #9-36 tap though.  However, I could find no reference to a #9-32 thread.  It is possible it could have been a proprietary thread by the manufacturer.


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## Frank O (Dec 4, 2019)

Superburban said:


> The diameter difference between the 8-32, and the 10-32 screws are only 0.03"



Thanks for everyone's comments. The quick answer is the mystery thread on my vintage device is 10-32.

Looking back at it now, I wouldn't call a 10-32 nut on an 8-32 stud a "good" fit. When I tried this again, the nut I tested does thread rather than slide, but it's pretty wobbly. By comparison, when I put the mystery nut on a 10-32 stud it's a very solid fit. And vice-versa for a 10-32 nut on the mystery stud.

In any case I appreciate the posts, and picked up a few ideas. BTW I do have several mikes and a thread pitch gauge, but what came in the handiest was when I was digging around in drawers I found a pair of inch and metric sets of "Thread Detective" testers. This allowed me to verify 10-32 and rule out at least the other common nearby sizes.


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## extropic (Dec 5, 2019)

Frank O said:


> Thanks for everyone's comments. The quick answer is the mystery thread on my vintage device is 10-32.
> 
> Looking back at it now, I wouldn't call a 10-32 nut on an 8-32 stud a "good" fit. When I tried this again, the nut I tested does thread rather than slide, but it's pretty wobbly. By comparison, when I put the mystery nut on a 10-32 stud it's a very solid fit. And vice-versa for a 10-32 nut on the mystery stud.
> 
> In any case I appreciate the posts, and picked up a few ideas. BTW I do have several mikes and a thread pitch gauge, but what came in the handiest was when I was digging around in drawers I found a pair of inch and metric sets of "Thread Detective" testers. This allowed me to verify 10-32 and rule out at least the other common nearby sizes.



Isn't the internet wonderful! We have access to all sorts of fresh and interesting challenges.


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## brino (Dec 5, 2019)

Frank O said:


> what came in the handiest was when I was digging around in drawers I found a pair of inch and metric sets of "Thread Detective" testers.



Hey, what a coincidence,  I just got delivery of some more of those today!

I initially picked-up the "Standard Combination in/mm" kit from KBCTools: https://www.kbctools.ca/itemdetail/1-842-405

I found that quite innovative and handy, so started looking for the other kits.
Their website is here: http://threaddetective.com/
(look under Products - Thread Identifiers)
They only want to sell in quatities directly, however they have two US suppliers.

I just got the mini and the jumbo combination in/mm sets delivered today from 
https://www.pegasusautoracing.com/group.asp?GroupID=DETECTIVE

No connection to them except a happy customer!

-brino


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## Superburban (Dec 5, 2019)

RobertB said:


> In context, that is a significant difference in size. A 10-32 nut will just about slide on to an 8-32 stud with no thread engagement.
> Note that the OP stated the nut he had was a good fit on the store's thread checker.  This also pretty much rules out M4 and 8-36, neither of which will thread on to an 8-32 stud without force and thread damage.


Key words, just about. I have seen many mix up the 8-32, and 10-32. The displays at the stores usually have the coarse SAE, and metric, another display down the isle may have fine threads, but when only coarse threads are avilable, the 10-32 nut on an 8-32 sample stud will look to be the best choice, and is easily mistaken, since there is no better choice handy.


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## higgite (Dec 5, 2019)

brino said:


> Hey, what a coincidence,  I just got delivery of some more of those today!
> 
> I initially picked-up the "Standard Combination in/mm" kit from KBCTools: https://www.kbctools.ca/itemdetail/1-842-405
> 
> ...


I got the same type thread checkers in SAE/inch and metric 2 or 3 years ago. Wish I had them 20 years ago. I found them awkward to use on the strings that they came on, so I cut the strings and keep them loose in 2 small boxes, 1 inch and 1 metric. They're super handy for testing threads as you cut them on the lathe and for determining the screw or bolt size needed to bolt a whatsit to the existing mystery hole in a thatsit.

Tom


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## MrWhoopee (Dec 5, 2019)

I'd never heard them called Thread Detectives, but we had a set. As higgite noted, they can be hard to use all strung together, particularly when checking a tapped hole in a part that is too big to spin. I'm not sure they would be accurate enough for testing threads you are cutting.


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## higgite (Dec 6, 2019)

MrWhoopee said:


> I'd never heard them called Thread Detectives, but we had a set. As higgite noted, they can be hard to use all strung together, particularly when checking a tapped hole in a part that is too big to spin. I'm not sure they would be accurate enough for testing threads you are cutting.


Why would they be any less accurate than using a nut to test fit lathe cut threads? Maybe not accurate for testing Class 1 threads, but I've had no problem using them to test fit "standard off the shelf" UNC, UNF  or metric threads. At least, not yet. 

Tom


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## MrWhoopee (Dec 6, 2019)

higgite said:


> Why would they be any less accurate than using a nut to test fit lathe cut threads? Maybe not accurate for testing Class 1 threads, but I've had no problem using them to test fit "standard off the shelf" UNC, UNF  or metric threads. At least, not yet.
> 
> Tom



I don't think they would be any less accurate than a standard (Class 2B) nut, which is a relatively loose fit for commercial fasteners. Using an off-the-shelf nut creates the possibility of threads that will fit the nut but not fit a tapped hole, which will generally be Class 3B. If you are threading to fit commercially produced parts, it should be no problem as long as the fit not too tight. In my shop we made a set of test nuts rather than use commercial nuts to avoid problems.


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## Frank O (Dec 12, 2019)

MrWhoopee said:


> Thread Detectives ... As @higgite noted, they can be hard to use all strung together, particularly when checking a tapped hole in a part that is too big to spin.



Good point. I work a lot with electronics gear, and wanted to check the diameter of some terminal studs on a large and heavy power supply sitting on a bottom shelf behind a lot of other gear. Getting the necklace-style Thread Detectives down there and finding enough room to rotate them quickly became a no-go. So I ended up trying some nuts, and verified my eyeball impression that they looked like 1/4-20. Per @higgite, cutting the Thread Detective test pieces loose is probably how I'll end up using them.


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