Determining unknown thread

[BSaggie]

I am trying to determine the thread size of this screw. All I have is a cheap pair of digital calipers. I measured the diameter of the threaded portion as 0.103” and the smooth portion as 0.0815”. The overall length is 0.55”. I’m pretty sure it is not metric.

Thanks

 

[FamilyTradition]

What did this screw come out of? Might help in identifying if we can locate specs for what it was made for.

As for making an educated guess, looks like you have 6 threads over about 11/64 of length, so that works out to approximately .0286 pitch, 36 TPI has a pitch of .0278, although .7MM is closer to that, at .0275 between threads.

#4-36 NS is around that major diameter, according to the table I have. I don't know if I have metric standards kicking around anywhere I could look up.

 

[L Vanice]

Thread pitch gages are pretty cheap if you go with Chinese made or a used one. But any handful of random size US screws and bolts can be used as pitch gages. Common machine screw sizes like 10-24, 10-32, 4-36 and 4-40 can be laid against your sample screw to identify the unknown pitch. A hardware store might have some 4-36 and other size nuts that you could try on your screw. Or get a cheap set of taps and dies, which may include a pitch gage and some dies you can try on your screw.

Larry

 

[EPAIII]

If it is an English measure (inch) thread then your 0.103" diameter probably means it is a #4 screw size. I also doubt it is metric as that translates to 2.616 mm which is half way between an M2 and an M3. While there is an M2.6 size, real world screws, especially one as poorly made as that one, are almost always smaller than the nominal size. Yours is greater than the 2.6mm size so it really can not be that.

That leaves the question of the pitch. A #4 coarse thread would be 40 TPI while a #4 fine thread would be 48 TPI.

Edit: I have to disagree with EducatedTradition's guess at the thread pitch. I see 4 threads over 6.5 64ths. With just a little math that comes to 39.4 TPI so I will guess it is a 4-40.

You do not have a location in your information so I can only assume/guess and I guess USA. A well stocked hardware store, like Ace Hardware, will have a variety of threads to check it against. Even poorly stocked hardware stores will have 4-40s you can check it against. If the nut fits without too much slop, that's it.

That hardware store will probably also have one or more thread pitch gauges. Ask about them.

 

[Luke Rickert]

A thread gauge is the obvious solution although if you are careful a photo with a reference can be used to calculate the pitch using digimizer (30 day demo version is free) or similar software.

 

[Scottl]

Thread pitch gages are pretty cheap if you go with Chinese made or a used one. But any handful of random size US screws and bolts can be used as pitch gages. Common machine screw sizes like 10-24, 10-32, 4-36 and 4-40 can be laid against your sample screw to identify the unknown pitch. A hardware store might have some 4-36 and other size nuts that you could try on your screw. Or get a cheap set of taps and dies, which may include a pitch gage and some dies you can try on your screw.

Larry

Yes, many times when away from a shop I've used known screws as pitch gauges. You hold both while facing a light colored wall or other bright background and try to "Mesh" the threads. An identical pitch will fit tightly, letting little light show through.

 

[BSaggie]

Since someone said this info might be helpful, This screw is a retaining screw from an upper Koch fitting used on a US Navy MA-2 torso harness. It’s a quick connect that attaches the pilot to the parachute on the ejection seat. My pair of fittings are used surplus parts and the threads on the screw from the other fitting are bad and it won’t screw in all the way. I assumed this was imperial thread because this is a surplus US military part. This fittings are hard to come by as most are recycled when a harness is retired. I have no idea where to find a replacement retaining screw so was thinking about alternatives.

 

[WizardOfBoz]

It looks like the crests and roots are rounded. Is that true?

 

[Joe Henderson]

You can check it with a Brownell's "Screw checker" or a 4-40 die if you have one. Where you're at there are lots of places you can go to get that checked.

 

[BSaggie]

It looks like the crests and roots are rounded. Is that true?

That was my first thought too when I first got a chance to zoom in. I have no idea if that’s by design or just poor workmanship

 

[DDoug]

Since someone said this info might be helpful, This screw is a retaining screw from an upper Koch fitting used on a US Navy MA-2 torso harness. It’s a quick connect that attaches the pilot to the parachute on the ejection seat. My pair of fittings are used surplus parts and the threads on the screw from the other fitting are bad and it won’t screw in all the way. I assumed this was imperial thread because this is a surplus US military part. This fittings are hard to come by as most are recycled when a harness is retired. I have no idea where to find a replacement retaining screw so was thinking about alternatives.

You-Would-Think....that this should have been included in your original post.....

 

[BSaggie]

You-Would-Think....that this should have been included in your original post.....

You don’t need to be rude. I didn’t really think it was relevant considering how obscure this part is and would have made the original post a lot longer. I thought the odds of someone being a Navy rigger or an employee of Koch & Sons were pretty remote and so was looking for general machinist expertise rather than first hand knowledge.

 

[CAMasochism]

You don’t need to be rude.

Nah, he pretty much has to. Birds fly, fish swim, Doug shits on people.

 

[FamilyTradition]

You don’t need to be rude. I didn’t really think it was relevant considering how obscure this part is and would have made the original post a lot longer. I thought the odds of someone being a Navy rigger or an employee of Koch & Sons were pretty remote and so was looking for general machinist expertise rather than first hand knowledge.

The thing is, you never know, sometimes those folks will come out through the woodwork when you need them, and they have all that info either in a book on their shelf, or stored away in their heads somewhere.

That information is very helpful, as I would second (or third, or fourth?) the recommendation that you go to your local hardware store and see if they have any screws that fit. If you can disconnect the fittings from the harness, even better - bring those and see what bolts fit in there. Hell, even if you have to bring the whole harness. The folks at my local hardware store have seen people bring all sorts of odd things into the store to try to find the right screw - engine parts, lawnmowers, appliances, you name it. But now you have a ballpark range of what you want to be looking for, at least. Good luck!

 

[Joe Henderson]

If that's been used over and over again those threads probably got rounded off from use.

 

[rons]

You don’t need to be rude. I didn’t really think it was relevant considering how obscure this part is and would have made the original post a lot longer. I thought the odds of someone being a Navy rigger or an employee of Koch & Sons were pretty remote and so was looking for general machinist expertise rather than first hand knowledge.

You ruined it for him. An entire bowl of Fruit Loops used up reading through your story.