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Nuts and bolts to the uninitiated

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heh101353
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Nuts and bolts to the uninitiated

Post by heh101353 »

Hi everybody

As I’m pretty much illiterate when it comes to nuts and bolts in the world of imperial measurements I’d like to ask for a bit of “enlightenment” on the subject.
In the SP250 spare parts catalog you may find part numbers like UFS.137/4R. Has to be a bolt or a screw, but what exactly was a mystery to me.
Poking around on the internet I could deduce the meaning of this notation.

Let me take this apart to clarify:

UFS.137/4R
==============
UF = U.S. Standard fine thread
S = Setscrew (threaded right up to underside of the head)
137 = 3/8” diameter
4 = 4 increments of 1/8” ==> 1/2” length
R = right hand thread

UFB.137/4R would be the equivalent of the above, but a bolt (partially threaded)
UCS = U.S. Standard coarse thread, setscrew
UCB = U.S. Standard coarse thread, bolt

So far so good.
What I’m still struggling with is that fine number in the middle which stands for the diameter. In the example above “137”.
By pure luck I stumbled across a piece of information which listed: 137 = 3/8”, 125 = 1/4”, 131 = 5/16”.
But there are quite a few more out there like 150, 362, 419, …
As I’m not a mechanical engineer, I’m at a loss , but I’m always willing to learn.

My questions:
Is there a way, a rule, some math, whatever to get from 125 to 1/4”, from 137 to 3/8”, from 131 to 5/16”, from 150 to ??, …?
Is there a simple lookup table somewhere?

All comments are highly appreciated.

Thanks a lot
Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

Vortex O'Plinth
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Re: Nuts and bolts to the uninitiated

Post by Vortex O'Plinth »

To determine the thread diameter from the bolt or screw part number, it's useful to ignore the initial figure, for example '1' in the designation. If you do this and then place a decimal point before the remainder the number now represents the first two figures of the thread nominal diameter in inches.

Thus 125 becomes .25" or 1/4" nominal, 137 becomes .37" or 3/8" nominal, 150 becomes .50" or 1/2" nominal and so on. I believe the initial figure in the designation may represent the class of thread but perhaps someone else can confirm this.

Your example of 419 may appear to be an oddball but is just a feature of the Unified thread system. One might think that the next standard thread size below 1/4" would be 3/16" in which case the designation should be 418. However, below 1/4" diameter the american National and the Unified National threads are designated by a number; 10,8,6,5,4,2,1and 0 are the standard sizes. The nominal diameter of the number 10 thread is .19", so ignoring the initial 4, the designation 419 refers to a No. 10 bolt or screw.
Nick

"Don't bother with the Air & Space Museum - there's nothing to see.......".

Dragon
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Location: Goodrich,between Ross-on-Wye and Monmouth, UK.

Re: Nuts and bolts to the uninitiated

Post by Dragon »

Hi Hans,
If you would like, I could e-mail to you a written piece about the threads used in the SP250.
Please e-mail me so I have your e-mail address; you will find me in PasSPort in Herefordshire.
Wilf,
SP250 104008, Herefordshire.

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heh101353
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Re: Nuts and bolts to the uninitiated

Post by heh101353 »

@ Nick, thanks a lot for the exhaustive reply.
@ Wilf e-mail is on its way.

Thanks again
Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

Ian Slade
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Re: Nuts and bolts to the uninitiated

Post by Ian Slade »

It's simple if you had a proper apprenticeship from the 60's 1/4 inch as previously stated, though why the 125 I'm not sure, the second two numbers are the diameter to two decimal places, so 137 is .375" or 3/8" and so on. Do not confuse UN or Unified with Imperial, it's not, Imperial is Whitworth for coarse, BSF or British Standard Fine and BA British Associated for very small bolts or rather screws far to small to be called a bolt, there are one or two BA nuts and screws on the SP mainly on the electrics. With UN sizes the head is measured across the flats for both UNF and UNC in inches, this is not the bolt or nut diameter, nor is it American Fread :D . we won't go into Metric yet, the SP has no metric threads though some UNC threads will take a metric bolt if tapped to the metric equivalent due to only the thread angle being slightly different
Owner since the 70's, Genghis is slightly to my left.

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heh101353
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Location: Basel, Switzerland
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Re: Nuts and bolts to the uninitiated

Post by heh101353 »

Ian Thanks for pointing out, to not confuse UN with Imperial. As I said I'm pretty much illiterate in this field. So everything that sets me straight is welcome!
Thanks Ian and thanks again previous respondents
Kind regards
Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

David S
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Re: Nuts and bolts to the uninitiated

Post by David S »

Ian,
with regard to your closing comment:
".......... some UNC threads will take a metric bolt if tapped to the metric equivalent due to only the thread angle being slightly different"
I think you refer to Whitworth threads having the same diameter and pitch as UNC threads but with a different thread angle [Whit & BSF are 55 deg. UNF & UNC are 60 deg.].
If you re-cut a Whitworth nut to increase its thread angle to 60 degrees and make it fit a UNC screw/bolt you would need another spanner for the nut in your box.
If you use Whitworth bolts or screws this would generally be ok, especially if you use allen head screws [or Cap screws] as they are normally made with stronger steel [steel strength is another can of worms, letters for imperial and numbers for metric] which would compensate for the 55 deg thread angle and then use std. A/F allen keys to fasten.
And, just for interest, the length of one flat on a Whitworth or BSF nut was designed to be the same as its thread's diameter. However during WW2 they changed the hex size to be one size samller to save weight and metal. This is why whitworth spanners are labelled with two sizes [e.g. 1/4 W & 3/16 BS].

David

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