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Thread: Unf, Bsf, Unc?

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    seph's Avatar
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    Unf, Bsf, Unc?

    What threads are used on series 2/3 jags?

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    Clive's Avatar
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    Hi Seph,
    BSF (British Standard Fine) was made obsolete just after WWII, so I don't think that should concern you. UNF & UNC are the standard imperial threads that you will come across. UNF (United Fine) is a fine thread usually used when a nut and bolt are required and UNC (United Coarse) is a coarse thread usually used when a component is bolted into a tapped hole.UNC will normally take a higher torque.
    Incidentally UNC and the older Whitworth are interchangeable up to under 1/2" (3/16" - 7/16") qafter which they aren't.
    3/16" UNF is the same as 3/16" BSF and 2BA.
    Cheers, Clive

    (If I'm not here, I'm either in my new workshop or on the golf course!)

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    UNF = Unified National Fine
    UNC = Unified National Course
    UNS = Unified National Special

    BSW = British Standard Whitworth
    BSF = British Standard Fine

    UN and Metric are both 60degree pitch
    Whitworth ranges are 55degree pitch

    (but i'm sure thats what Clive meant to mention )

    Inrespect to the question, i would be inclined to use BSW or BSF for jag up until 1983, but don't quote me on that

    Si
    Dax De Dion c/w Chevy 383 - Scaring old ladies, animals and children with the popping from my exhaust

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    mylesdw is offline Chief Engineer
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    Quote Originally Posted by Clive View Post
    Hi Seph,
    Incidentally UNC and the older Whitworth are interchangeable up to under 1/2" (3/16" - 7/16") qafter which they aren't.
    Not really, they're a different thread form. WW is 55 degrees with rounded profile, UN is 60 degrees with flat tops/bottoms.
    Cheers

    Myles D-W

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    Sparticus's Avatar
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    actually UN does not to have flat peaks and troughs in its pitches its just a different angle (you're thinking about ACME or STACME threadform). I think what Clive is saying is that you BSW will fit UNC till a certain size but it isn't an EXACT fit, its close but not exact.

    best thing for you to do is get a thread pitch guage and check the sizes exactly. they only cost a few quid
    Dax De Dion c/w Chevy 383 - Scaring old ladies, animals and children with the popping from my exhaust

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    Purple AK's Avatar
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    Quote Originally Posted by seph View Post
    What threads are used on series 2/3 jags?
    Mostly UNF, But some are Metric Fine (steering arm bolts for instance)
    Chris
    AK427 351Cleveland T5WC

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    andicole0's Avatar
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    If I ever build another kit it's going to be metric, come hell or high water

    Andi.
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    Dax Tojeiro, Chevy 350, Tremec TK05, 3.54 Powerloc diff. MoT'd, SVA'd, registered, taxed and insured. Now with RAC cover, just in case
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    Neil O's Avatar
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    Isn't the front end of the Jag suspension/brakes metric and the rear imperial.....or vise versa?
    Thought I'd heard this somewhere before?
    2003 Dave Brookes Dax 350ci Chevy SB.
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    stu e's Avatar
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    Rover V8 is Imperial LT77 gearbox bolted to it Metric.

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    Sorry to be an anorak here, but its easy to make some dangerous combinations with incorrect but 'fitting' fasteners, and there are a couple of common misconceptions on this thread.

    Thread pitch is the distance between thread forms, it is not not the thread angle.


    BA threads should not be mixed with UNF or BSF as the thread forms just dont match; yes they will screw into each other, but they are weak, and are likely to either strip or jam if the correct torque is applied to them.

    BA (British Association) 16BA to 0BA have a 47.5 degree thread angle; you'll find these fastenings on equipment from 1884 on (mainly small electrical items)

    BSW British Standard Whitworth 1/16 to 2.5 inch have a 55 degree thread angle you'll find these fastenings on equipment from 1841 on, BSW starting out with general machinery use, until BSF was created in 1908 after which BSW became the accepted norm that BSW threads were used when tapping into soft materials such as aluminium castings.

    BSF British Standard Fine 3/16 to 1 inch have a 55 degree thread angle you'll find these fastenings on equipment from 1908 on, they can take higher torques that BSW, and the finer thread pitch makes BSF more useful in applications where there is significant vibration

    UNC Unified National Coarse 0.05 to 1 inch have a 60 degree thread angle you'll find these fastenings on equipment from 1918 on, application as, but not interchangeable with, BSW.

    UNF Unified National Fine 0.05 to 1 inch have a 60 degree thread angle you'll find these fastenings on equipment from 1918 on, application as, but not interchangeable with, BSF.

    Regarding torque values, its a very common misconception that a BSW fastener can take a higher torque than the same dia BSF; it cant. The BSW bolt is weaker due to it having a smaller minor diameter. The BSF bolt can however take a higher torque into a cast material such as aluminium where the tapped material is weaker and the larger thread area of BSW comes into its own.
    For a grade A bolt into a plain nut you can apply more torque to a BSF than BSW, and more torque to a UNF than a UNC.

    God bless Joseph Whitworth, we'd be in a right pickle without his system!!
    Regards Steve

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