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RequiredOutside Diameter to Cut 5/16 BSF Thread

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James Alford23/06/2019 13:49:36
361 forum posts
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I am making a stepped stud for the water manifold on an Austin Seven and wish to cut a 5/16 BSF thread on one end. I can find charts showing the size of hole to drill to tap a thread, but nothing for the diameter for studs and bolts. Would it be the same as the tapping diameter or something else?



blowlamp23/06/2019 13:53:30
1218 forum posts
82 photos

0.3125" or 5/16"


JasonB23/06/2019 13:54:05
16870 forum posts
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5/16" or 0.3125"

larry phelan 123/06/2019 13:54:38
553 forum posts
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Would it not be 5/16" ?

Just asking.

James Alford23/06/2019 13:58:31
361 forum posts
72 photos

Thank you both.

So, showing my utter ignorance: is the outside diameter always the same as thread size eg a 7/16 BSF thread would need a 7/16" shaft?



Brian Sweeting23/06/2019 14:00:23
392 forum posts
1 photos

Sometimes known as the major diameter IIRC.

Table here **LINK**

JasonB23/06/2019 14:01:37
16870 forum posts
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In most cases it is but you also get threads that are specified by number eg 2BA, 10-32 UNC both of which are approx 3/16" diameter. Pipe threads are also different in that they tend to go by the bore, for example 1/4BSP has a OD of 0.518".

James Alford23/06/2019 14:23:14
361 forum posts
72 photos

Thank you.

Paul Kemp23/06/2019 14:26:02
361 forum posts
18 photos


All above answers correct. Some other considerations for you though depending on how you plan to cut the thread. If you measure commercially made fastenings depending on the tolerance and thread engagement you will find they have a major diameter slightly smaller than nominal.

if you are going to screw cut then starting with the full nominal is a good idea as your depth will be related to that. If you are going to use a split circular die then being a few thou under the nominal won't hurt as there will be a degree of extrusion in the die cutting process and it will be easier to cut.


old mart23/06/2019 17:11:09
962 forum posts
104 photos

There is a site with some of the most common thread charts :

James Alford23/06/2019 20:10:28
361 forum posts
72 photos

Thank you for the information. I have turned the shaft down and tapered the end a little to ease the die. The die is now cutting nicely.



Hopper24/06/2019 04:05:04
3888 forum posts
83 photos

In the commercial machine shop world it is common practice to make the OD about five thou under the nominal size, so somewhere around .307" (in practice, about .305" will do).

This provides a bit of tip clearance on the threads but does not reduce thread strength measurably. You die will thread the job much easier. With screwcut threads, parts will fit together better without riding on the thread tips.

Larger threads are usually made ten thou or more undersize.

You only need 65 per cent thread depth engaging to have about 95 per cent strength of a full depth thread so there is no danger at all in going a few thou undersize..

JasonB24/06/2019 06:58:42
16870 forum posts
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I can't see that making any commercial sense. If a jobbing shop got an order to make say 1000 studs it would just about double the cost if they first had to turn down each end rather than going straight at them with a die head or even a die.

Also beware of generalisations of" 5thou less". What if you were cutting 40tpi, by the time you have taken 5thou off that's almost 20% thread depth and then you start doing the same with the tapping drill you won't have much % engagement.

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