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Whitworth v UNC

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Dr. MC Black17/11/2020 18:51:17
286 forum posts
1 photos

Ladies & Gentlemen

What's the difference between 3/8 x 16 BSW(Whitworth) and 3/8 x 16 UNC, please?

Are they interchangeable?

By that I mean will a 3/8 Whit thread screw into a 3/8 UNC threaded hole?

Very many thanks for taking the time to read this


Pete Rimmer17/11/2020 18:53:34
1219 forum posts
63 photos

The two threads have different flank angles. 55 degrees vs 60 degrees. They also have different profiles, Whitworth having rounded crests.

JasonB17/11/2020 18:53:37
22588 forum posts
2641 photos
1 articles

Whit is 55deg, UNC 60deg. you may get lucky if it's a slack thread or you may get it stuck

David George 117/11/2020 18:58:53
1808 forum posts
503 photos

Hi MC you could screw a UNC screw into a Whitworth hole but the thread angle is different 55 deg to 60 deg. But you can't do the reverse as the route angle foules but you could tap the hole Whitworth if it dosnt compromise the hole.


Edited By David George 1 on 17/11/2020 18:59:45

Howard Lewis17/11/2020 19:11:05
6024 forum posts
14 photos

Many years ago, our Chief Engineer would not have Unified threads, so vehicles with Unified threads soon became hybrid. It was standard practice to tap the securing holes for the compressed air brake unloader valve from UNC to BSW so that we could use his beloved Whit form fixings.

One problem that having been made to change, on delivery, all prop shaft bolts from UNF to BSF, after having discarded the UNF nyloc nuts, we had to drill the BSF bolts so that split pins could be fitted into his beloved castellated nuts.

Fortunately, his successor had more sense!


Stuart Bridger17/11/2020 19:40:25
538 forum posts
29 photos

In the UK aircraft industry, unified threads were seen as the spawn of the devil. Purely due to the potential for mismatching resulting in dangerously reduced strength. All aircraft grade fasteners with unified threads had to be specifically marked with a symbol with three circles.


Edited By Stuart Bridger on 17/11/2020 19:48:02

colin brannigan17/11/2020 20:02:53
108 forum posts
18 photos

Any Unified nuts made for MOD were marked with the circles above, the hex bar before issue to machine shop was pushed through a rolling die which marked one flat for the whole length, that was back in the 60's.

old mart17/11/2020 20:41:32
3721 forum posts
233 photos

I have a Pratt 6 3/4" three jaw chuck with serrated jaws, It has 1/4, 5/16 and 3/8 inch coarse threads, and as whitworth and UNC are similar in these sizes, I had difficulty identifying them.

Airbus use both Unified and metric in their aircraft. There should be no danger of mixing them up, as you don't get the fasteners out of a sack when working on aircraft. It was a cardinal sin committed when somebody put screws that were too short in that airliner windscreen.

Ian P17/11/2020 20:48:49
2578 forum posts
114 photos

3/8"x16 is a thread size that has become standard in the film and video industry (as has 1/4x20).

I know there is an ISO standard for the 1/4" thread (probably one too for the 3/8" so all camera and grip equipment have male and female threads that work with legacy UNC and Whitworth kit.

Must add though that these fixings are mostly hand tightened and not under great stress.

Ian P

Edited by punching that smiley up its bracket


Edited By Ian P on 17/11/2020 20:50:12

Bazyle17/11/2020 21:05:35
6297 forum posts
222 photos

In case of confusion in the above it may help someone to know that a SoutBend lathe chuck backplate (UN) does NOT screw onto a Boxford spindle (W) although they are the 'same' size.

Andrew Johnston17/11/2020 21:31:49
6575 forum posts
701 photos
Posted by old mart on 17/11/2020 20:41:32:

.............screws that were too short in that airliner windscreen.

The real problem was that 84 of the 90 bolts were 8-32 UNC rather than the correct 10-32 UNF, so were too small a diameter. The remaining 6 bolts were the correct 10-32 UNF but were 0.1" too short.


old mart17/11/2020 22:02:28
3721 forum posts
233 photos

I knew it was something like that with the windscreen. I would have expected that all new fasteners with the correct part number would have been issued for the job. I'm also surprised that 8-32 screws did not strip out of the 10-32 holes when they were torqued up.

Brian H17/11/2020 22:35:32
2312 forum posts
112 photos

The fasteners were not issued to the job, they were picked out by the maintenance fitter from two locations, one of which was so dark that he couldn't see what he was picking. At the main stores he also ignored the storeman who knew what the correct bolts were.

An absolute classic in how not to maintain aircraft (or anything else).


Dr. MC Black17/11/2020 23:53:13
286 forum posts
1 photos

There has been some interesting things posted BUT...

Back to my original question and clarification

I am thinking of buying some inexpensive 123 blocks here:

Link Removed see CofC

The description indicates that some of the holes are threaded 3/8 x 16 - but I can't see any indication if they are BSW or UNC

I have asked the local Fastener supplier if they stock machine screws in 3/8 BSW or UNC and await a reply

Buying a handful of screws by post would be uneconomic.

Very many thanks for all the hel and comments


Edited By JasonB on 18/11/2020 10:07:19

oldvelo18/11/2020 00:00:35
294 forum posts
56 photos

Good luck trying to put 1/2 inch U.N.C. nut on whitworth and vice versa

Dr. MC Black18/11/2020 00:04:07
286 forum posts
1 photos

It's 3/8 inch x 16 NOT 1/2 inch

I apologise for NOT writing clearly

Peter Greene 🇨🇦18/11/2020 01:17:39
491 forum posts
6 photos
Posted by Andrew Johnston on 17/11/2020 21:31:49:

The real problem was that 84 of the 90 bolts were 8-32 UNC rather than the correct 10-32 UNF, so were too small a diameter.

Did this come out of some kind of official investigation or is it simply internet "information"?

I commonly use UN hardware on this side of the pond and based on my experience, I was having real trouble imagining this so I just popped into the shop and checked. I'm still having real trouble imagining it.

An 8-32 screw in a 10-32 hole is as loose as the proverbial pigs' droppings. Wobbles around so much it feels likely to drop right through. Presumably the "fitter" who assembled the screws was at least slightly qualified for the job, in which case he couldn't fail to notice something was wrong IMO.

And since it's an aircraft assembly, was there no QC person in attendance?

peak418/11/2020 02:31:44
1676 forum posts
179 photos

It's all explained in the formal final report HERE

I particularly like section 4.7 of the safety recommendations near the end of the document.


Brian H18/11/2020 07:10:56
2312 forum posts
112 photos

Quote" And since it's an aircraft assembly, was there no QC person in attendance?"

The maintenance fitter was considered to be qualified hence no one else looking over his shoulder.


Nicholas Farr18/11/2020 08:29:50
3317 forum posts
1530 photos

Hi MC, my 1-2-3 blocks have 3/8" UNC threads in them, but 3/8" Whit bolts will screw into them OK, although they will wobble about slightly more than a UNC bolt does. I would say that for the most work that a home workshop is likely to do, a Whit bolt would probably be OK to use.

Incidentally, the almost complete circle with the figure 1 in the gap as seen in the photo below, I have only even seen on UNF and UNC nuts.


3/8" UNC threads will go tight into a normal 3/8" Whit nut.

Regards Nick.

Edited By Nicholas Farr on 18/11/2020 08:49:35

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