20 TPI threads

Can anyone tell me the major difference if any between 7/16" 20 unf and 7/16" 20 bcy, I know they are both 60°. I need to find a left hand bolt / stud for my BSA left hand footrest. The stud I require doesn't seem to be available.

Regards Mark P.

You would have to closely compare the specifications, the differences would be the rads at the top and root of the threads. There is not much difference in practice, and as long as the nut is a decent length, say 1.5 diameters and not loose on the male, it should be fine.

Is this what you're looking for?

BSA A50/A65 LH Footrest Stud

or

Unit Singles

or

Draganfly Motorcycles

Edited By Nick Hughes on 06/11/2022 15:47:03

Nick Hughes, I bought one of those but it's wrong for my frame I need a plain 7/16" left hand stud. Unfortunately as it's an unused frame I don't know the year as it's never been stamped.

Yes Old Mart that was my thinking too, for what I need unf should be fine.

Regards Mark P.

The last one is a plain LH Threaded stud, for the early Plunger A series, as shown in this diagram:-

**LINK**

The most obvious difference to me is the thread shapes are different. Diameter, TPI, thread depth and angle all seem compatible, but UNF has squared off peaks and valleys whilst BSC threads are rounded top and bottom. This makes them somewhat stronger and less likely to vibrate loose.

However, I think a close tolerance male BSC would be too tight to fit into a close tolerance UNF female. Never tried it but I guess a UNF male will fit into a BSC female. There's a degree of uncertainty, and I don't think anyone can guarantee a fit. Though if failed, running a cheap tap and die over the partners would fix it - judging by the spec very little metal needs to be removed.

I'm a bit nervous of this sort of compromise on vehicles because of the risk stuff will come apart at 90mph and cause a crash! Much safer to source the correct part. But a footrest doesn't seem safety critical at all. If UNF fits, even as after a bodge, why not?

Dave

Hi Mark , I have a BSA A10 and all the bcy nuts and bolts were 26tpi. I have just checked on Google and there isn’t a 7/16 20 tpi listed . I am not criticising ,but please check again.

Maurice

Maurice Taylor, it's definitely 7/16" 20. I'm now sorted with what I need.

Mark P.

Edited By Mark P. on 06/11/2022 16:26:04

Basically BSC is to whitworth specifications re written for 60° flank angle.

According "Guide to World Screw Threads" by PA Sidders

Triangular height (H) = pitch x 0.8660254

Shortening is symmetrical at pitch divided by 6, = pitch x 0.1666667

Radius of crest and root = pitch x 0.166666

Depth of thread (h) = pitch x 0.5326921

There is no truncated form.

For 7/16 " 26 tpi BSC depth is near enough 0.021" (21 thou)

Major Diameter is 0.4375",

Effective Diameter 0.4170",

Minor Diameter 0.3956".

According to Sidders tolerances are quite wide at near enough 0 to- 0.006" (6 thou" ) on screws for medium fit and 0 to+ 0.0117" on nut minor diameter. Which seems way to great. I suspect a mis-print

The major difference between UNF and BSC is that the UN thread is flat topped and the shortening assymetric, H/8 outside for a male thread as against H/6

UNF data for male thread

Triangular height (H) = pitch x 0.86603"

Height of external thread = 17/24 x H = pitch x 0.61343

Height of internal thread = 5/8 x H = pitch x 0.54127

Shortening :-

At crest of screw = 1/8 H = pitch x 0.10825 at root of screw = 1/6 H = pitch x 0.14434

At crest of nut = 1/4 H = pitch x 0.21651 at root of nut = 1/8 H = pitch x 0.10825

For 7/16 " 26 tpi UNF depth is near enough 0.021" (21 thou)

Major Diameter :- screw = 0.4375", nut = 0.4375"

Effective Diameter :- screw = 0.4050", nut = 0.4050"

Minor Diameter :- screw = 0.3762", nut = 0.3834"

Sidders gives tolerances without ± indications for the Unified 20 thread series.

Screw :- major diameter 0.0081" (8.1 thou}, effective diameter = 0.0042" (4.2 thou)

Nut :- effective diameter 0.0054" (5.4 thou}, minor diameter = 0.0115" (11.5 thou)

Still seem quite wide

Plenty of room for tolerance stack up to give you a very poor fit if you use a UN form thread.

Clive

Edited By Clive Foster on 06/11/2022 16:33:54

I nearly fell into the same trap. Although several tables on the web and one of my books suggest BSC threads are all 26tpi, and these are most common on bikes, the standard also has some 20tpi too. Apparently 20tpi was inherited from the earlier CEI standard, and we're still using them!

Dave

Hi Dave, I thought they were all 26tpi ,I have looked again on Google and found 20 tpi bcy bolts.

Thanks for your post ,I’ve learned something new .

Maurice

I seem to remember that as originally devised, cycle threads had two standards, 26 tpi and 20 tpi but the 20 tpi thread was abandoned in 1935. Despite this the 20 tpi thread was found to be useful by motorcycle manufacturers and continued in use even after WW2.

We can get too precious about this - if the footrest thread is old then there will be some wear which will affect thread dimensions anyway.

A while back Hopper commented that despite carefully defined standard tolerances, sometimes sloppy manufacturing (oriental suppliers?) and worn manufacturing equipment came into play, and theoretical thread forms were not always met in practice. Wear on UNF thread manufacturing equipment would lead to a more rounded crest and root form anyway. Certainly for non-safety related purposes I would use a 7/16 UNF nut on a 7/16 BSC thread

Chas

Indeed. And commercial threads are made with plenty of clearance, so it matters not if the crest is a radius or a flat, there should be no contact in that area between the two threads. All contact should be on the flanks. If the crests and roots are in contact, any tiny piece of grit will prevent the two threads screwing together. If worst comes to worst, you can run a file over the male thread in the lathe and knock quite a few thou off the OD and all questions of crest profile are moot. Remember, according to Tubal Cain's Model Engineer's Handbook, you only need 65 per cent thread depth engagement to maintain 95 per cent of full thread strength.

My good old standby from Motalia who have a range of motorcycle goodies:

**LINK**