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BSA A50/A65 LH Footrest Stud

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Unit Singles

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Draganfly Motorcycles

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

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

**LINK**

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

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

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

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.