Difference between 9/16”x 20 Cycle Pitch and 9/16”x 20 UNS.

Advert

New Replies

Report Bug

Help/FAQs

Difference between 9/16”x 20 Cycle Pitch and 9/16”x 20 UNS.

This topic has 46 replies, 12 voices, and was last updated 29 October 2024 at 22:45 by Michael Gilligan.

Viewing 25 posts - 1 through 25 (of 48 total)

1 2 →

Author

Posts
18 October 2024 at 00:17 #759848
Bill Phinn
Participant
@billphinn90025
Can anyone tell me what difference, in theory and practice, there is between these two thread types. I notice Tracy Tools list them both but at prices that don’t quite coincide.

I’m looking into the options for repairing bicycle crank arm threads, and no option looks exactly cheap at present.
Advert

18 October 2024 at 00:22 #759850
Michael Gilligan
Participant
@michaelgilligan61133
I think it’s a subtlety of the thread-form, Bill

This might help:

https://en.wikipedia.org/wiki/British_Standard_Cycle

MichaelG.

.

Edit:__ if you thought the taps were expensive:

https://knowledge.bsigroup.com/products/specification-for-cycle-b-s-c-threads-formerly-known-as-c-e-i-threads?version=standard
18 October 2024 at 00:31 #759851
Michael Gilligan
Participant
@michaelgilligan61133
Salvation:

https://ut.com.sg/ut/thread-2/truthread-gauging/bs-cycle-thread-gauges/

MichaelG.

[ sorry, need to get some sleep now ]
18 October 2024 at 03:04 #759854
jimmy b
Participant
@jimmyb
9/16 20 UN, is not a UNS thread size, that is for different pitches (14 and 27 tpi), info below on 9-16 20 UN-

The only info I have on cycle thread is-

Based on the above, I’d just use the cheapest option.

JImB
18 October 2024 at 06:34 #759856
Speedy Builder5
Participant
@speedybuilder5
9/16 BSC (British cycle) is 20 or 26 tpi and 60 degree thread form,

This makes some interesting reading on thread form differences between UNS, UN, UNF

https://www.ring-plug-thread-gages.com/ti-N-vs-UN.htm

https://www.ring-plug-thread-gages.com/ti-N-vs-UN.htm

Bob
18 October 2024 at 09:40 #759883
Tony Pratt 1
Participant
@tonypratt1
In practical terms and your application either would be acceptable. At one company I worked at back in the 70’s there was many different threads in use and we regularly swapped BSW and UNC fittings with no detriment to the work, not totally correct but when production was waiting time was money.

Tony
18 October 2024 at 11:06 #759906
Howard Lewis
Participant
@howardlewis46836
From memory, BSC and Unified have the same 60 degree thread form. (Like several sizes of Cycle thread, British Standard Brass is 26 tpi but is Whitworh, 55 degree, thread form, so not interchangeable.)

So if both items have the same tpi, they should be interchangeable.

FRom memory, UNC will screw into a Whitworh tapping of the same pitch, but not the other way round.

(Our Chief Engineer would only countenance BSW or BSF, so often replacing fixings on UNC threaded items, we had to tap out to BSW. He used to insist that the UNF prop shaft bolts on new vehicles were replaced with BSF before being allowed to go into service!. Kuckiliy, he never knew that the fixings for the bell housing on the Bristol RE were a mixture of 3/8UNF and 3/8 BSF!)

Now back to the original thread!

Howard
18 October 2024 at 13:22 #759932
Speedy Builder5
Participant
@speedybuilder5
BSW and BSF are 55 degree thread angle, Unified 60 degrees, so although there are some “bastard” fits neither should be miss matched in any form of safety environment. As “engineers”, we would never mix them (would we ??).

Bob
18 October 2024 at 13:29 #759935
Michael Gilligan
Participant
@michaelgilligan61133
On 18 October 2024 at 11:06 Howard Lewis Said:

From memory, BSC and Unified have the same 60 degree thread form. […]

Not quite true, Howard ^^^

They have the same angle, but NOT the same form.

[ which is why I posted my “Salvation” link ]

MichaelG.
18 October 2024 at 16:02 #759955
jimmy b
Participant
@jimmyb
On 18 October 2024 at 13:29 Michael Gilligan Said:

On 18 October 2024 at 11:06 Howard Lewis Said:

From memory, BSC and Unified have the same 60 degree thread form. […]

Not quite true, Howard ^^^

They have the same angle, but NOT the same form.

[ which is why I posted my “Salvation” link ]

MichaelG.

As far as I can find out, 20UN has an external root rad of 0.007″, whereas 9/16th 20tpi has a root rad of 0.0083″.

JimB
18 October 2024 at 16:15 #759959
Michael Gilligan
Participant
@michaelgilligan61133
More to the point [?] JimB

the crests of UN threads are typically flattened/truncated … as per the table that you posted.

MichaelG.
18 October 2024 at 16:31 #759961
Robert Atkinson 2
Participant
@robertatkinson2
In safety critical applications it may not be just the thread form that matters but how the thread is formed.
There were a couple of Tiger Moth crashes caused by failure of the lower wing tie bar. This bar has male threads on the end and connects the inboard ends of the lower wing. The original threads were rolled but some replacements had cut threads and failed due to fatigue cracking.
Some also had different material see:
https://asasi.org/wp-content/uploads/2021/05/Tiger-Moth-In-flight-Break-up.pdf
I’d consider a bicycle crank a fairly critical application. Quite high loading and failure at a critical moment e.g. turning into traffic, could have serious consequences.

Robert.
18 October 2024 at 16:58 #759969
jimmy b
Participant
@jimmyb
On 18 October 2024 at 16:15 Michael Gilligan Said:

More to the point [?] JimB

the crests of UN threads are typically flattened/truncated … as per the table that you posted.

MichaelG.

I have spent too much time on this one, looking into the root rads, I must now leave this one. Unless someone has a copy of BS811:1938…..

I’ve been through my extensive thread information and my Guide to World Screw Threads on this one!

Jimb
18 October 2024 at 17:04 #759972
jimmy b
Participant
@jimmyb
On 18 October 2024 at 16:31 Robert Atkinson 2 Said:

In safety critical applications it may not be just the thread form that matters but how the thread is formed.
There were a couple of Tiger Moth crashes caused by failure of the lower wing tie bar. This bar has male threads on the end and connects the inboard ends of the lower wing. The original threads were rolled but some replacements had cut threads and failed due to fatigue cracking.
Some also had different material see:
https://asasi.org/wp-content/uploads/2021/05/Tiger-Moth-In-flight-Break-up.pdf
I’d consider a bicycle crank a fairly critical application. Quite high loading and failure at a critical moment e.g. turning into traffic, could have serious consequences.

Robert.

This is something I’ll hopefully not overlook again.

about 25 years ago we were making a large batch of fasteners that were to be thread rolled, I grabbed the wrong gauge and worked to finished size, inspection didn’t pick on the wrong gauge. £10,000 worth of scrap that time…..

Possibly my most costly scrap session…

When I’m asked to cut a thread (internal or external) I always check if the thread has been rolled.

JimB
18 October 2024 at 17:07 #759974
Michael Gilligan
Participant
@michaelgilligan61133
From the link that I posted:

.

.

MichaelG.

https://ut.com.sg/ut/thread-2/truthread-gauging/bs-cycle-thread-gauges/
18 October 2024 at 17:19 #759978
jimmy b
Participant
@jimmyb
I think that must be a mix of internal and external profile.

Just about every speck calls for a root rad at the minor on external and on the major on an internal thread.

That’s why I’d like to see the old spec.

JimB
18 October 2024 at 18:11 #759989
Michael Gilligan
Participant
@michaelgilligan61133
I could probably access the latest [1950] issue, as per my earlier link, but my authorisation is for personal use, and I would not be at liberty to share it.

I have therefore contributed what I can to this discussion and will leave others to continue the debate.

MichaelG.
18 October 2024 at 18:15 #759991
Michael Gilligan
Participant
@michaelgilligan61133
On 18 October 2024 at 00:22 Michael Gilligan Said:

[…]

Edit:__ if you thought the taps were expensive:

https://knowledge.bsigroup.com/products/specification-for-cycle-b-s-c-threads-formerly-known-as-c-e-i-threads?version=standard
18 October 2024 at 19:14 #760001
Tony Pratt 1
Participant
@tonypratt1
On 18 October 2024 at 16:31 Robert Atkinson 2 Said:

In safety critical applications it may not be just the thread form that matters but how the thread is formed.
There were a couple of Tiger Moth crashes caused by failure of the lower wing tie bar. This bar has male threads on the end and connects the inboard ends of the lower wing. The original threads were rolled but some replacements had cut threads and failed due to fatigue cracking.
Some also had different material see:
https://asasi.org/wp-content/uploads/2021/05/Tiger-Moth-In-flight-Break-up.pdf
I’d consider a bicycle crank a fairly critical application. Quite high loading and failure at a critical moment e.g. turning into traffic, could have serious consequences.

Robert.

Your post made me chuckle, from aviation to a push bike application! I’m moving on also.

Tony
18 October 2024 at 20:06 #760021
Michael Gilligan
Participant
@michaelgilligan61133
JimB

I couldn’t possibly encourage so-doing … but if you happen to have a Scribd account [licence to steal?] you may find enlightenment here:

https://www.scribd.com/document/675387539/BS-00811-1950

MichaelG.
18 October 2024 at 20:52 #760031
old mart
Participant
@oldmart
Bicycle crank arm threads may differ left and right, as I recall the left crank has a left hand thread. Unless the cranks are very old or rare, it would be cheaper to buy new rather than buy and use taps which cannot restore a damaged thread to original strength anyway.
19 October 2024 at 02:48 #760046
Bill Phinn
Participant
@billphinn90025
Thanks a lot for everyone’s contributions.

Distinctions between the two are evidently slight in practice (depending on the application), but they are there nonetheless. It would be interesting to know what thread individual pedal manufacturers consider they’re putting on their pedal spindles.

If you search on Ebay for 9/16”x20 crank thread repair taps, in some listings you see images of taps marked 9/16”x 20 BSC and 9/16”x 20 UN interchangeably.

As Old Mart suggests, it probably isn’t very cost effective to do this particular repair – unless you’re doing quite a few of them. And the repair has to be done well if the crank is to be strong and safe in use.

I found the following video entertaining.

The man doing the LH thread repair shows remarkable determination, but have a look at the point where he is cutting the thread (about 38 minutes in), where it all appears to go badly wrong, though apparently not in the eyes of the bike owner. Realising his mistake, at 39.08 he turns the chuck the other way but the threading tool is still cutting on the near side of the bore. Presumably it won’t cut at all under these circumstances unless he’s ground it to cut equally well from the bottom as well as from the top. It looks to me as if when he comes to actually screwing the pedal on to the crank (39.30) all he’s doing is using the threads on the pedal to tap the crank.
19 October 2024 at 03:36 #760050
Hopper
Participant
@hopper
^^^There’s that. Instead of buying a tap, if you have a spare pedal spindle, you can put a shallow axial hacksaw slot along the threaded section and use that to repair a damaged thread in the crank arm. It acts as a crude tap, good enough in most cases to fix a distorted or damaged existing thread.

Agonising over whether thread forms have a rounded or flat crest has no practical application on bicycle work (or most other work for that matter!). They fit together just fine on mass produced threads, as the crests are a clearance area and not a load-bearing area.
19 October 2024 at 03:50 #760055
Bill Phinn
Participant
@billphinn90025
On 19 October 2024 at 03:36 Hopper Said:

Instead of buying a tap, if you have a spare pedal spindle, you can put a shallow axial hacksaw slot along the threaded section and use that to repair a damaged thread in the crank arm. It acts as a crude tap, good enough in most cases to fix a distorted or damaged existing thread.

Yes, I toyed with the idea of putting three slots down an old pedal spindle to do the repair, but when I saw the crank today (the drive-side crank for a triple chainset) the thread is too thoroughly reamed out for a straightforward re-tap.

Maybe I can melt some Alusol into the hole or use some of those aluminium “welding” rods, as seen here.
19 October 2024 at 03:55 #760056
Hopper
Participant
@hopper
Maybe. Or maybe that might affect the heat treatment and strength of the crank arm? I think I would look at boring and threading the hole oversized and make an insert. Which gets back to buying a tap!

Even if you have to use a RH thread on the OD of the insert, it could be secured with Loctite/peening/Dutch key. Or made with an appropriate small step on the end to prevent unscrewing in that direction under load (which is the opposite direction to what you might intuitively think because of some confusing physics aka “precision”.)
Author

Posts