Thread Details For Vintage Car Wheel Spokes

[n.glasson]

Can anyone tell me what the thread size is for spokes for car wheels? There is plenty of information online about bicycle spokes and I have found reference to these being a thread designated BC2.x, where x is 0, 1, 2, 3 etc.

I also came across this which refers to car wheel spokes having 32 tpi but being a BA threadform.

http://www.vintagerims.com/Spokes_and_nipples_Vintage_Rims.html

I am in the process of rebuilding some Austin 7 wheels and the spoke threads are 32 tpi as stated on the link. I'd quite like to buy a tap to tidy up the re-usable nipples and a die nut to tidy up the re-usable spokes. No. 10 UNF is close - it is the same pitch but smaller diameter than the spokes which measure very close to 5mm major diameter. M5 is closer but still not quite right. Does anyone have a more definitive description of the threads typically used on spokes for cars/motorbikes?

 

[Bob Korves]

I never had to mess with the chrome wire wheels on my '65 MGB. It is quite possible that the spoke threads of your Austin 7 have Whitworth threads, which are 55 degree and have rounded crests and troughs on the threads. It is not just the pitch and diameter...
I see the reference you are looking at, perhaps:
http://vintagerims.com/Spokes_and_nipples_Vintage_Rims.html
No real mention of the thread form there, and I found nothing else with a cursory search. I searched on chasers as well.

 

[Bob Korves]

On bicycle spokes the threads have rounded crests and valleys as well, and the threads are usually rolled into quality spokes so they are less susceptible to fatigue and breakage. It might be smart to just use a rotary wire brush on the external threads and a brush like a rifle bore brush on the internal threads to clean them up without introducing stress risers in the threads. If they are badly pitted or corroded you might need to find good replacements instead.

 

[Wreck™Wreck]

If you have a sample of the external thread measure it and you will know. It is possible yet unlikely that it is a proprietary thread form and size, not impossible however. The Unified Thread dimensions published in the United States ensure that threaded fasteners play well together regardless of manufacturer.

There is nothing to stop a company from producing a product using a thread system of their own invention.

 

[n.glasson]

I have samples and I have measured them. The spokes are 5.0mm major diameter on the threads and 32 tpi. I don't have the kit or the eyes to measure the thread angle properly. The threaded section is larger diameter than the wire of the spoke because the threads are rolled. Spoke threads are cold rolled rather than cut so that the thread is stronger than the rest of the spoke rather than weaker. The whole idea of spoked wheels is to save weight so the wire can't be any larger diameter than necessary. As the spokes threads are rolled - there may be some variation in the overall thread diameter. The nipples however will be threaded with a tap and I very much doubt that the size is unique to Austin. I know it is not M5 and I know it is not 10 UNF, although it is close to both of these. I strongly suspect there is a specific spoke thread standard that is not well documented as Whitworth, BSF, UNC, UNF/SAE etc are. I might try inspection of a new spoke under a USB microscope and see if I can get an accurate measure of the thread angle. Then I'd be able to make my own tap and die to suit cleaning up the reusable spokes. I know it would be less hassle to just buy new spokes but I'm picking they will be a few $$$ each and with 36 spokes per wheel and 5 wheels to recondition, I want to save as many spokes as I can.

 

[Wreck™Wreck]

I have samples and I have measured them. The spokes are 5.0mm major diameter on the threads and 32 tpi. I don't have the kit or the eyes to measure the thread angle properly. The threaded section is larger diameter than the wire of the spoke because the threads are rolled. Spoke threads are cold rolled rather than cut so that the thread is stronger than the rest of the spoke rather than weaker. The whole idea of spoked wheels is to save weight so the wire can't be any larger diameter than necessary. As the spokes threads are rolled - there may be some variation in the overall thread diameter. The nipples however will be threaded with a tap and I very much doubt that the size is unique to Austin. I know it is not M5 and I know it is not 10 UNF, although it is close to both of these. I strongly suspect there is a specific spoke thread standard that is not well documented as Whitworth, BSF, UNC, UNF/SAE etc are. I might try inspection of a new spoke under a USB microscope and see if I can get an accurate measure of the thread angle. Then I'd be able to make my own tap and die to suit cleaning up the reusable spokes. I know it would be less hassle to just buy new spokes but I'm picking they will be a few $$$ each and with 36 spokes per wheel and 5 wheels to recondition, I want to save as many spokes as I can.

I had already assumed that they are rolled, 90% of fasteners are today. Being a UK product of that era it may be a Whitworth 55° form.
However 5MM is .196" diameter ,the American standard thread 10-32 has a .190 Major diameter, 3/16" is .188 so this may well be a Whitworth 3/16-32 which is almost identical. aside from the included angle and the root and crest features.

 

[n.glasson]

The thing that makes me think it might be BA thread form is the website I referred to in my initial post where they say "The thread is the standard British BA screw thread system with 32 TPI". 32TPI is most definitely not a standard BA pitch so I assumed they were referring to the thread profile which for BA has an angle of 47.5 degrees.

 

[Wreck™Wreck]

I once came across some bicycle related best practice for certain threaded products, BA does indeed ring a bell now that I think about it.
Thanks

 

[T Bredehoft]

Do you have a perfect spoke to spare? If so, find a mate with a milling machine and have grooves cut in the threads, then have the spoke threads hardened and use that as a tap to clean out the nipples.

 

[n.glasson]

Good idea. I figure I'd be able to grind (rather than mill) flutes in a spoke to make the tap and case harden it. I could also make a rudimentary die by using a thin cut-off wheel and cutting a slot or two into a steel nipple and case hardening that too.

Out of interest I have attached a microscope image that shows the thread form of a new spoke. I took the image into CAD and overlaid some lines to match the profile so I could get an idea of thread angle. It seems it is around 65 degrees and the radii are quite generous. It is nowhere near the expected BA or BSF/Whitworth thread angle.