ET: Engine (Twin) Black Shadow Crankcase Threads

[Simon Dinsdale]

Is ADM the same thread? I just googled BSC and the 5 links I viewed say 60 degrees?

Don't believe any of those websites as most are wrong info and are modern interpertations written since the internet was invented. Especially the british fasteners website which is an American interpertation of cycle thread where they have just assumed all was 60 degrees which is wrong.

I have a copy of a the factory drawing for the driveside crankcase half here which I have just dug out. On it the threaded holes for the cylinder studs is specified as 9/16" x20 CEI where CEI is one way of writing cycle thread but the drawing doesn't say the thread angle.

The defining document that explains what cycle threads are is exactly is BS811. In the UK where these bikes were made, threads and other engineering specifications were defined in the British Standards documents.

Here is a link to a copy of BS811 and its dated 1950 with a 1951 revision which is perfect as its exacly from the time when Vincents were been made.

https://allcivilstandard.com/wp-content/uploads/2019/02/BS-00811-1950-1999.pdf

If you read the document it shows a diagram which does say 60 degrees which is where the confusion comes from because on the same page it also says the 20 TPI series uses an alternative Whitworth form. Now Whitworth form was always 55 degrees.

If you read the rest of BS811 what you will see in table 1 that they start at 40 TPI for 1/8", then 32 TPI for 5/32 & 3/16 and then settle at 26 TPI right up to 3/4" size. Now this is the standard 60 degrees form and nowhere on that list is 9/16x20 TPI which is what we are looking for.

There are then other tables for strange CEI threads including spoke threads.

Further down the document Apendix A is the first time 20 TPI is mentioned and in that appendix it shows 9/16 20 TPI. At the top of that appendix table it says these are British Standard Whitworth form. Now Whitworth form was always 55 degrees.

So according to BS811 which is the document that defines what cycle threads are all cycle threads are 60 degrees except the 20 TPI series which were Whitworth form and so 55 degrees.

None of this is to be confused with 9/16" 20 TPI UNF which is an American thread and is 60 degrees.

This also agrees with Know Thy Beast which lists the thread as 9/16 20 TPI Whitworth form, so I rest my case. This is the last I will comment on it as I'm getting bored and also got the Winter Pennine rally to go to this weekend.

 

[fogrider]

During one interesting chat with Bob Dunn who did the major machining work on my first twin, he told me the reason there are 20 TPI threads, in all sizes, small to large, was because Vincents routinely used thread chasers which were all 20 TPI BSF, ie 55 deg.

I think that supports Simon resting his case....... while he freezes at the winter Pennine rally. !

 

[oexing]

Firstly 9/16-20 thread is NOT UNF, that is 18 tpi at 9/16. That is why my tap from Aliexpress is marked UN 9/16-20 and is cycle thread for bicycle pedals - still. And no, it looks like at Vincents they had thread rolling heads for producing cylinder studs. Why : I looked at my 5 studs from the Argentinan engines, two will most likely be standard, the two at bottom of first photo. They got waisted tubes for ca. 13 mm for working with the rolling head, you have to reduce the 9/16 stud for this diameter certainly. The three above them are repairs most likely done in Argentina, by chasers or thread cutting, not rolled . It is very unlikely that in Argentina they had a rolling head for that cycle thread , so the two rolled types are originals I say.
We do not know today the reasons then to have 60 degrees thread form which I verified by a little use of the microscope. See yourself and draw your conclusions. One reason is the remaining meat around the rolled thread when having 60 degrees AND having a through hole of 3/8 inside with the two part studs post war. That gives less than 2 mm wall thickness, so 60 degrees is better than 55 degrees. . Plus we do not know about equipment at the works, maybe thread rolling heads from cycle threads - which got 60 degree rollers. Or maybe heads from USA that got the 60 degree types from UNC or UNF - unlikely as they don´t offer the 20 tpi type ? But they may have come to that available bicycle pedal thread as best compromise when having the two part studs and maximum remaining meat at threaded ends, not an argument with the later one part studs.
In my photos a lathe tool with 55 degrees profile is shown , next to the UN 9/16-20 tap and the obviously standard B-type two part studs. Your choice if you can see 55 degrees or rather 60 degrees . . .

Vic

 

[fogrider]

Not just trying to have the last word, but I must point out that thread charts simply list common sizes and threads. 9/16x 20 can be ANY thread form you want, just use a tool of the form you wish and set the TPI you wish.
As for thread rolling a cylinder head stud, the machine cut female in the crankcase will be a different profile to the rolled male, as seen in Oexings' excellent photo's. Not a very good thread contact in my mind.
My point is, you can't assume what a thread is simply from its' diameter. Its' a clue , that's all.

 

[Vincent Brake]

Tap 60 degr cyle thread.
Crosswires just visable on top of tap.

 

[Vincent Brake]

Did that for 55 degr tap.

And an old hollow cyl stud.
Checked all 4 of them.

more to 60 than 55 degree.
So as usual, factory did as they suited.

Note the Moon in the background.

 

[oexing]

Do we agree the Vincent works had a thread rolling head for the cylinder studs then ? So they had rollers with 20 tpi for the head and that pitch was found in BSC British cycle thread as well, with 60 degrees profile certainly. With a rolling head you can do a few sizes of threads with same rollers, depending on the brand of heads, I only know the Fette types.
So then it looks quite logic to find the 60 degrees type on Vincent studs and the best solution when dealing with thin walled studs they got on early B bikes. The Spares Co. will have that type of thread studs on their stock I guess . So when repairs are made on stripped engine threads you want to do adapters for the 60 deg BSC 9/16-20 studs. And taps are cheap to get from Aliexpress like in link below, once again, dies as well. But I would not suggest dies , the threads would not come out very clean, better have a lathe tool for the studs. Or even better get studs form the Spares Co as they may have rolled threads ??
The thread profile in my micro photos looks a bit rounded in the i.d. but is correct like this. You do not want a sharp core diameter when there is high tension on the bolt. The radius got clearance in the tapped adapter from predrilling correct oversize. So then the bolt/stud sits nicely in the tapped thread even when that looks a bit strange to Joe Public.
In photos some studs I did with my own rolling heads for showing correct profiles. You can modify diameters as you like them for your nuts but take care for dirt so some slack should be present .

Vic

Cycle thread 60 deg. UN 9/16-20:
UN 9/16-20

 

[clevtrev]

Taken from 1948 factory drawing of Cylinder Head.

 

[rapide049]

Taken from 1948 factory drawing of Cylinder Head.

55 degrees Whitworth form