Thread of Atlas / Early Commando steering stem

[Rohan]

We diverge here somewhat, but we'll let folks see for themselves.

This is a BSF bolt and not a BSF bolt - BSF as in British Standard Fine.
Which is which ?
Both 5/16"
The other one is a Norton bolt..

 

[Triton Thrasher]

Is it a trick question, because one's a screw?

 

[Rohan]

You get extra brownie points - ones a setscrew, as it correctly would be called. (threaded all the way to the hex ).

Actually, we are wondering if this mention of 28 tpi on Norton fork yokes is a trick question.
Can't find that thread size (on that size shaft) on any thread charts.
Anyone verified this 28 tpi with a thread gauge ??
Or is it a 26 tpi cycle thread spec, that someone hasn't quite identified correctly.

Just FYI, this is a featherbed yoke and an 850 Commando yoke.
The 850 Commando yoke is NOT a BSF thread, but is very close in tpi (threads per inch) to what the equivalant BSF thread would be.
Not a FINE thread at all, is it.
And doesn't look like a fine thread either...

Pardon the grease.

 

[madass140]

if I remember correctly the later Commando thread, the one on the underside is UNF
but the other 2 are their own respective threads , 26 TPI x 13/16" and 28 TPI x 13/16" , you would be doing well to find taps and dies for either.

 

[Rohan]

if I remember correctly the later Commando thread, the one on the underside is UNF

That Commando yoke I show there is an 850 type, with the threads down below.
I'm not sure what size that thread is, didn't measure it, but its going to be something like 7/8" x 14 tpi. UNF like you say.
The nut has the 3 circles stamped onto it - well it would if there was room for 3 circles, its rather slim so only 2 circles fit.
(BTW, if that Commando yoke was threaded BSF, it would only be 11 tpi in that size. What was that about FINE threads for BSF).

So there is a 28 tpi in the Commando/featherbed style yokes then - you have seen ?
What threadform would they call that ?

 

[madass140]

"So there is a 28 tpi in the Commando/featherbed style yokes then - you have seen ?
What threadform would they call that ?"
Yes, sometimes these extra fine threads had something like EF or FF after their description, eg 13/16" BSFEF. or 13/16" BSFF
not saying that the Norton stem threads were BSF. what these extra letters meant was that it was an extra fine pitch of a particular thread.
But I guess some manufacturers just made their own threads for their own purpose.

 

[Rohan]

Nortons almost never used BSF, so it would be unlikely they would suddenly jump on that bandwagon ?

It could possibly be UNF-EF, maybe
28 tpi is twice 14 tpi for UNF.
Although 13/16 is an oddball size in that range ...

 

[manx-alike]

Doesnt solve the issue of how to 'restore' the thread, but:

Copied from the MPL stainless calatolog

"IMPORTANT NOTES ABOUT NORTON FORKS:
1. Featherbed frames used 3 types of yokes for solos;
(a) 7" stanchion centres, all 26 tpi
(b) 7.3/8" stanchion centres, 26 tpi, introduced in 1964
(c) 7.3/8" stanchion centres, 28 tpi, introduced in 1968
As all the above can be interchanged the year of manufacture can only be used as a guide.
2. Norton/Matchless hybrids of the mid '60's used longer 'Short Roadholder' forks. The main
difference is the fork seal holder, our part no. G66, price £35.00 ea.
3.1968-70 Commando models use featherbed type (c) yokes. These have the column nuts
above the headstock, the later models have one nut under the bottom yoke. Early Commando
models also use FW23, FW24, FW25, FW26."


I think stamped circles will denote 28TPI for the 'featherbed style' yokes . This marking will be stamped on the appropriate stem nuts (if from reputable source like MPL and on the OEM nuts I believe). On the ones I got from MPL, the lower nut (below top yoke) had most of 2 circles and the top one (above top yoke) had a single circle stamped on it. I used a 28TPI 'featherbed style' triple tree (from a 750S) on my atlas and it works fine. If I remember correctly, the stem of that triple tree was marked with the stamped overlapping circles (but I cant say if it was 2 or 3 overlapping circles). That might help distinguish the pitch. Took me a while to work out that the circles probably designated the thread pitch. I actually measured mine with a tread pitch gauge.

I have a OIF B25 BSA triple tree sitting on the bench and the botton nut on that (same arrangement as the late commando) has 3 overlapping circles stamped on it. Same thread form/nut??

 

[madass140]

What I have done in the past to restore the thread is a bit crude but worked, with a used nut with good thread I made 4 longtitudinal hacksaw cuts on the inside of the nut, I then case hardened the nut, that was my crude die nut that worked for me, of course some pre filing with a small 3 sided file will help before applying the die nut.

 

[gjr]

So there is a 28 tpi in the Commando/featherbed style yokes then - you have seen ?
What threadform would they call that ?

The early Commando (featherbed style) yokes are 28 TPI. I just measured them the other day. I have no idea what threadform it is, UNEF is 20 TPI.

Greg

 

[acotrel]

I think only 1/4 BSF is 26 TPI the same as 1/4 26 TPI cycle thread. In larger diameters the threads are coarser.

 

[L.A.B.]

13/16 - 28

UNR ?

http://www.gewinde-normen.de/en/unified ... hread.html

 

[Rohan]

UNR ?

Interesting. Thats a new one on me.

I suppose we should ask why Nortons would switch from 13/16 26 tpi cycle thread, which they had been using for nearly 2 decades, to this new version ?
Must have been a VERY convincing travelling salesman !

Perhaps we should also ask who, and where, the making of the fork yokes and stems was contracted out to ?
Assuming it was contracted out.

 

[L.A.B.]

I suppose we should ask why Nortons would switch from 13/16 26 tpi cycle thread, which they had been using for nearly 2 decades, to this new version ?

This was around the time when Norton begun converting from British Standard to Unified threads.

 

[Rohan]

This was around the time when Norton begun converting from British Standard to Unified threads.

I'm sure you are right.
But if this is what they moved to, with 20/20 hindsight it seems like an obscure thread type they selected ?!
You'd sure need to go to your Norton Dealer for replacement bits.

(but before anyone comments, thats probably true no matter what thread was used).