The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: bikerboy on 31.08. 2017 23:52

Title: Dynamo screw threads
Post by: bikerboy on 31.08. 2017 23:52
Does anybody know the thread sizes that an E3L dynamo uses particulary the little countersunk screws that hold the field coils in?
Title: Re: Dynamo screw threads
Post by: duTch on 01.09. 2017 06:45

 Call Andrew (Beezamacc on here) at priory magnetos.

 I would expect Tyne field coil owes tho be BSF, as is the drive-sprocket nut, and the others to be BA*
Title: Re: Dynamo screw threads
Post by: JulianS on 01.09. 2017 09:21
The pole shoe screws are 5/16 x 22 tpi BSF.

The 3 countersunk screws in the end plate at drive end are 5BA.

The armature thread is 3/8 x 20 tpi BSF for sprocket retaining nut.
Title: Re: Dynamo screw threads
Post by: trevinoz on 02.09. 2017 00:31
The through bolts are 2BA, the screws which attach the brush plate are 6BA, the earth screw is 5BA.
Title: Re: Dynamo screw threads
Post by: coater87 on 02.09. 2017 14:13
 So the single screw in the end of the tin brush cover is what size?

 The lucas part number is 200,902 for this cover, and the cover is called "one piece end cover".

 I dont think I have ever owned a screw for that end cover, it was held on with black electrical tape when I got it. *conf*

 Lee
Title: Re: Dynamo screw threads
Post by: beezermacc on 02.09. 2017 20:30
2BA x 1/2"
Title: Re: Dynamo screw threads
Post by: coater87 on 02.09. 2017 22:54
2BA x 1/2"

 Thank you very much, I could have guessed at that until the cows came home and still wouldnt have gotten it. *smile*

 Lee
Title: Re: Dynamo screw threads
Post by: duTch on 02.09. 2017 23:29

 
Quote
So the single screw in the end of the tin brush cover is what size?

 The lucas part number is 200,902 for this cover, and the cover is called "one piece end cover".

 I dont think I have ever owned a screw for that end cover, it was held on with black electrical tape when I got it. *conf*

 Lee

 Not to do Bezzamacc out of a highly lucrative international transaction, but fairly sure Birmingham Triumph unit points covers  are 2BA, so you may be able to procure one of them locally, fairly easily

Title: Re: Dynamo screw threads
Post by: coater87 on 03.09. 2017 00:58
 I was just going to cut my own.

 Then I looked up the thread form and TPI for BA screws......what were those people thinking? *sad2* *eek*

 Lee
Title: Re: Dynamo screw threads
Post by: Black Sheep on 03.09. 2017 06:46
"they were probably the most "scientific" design of screw, starting with 0BA at 6.0mm diameter and 1.0mm pitch and progressing in a geometric sequence where each larger number was 0.9 times the pitch of the last size"
Thus quotes Wikipedia.
Title: Re: Dynamo screw threads
Post by: Rex on 03.09. 2017 12:08
  Then I looked up the thread form and TPI for BA screws......what were those people thinking? *sad2* *eek*
 Lee

BA is/was an extremely good thread system. Although it's been fashionable for years for the Great Unwashed to laugh at something they have little knowledge of, it worked well in it's time, and gave a range which extended from small enough for watchmakers to universal in the electrical/electronic industries and all in-between.
 What they were thinking was, "this is a good thread system for small industrial threads and fixings and will endure for centuries" and they'd be right.
Title: Re: Dynamo screw threads
Post by: groily on 03.09. 2017 17:14
Spot on Rex. It is a good series and well thought out, although the tpi, major and minor diameters do seem 'funny' at first glance, and 47.5° is not exactly an angle you'd ever guess at.
Screws of the various sizes/head forms and all the tackle to make them is easily available in the UK at least. BA Bolts is one good source, and that old favourite Tracy Tools has all the necessary in HSS and carbon steel, boxed sets included. 
Hex bar in stainless, mild, brass in the right, or very close to right, sizes for nuts and for the heads of bolts and setscrews is also out there.
(Not really threads to be cutting on a lathe - but it's not hard to turn stock to the right major diameters and use tailstock mounted dies to get pretty decent results.)
Title: Re: Dynamo screw threads
Post by: bikerboy on 03.09. 2017 23:44
Thank you everyboody for your replies but now the real teaser

What size are the tiny self tapping screws that screw to the terminals on the face plate  *smile*
Title: Re: Dynamo screw threads
Post by: coater87 on 04.09. 2017 01:14
 OK you two,

 Being an American I can safely say you two have drunk too much of the Cool-Aid.

 A good engineers can take something complicated and make it simple.

 He does not take something simple like a screw thread and make it complex.

 47.5 degree included angle, 31.36 threads per inch, and a .185 OD for a threaded screw form sounds to me like a paper tiger, some engineer developed this using nothing but a math calculation. Someone should have kicked his ass right then and there. ;)

 Lee
Title: Re: Dynamo screw threads
Post by: beezermacc on 04.09. 2017 08:57
The self-tapping screws are no.4 x 1/4". No.4 is about 2.9mm. You're better off using no.6 which is 3.5mm as the no.4 holes are usually stripped. Next question?!
Title: Re: Dynamo screw threads
Post by: Rex on 04.09. 2017 09:20
He does not take something simple like a screw thread and make it complex.
  Lee

That's one of those philosophical arguments that I've never really understood.
When I'm looking for, repairing, recutting, rethreading, drilling and tapping or any other operations involving BA threads,  why would I give two fecks over the included angle or mull over the chosen major diameter, etc? It's irrelevant.
So it's different to other screw threads.....so what? No-one's going to mistake it for anything else, but it works, and works well.
It's like complaining that a foot is made up of twelve inches and saying that it should be nine or fifteen to the foot.
It's the past and they did things which we wouldn't do now.
Title: Re: Dynamo screw threads
Post by: BSA_54A10 on 04.09. 2017 10:27
He does not take something simple like a screw thread and make it complex.
  Lee

That's one of those philosophical arguments that I've never really understood.
When I'm looking for, repairing, recutting, rethreading, drilling and tapping or any other operations involving BA threads,  why would I give two fecks over the included angle or mull over the chosen major diameter, etc? It's irrelevant.
So it's different to other screw threads.....so what? No-one's going to mistake it for anything else, but it works, and works well.
It's like complaining that a foot is made up of twelve inches and saying that it should be nine or fifteen to the foot.
It's the past and they did things which we wouldn't do now.

No it is EXACTLY the same as smashing a round peg into a square hole and is the reason why holes split,  small bolts corrode into place and snap off or just plain fall out
You can hammer a screwdriver into the key hole in your front door and turn it with a foot long spannar and that will work as well.
Back in the day, people worked with their hands and understood mechanical things, now days wea re soo much smarter and work with out brains so smashing something into a hole where it does not fit is a sure sign of a higher level of interlect.

IF understand it is difficult, draw a pair of lines 1" apart then pinch the kids, grandkids protractor and make a zig zag 60 deg and another 55 deg  between the two lines.
Cut them out and lay them on top of each other.
You will then see in place of the threads contacting full length they just touch each other at either the top or the bottom.
A bolt is held in place by friction and if the threads don't touch anywhere other than at a knife edge you dont get any friction.

Now because 1/4" is small it is well withing the strength of even the most imaciated brain dead to deform which ever is the softest material if they use a big enough lever.
The first time you do it you bend over the tips.
The 4th time you screw them together you brake off the tips of the threads and it strips.
Now because you have made the hole deeper when you try to put a helicoil into the buggered hole the helicoil dose not have enough material to crush so the helicoil then come out with the bolt and you have totally destroyed what ever you you were working on.
Now it is your bike so if you want to wreck it then fine because you own it thus you are entitled to destroy it.
Title: Re: Dynamo screw threads
Post by: groily on 04.09. 2017 10:28
You kind of get used to it over this side Lee!

Nowadays it's all ISO metric and ISO metric fine for these things out this way. Good stuff I'm sure . . .  even if 'Metric' doesn't mean 'simples' and 'uniform' though. Far from it. 

Not sure what you guys use over there (metric too maybe these days?), but I get confused by the nomenclature of the tiny sizes in UNC and UNF to be honest, simply due to my lack of familiarity (although their sizings are a bit more transparent I do agree).

Come to that, the Brit imperial focus on shank diameters not spanner sizes doesn't make life easy for many people - my French friends struggle with non-across-the-flats measurements for wrenches. Can't figure why the US imperial system is different from the British. Think 55° is also an odd angle.  Can't understand why BSC and BSF would be 5° apart, all being 'anglais'. Think the near-interchangeability of some WW and UNC sizes was designed specifically to drive them, er, nuts . . .!  The only constant is change, indeed.

And as Rex says, the past is another country.

The fact that an inch is 25.4mm and that it only divides once before hitting a prime number just adds insult to injury for those wanting to lathe-cut perfect threads across the divide on small machine tools.  Even if the French mathematicians had got the earth's dimensions right when calculating the metre, the relationship would probably still have been uncomfortable or bizarre. As it is, couldn't have made things more awkward if they'd tried - and BA would still be near-impossible to lathe-cut at pitches other than 0!

Just have to accept the rich diversity of it all and tool up accordingly!


Title: Re: Dynamo screw threads
Post by: BSA_54A10 on 04.09. 2017 10:36
OK you two,

 Being an American I can safely say you two have drunk too much of the Cool-Aid.

 A good engineers can take something complicated and make it simple.

 He does not take something simple like a screw thread and make it complex.

 47.5 degree included angle, 31.36 threads per inch, and a .185 OD for a threaded screw form sounds to me like a paper tiger, some engineer developed this using nothing but a math calculation. Someone should have kicked his ass right then and there. ;)

 Lee

A good engineer would not take something that was metric then complain it seems too complicated for them to comprehend when converted to inches.
Cutting a thread forming tool to 47.5 is no different to cutting a thread forming tool to 55, 60 or 45
And if you want stupid complication how about an inch thread form that goes down in 1/16" then for not good reason suddenly switches over to wire gauge for diameters.
And from memory BA thread count is  mm/ per 10 threads  *conf*