Speedo rotating

[Angus]

Noticed a new rattle/buzz from the A7 last night on the VMCC ride. So had a look the morning. The knurled nut that hold the speedo cable onto the Speedo had come off and slipped down the cable. So I put it back and did it up and the whole speedo moved inside its housing. There appear to be two screws into the bottom of the housing that hld it in place, the one next to the speedo cable was missing (see picture).
Does anyone know the size, thread and length of this. Does it need to be done up tight.
The A10 had one but I dont want to undo it in case it hold something internally.
 

[Angus]

Sorry to chase I read RichardL's article about his disassemble etc and other have said they have done similar, I have found a screw that fits but don't want to just screw it in, in case its too long and fouls something. I am going to the BSA national next weekend and would really like a speedo that does not move about.

[RichardL]

Angus,

Wish I could help but I'm away from home for a week and can't lay hands on my loose speedo.

Richard L.

[morris]

3/16" x 3/8" BSF 32 TPI
I fitted a spring washer. I know why now... 

[Angus]

Thankyou Morris  . I was about to take the one out of the A10, but really did not want to as it could have held anything inside the housing and then I would have been two bikes down. I assuming it is done up tight.

[morris]

I assuming it is done up tight.

Yep. To put it in the immortal words of Muskrat " tighten until you strip the thread, then back of a little..."
Just joking there, I use a light spring washer and normal screwdriver force.

[duTch]

 I had a feeling Smiths gauges used metric threads (Smiths Jaeger) - or am I thinking of something else

[trevinoz]

They are definitely metric threads.

[morris]

They are definitely metric threads.

Did a search on the web, and to add to the confusion this came up from http://thevincent.com/vininst.htm ;
"The two screws are a very unusual metric thread 0.75 pitch x 4.8mm diameter, they do not conform to any thread table I have.
But help is at hand by shear coincidence the American wire gauge thread 10 x 32 UNF will screw right in, it is - in metric 0.78 pitch x 4.83mm diameter - make sure the thread is no longer than the original or there is a danger of the end of the screw foaling the movement the length of thread under the head is 9mm or 0.355in and the washer was a star washer 40 thou thick"

[kiwipom]

hi guys, now that`s interesting,
..An Electronic Tachometer from a magneto is a relatively simple instrument.  The primary winding in the magneto armature can be regarded as a low powered generator.  If you have one of the plastic end caps on the magneto of the type that has an electrical pickup coming from it - on the inside of this cap is a carbon brush that rubs on the head of the captive bolt for the points block - good reason for not having a screwdriver slot in the head of this bolt!
This type of end cap is normally connected to a kill switch - mounted up on the handle bar - the kill switch is used to short the centre of the armature to earth and thus kill the engine - horrid things and not recommended - the switch will inevitably goes wrong and is guaranteed to give you a sore right leg. Vincent's are supposed to be switched off by holding the valve lifter open when the engine is at idle speed.  If you were to connect a multimeter - select AC Volts - to this pickup and the body of the magneto you would be able to record a voltage that is directly proportional to the speed of the magneto. Unfortunately the voltage is in Alternating Currant - AC - form and will require a full wave rectifier so that a standard Direct Currant - DC - voltmeter can be used.

Select a suitable rugged vibration resistant voltmeter.  It is now possible to paint a dial to indicate Revolutions Per Minute - not an underground political organisation! - to fit the voltmeter.    Andrew  Rackstraw  2/20/02
 

[Angus]

Very strange about the screw thread. I found one that fitted fine in my spare BSA nuts and bolts tray. It was a bit long and so did not use it until I saw Morris's answer on length and tightening. I added a nut to it to adjust the length (will do it right eventually) and did it up. Its working fine.
Thanks again Morris, I could have just used the long screw and damaged it.

[duTch]

"The two screws are a very unusual metric thread 0.75 pitch x 4.8mm diameter, they do not conform to any thread table I have.
But help is at hand by shear coincidence the American wire gauge thread 10 x 32 UNF will screw right in, it is - in metric 0.78 pitch x 4.83mm diameter -.................

 The thread chart I use from Classic Fasteners shows 10G as

                        Imperial Thread Chart (Threads per inch)
           Major        Major        BSF      BSCY     Brass      BSW    UNC    UNF       BA
Size      Dia "        Dia mm      55°        60°       55°        55°      60°     60°     47.5°

10G    0.1900        4.826                                                          24      32                 

 Or the Metric Chart;

                                           Metric Thread Chart
                                                           A'mobile
Dia      Dia           ISO        ISO        SI           Fine,      SI (DIN)   Lowenherz     Swiss        Swiss
mm    inches      Coarse     Fine      French    French    German                            "A"           "B"
4.5    0.1772        0.75                   0.75                                           0.75        0.75           0.5
5       0.1969        0.8                     0.9         0.75        0.8                 0.8          0.8            0.5