Classic cars forum & vehicle restoration.

[Ray White]

Hello everyone. This horn button, combined with a main beam dip switch, is normally (on a standard T Type MG) mounted on the metal instrument panel. On my car it is to be mounted onto the wooden dashboard and I think I may have an issue with regard to earthing. I have taken some photos to give an idea of how I think it should work.

I am assuming the feed to the horn button should be soldered to the middle of the insulated disc?


When the button is depressed against the spring pressure a connection is made with the case and should (I think) be earthed through the two brass machine screws that would normally be attached to the metal instrument panel?


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The question I have is how should I organise an accessible earth wire from the horn button given the fact that the machine screws don't go all the way through the wood.? I don't know how readily available some longer 4 BA screws might be.



I am making assumptions about how I think the horn works as I have not any original fittings or wires to copy. I am quite prepared to accept that I may be wrong and I am open to suggestions but I do need to keep the wiring behind the dash.

[ukdave2002]

Counter bore enough to fit a nut and terminal?

[Ray White]

[Bitumen Boy]

I think longer screws from studding - well, I guess you'd only really need one - would be perfectly acceptable. Or as you say a soldered connection to the end of a shorter screw, though that might be fragile and I think you'd probably want a suitable die to clean up the end of the thread and allow a nut to screw on to it - I assume you'd be wanting to fit the nut over the length of cable after soldering, but I could be wrong there.

Another thought, though, is that it might be worth tracking down and speaking to the folks that make the studding - given the limited demand for anything threaded 4BA these days I'd hazard a guess that the studding you link to is made on a lathe in small quantities, and it may well be quite straightforward for them to produce longer screws as specials.

[Ray White]

Well, I have ordered a 12" length of 4 BA brass studding.
Fortunately I have a good supply of BA nuts and washers.

I have yet to decide on how best to create a slotted head. In the past I have soldered on a nut and cut a slot with a junior hacksaw. If I want to re create the same head shape it is a fairly easy matter to file the corners off... but it is also easy to mess it up. !

If I am unhappy with my efforts I do know a model engineer who would be able to come to my rescue

Funny how things always seem to happen in the wrong sequence. At least that seems to be my experience!

[ukdave2002]

If all else fails you could use the screws that are used to hold the fronts on electrical sockets.

They are M3.5 so only 0.1mm narrower than 4BA, countersunk slotted head, made of nickel plated brass, so you could polish the nickel from the head if desired, and readily available in various lengths up to 100mm.

Best of all the screws and nuts cost pence

Dave

https://www.screwfix.com/p/easydrive-electrical-screws-m3-5-x-75mm-50-pack/2792h

[Ray White]

[ukdave2002]

[Ray White]

Dratts! I only have one long screw!

Not only that but I have just realised that the original horn button screws are smaller than 4BA. I thought they were the same size as the indicator screws but I suppose they must be 5 BA....??

Not only that but I checked and they are not even brass!

What I am thinking of doing is soldering an earth wire to the underside of the fixed brass middle section (collector ring?); taking it down the same route as the live wire and leaving the original screws in place. I think this will work.

[MVPeters]

BA threads in 'odd' 1 3 5 7 sizes are rare - try a local hobby shop.
_________________
Mike - MVPeters at comcast.net
2002 MINI Cooper 'S'

[Ray White]