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MG MGB Technical - Guage mounting screws

Ijust discovered that one of the mounting screw nuts (round thumb nuts) on the speedometer of my 73B is missing. I had the tach out and tried to determine the thread size but it didn't appear to be metric or 10-32.

Is it Wentworth (do I have that right?)......or perhaps something else? I just want to go to the hardware store and get a nut to spin onto the threaded mounting post. The stand off bracket is still in place.

Thanks,
BH
BH Davis

It is neither. It is 4BA or something like that. I bought a tap years ago to make new ones from brass rod. It is just a tad off from 10-32 as I remember, just enough so that they will not work together. BH, I may be able to find that tap again in my stuff. If I can, I'll make you a couple
gerry masterman

Gerry,

As I said in my email to you THANKS !!!! for helping out with this. You're dead on in the 10-32 being close but not quite there.

I would like to make sure everyone who reads this knows that you made me a couple of these thumb nuts within hours of my posting this question.

If you haven't bought any of Gerry's custom parts before you should visit his website at:

http://www.gem-enterprises.net/

He's got some great after market components that solve a number of problems. I know as I've used several of his parts on my 73B. And he is ALL customer service......as you can see here!

Thanks again!
BH

BH Davis

It won't be a 4BA, that is too small. If it is close to a 10-32 it would be a 2BA, but a 10-32 nut will thread right onto a 2BA - they are so close that they can be used interchangably. I have been told that they are a 3mm, but I don't know that for sure.

So Gerry, what size did you find it to be? Cheers - Dave
David DuBois

I'd just call Moss and have them ship another - then who cares, its a 5c piece ... or maybe $1 or $2.
Paul Barrow

I tried 4mm and 5 mm nuts. 4mm is too small and 5mm is too big. 10-32 is right there size wise, but after about 2 full turns it binds up and won't thread on any further.

BH
BH Davis

It's actually 2BA. The 4BA just came off the top of my head when I was making my first post

S far as Moss goes, I have not noticed them in their catalog. Do they carry them?
gerry masterman

I looked quickly in Moss before posting here and didn't see them listed.

BH
BH Davis

Guys, When I checked a set of gauges that I had in storage, I find that it is NOT 2BA. Measuring I come up with metric 4 X 0.70. I do not know who told me the 2BA size years ago but it is not correct for what I have on hand. Just wanted to clear this up before anyone else made nuts of the wrong size, like I did. When I get my M4 X 0.70 Taps in, I'll repost to be sure that this is the correct size for everyone's records.

I wonder-is it possible for Smiths to have uses two different stud sizes through the year?
gerry masterman

Jerry,

I tried a 4mm and 5mm nut on the tach. 4mm was too small. I suspect that there were different threaded studs used over time. I'm hoping my tach and speedo have the same size threaded studs. I didn't try putting a nut on the speedo which I didn't take out.

BH
BH Davis

Bernie, I will send you what I made and also a pair of spares that I pulled of a tach I had in storage. Are you willing to send back the ones that do not fit? Maybe we can get to the bottom of this easily. The ones I made are 2BA according to the markings on the taps I used to thread the holes. They go out in the morning!
gerry masterman

Not all the cars had Smiths gauges. The early cars had Jaeger (sp?). The change point was very early, not sure when but perhaps before the change from strap tank to bolts around the flange.
David Witham

IIRC BA uses a metric thread profile with an imperial diameter. So confusion is to be expected. Smiths are almost certain to be BA but I don't know about Jaeger.
David Witham

Gerry - I found out that some of the gauges use a 3BA thread and some of the earlier ones may have used 3mm. If you have a selection of gauges on hand, see my Standard Thread chart at: http://www.mgexperience.net/article/thread-reference.html
That chart has the thread count for American, British and Metric fasteners (including hte metric equivalent for the BA fasteners). Cheers - Dave
David DuBois

Out of curiousity I checked a Victoria British catalog last night. Don't know why I didn't think to do that earlier.

They have the guague components available separately.....including glass faces, chrome bezels and the thumb nuts!!

Gerry, I will certainly test things out on my speedometer and send back the ones that don't fit. That's the least I could do for all your help!

BH
BH Davis

Well Guys, they are not M4 X 0.70 either, but that is damned close. TPI looks right but the major/minor dia is a tab bit large. It will thread on if forced. The tap is just a bit loose in an original nut. The search goes on...
gerry masterman

Gerry - A 4mm stud has a dia of 0.1575" and could have either a 0.70mm ISO thread, a 0.50mm fine metric thread or a 0.75mm French Standard (note this is an old obsolete thread that was used by MG in the 40s and early 50 in their engines). A 3BA stud would have a dia of 0.1614" and a thread of 0.73mm. Based on that, I would guess that we have a 3BA thread, but it could also be one of the odd ball 4mm threads. Cheers - Dave
David DuBois

Just checked the studs on a couple of speedos that I hava on hand, The diameter is correct for a 4mm (0.1575), so I broke out a 3BA tap and checked the thread - no match. Then I dug out my set of British and Metric thread gauges and found the 0.75 thread matched perfectly. It looks the studs are a 4 X 0.75mm - back to those obsolete French Standard Metric. Cheers - Dave
David DuBois

Gerry,

Good news and bad news.

Good........one of the sizes fit perfectly.
Bad.........it was one of the thumb nuts from your spare tach. So I'm afraid that unless you know the thread type of that we're no further along in solving the stud thread size issue on a 73B speedomenter.

Thanks again for sending these. I'll get the 2 brass thumbnuts and the 2nd spare from your tach back to you tomorrow along with a bit of a thank you enclosed.

Regards,
BH
BH Davis

Bernie, I think we are in good shape. David says the thread is M4 X 0,75 so that's good enough for me. I will source up a proper sized tap and give making them a another try just because I refuse to let it beat me. Sure, now we know they are available from VB but it's the principal that keeps me moving-I do things just to prove that I can, usually all the while someone is telling me I can't do it
gerry masterman

This thread was discussed between 21/07/2007 and 26/07/2007

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