I have read several threads on this subject that has left me more confused than what i was before. I apologize for bringing this up again. Hopefully someone can offer some help. I removed the Smiths clock from my S2 e-type to send it to Mike Eck to get it working. In the process one of
the knurled clamp nuts flew out of my hand, never to be seen again. I need to replace it, but confused regarding where to get a replacement and what thread size it actually it is. I have found on ebay, supposedly for this by one seller. The problem is that one he refers to as a 2" Gauge Clamp Thumb Nuts Aluminium 3BA Small 8mm x2. The other as a Smiths Instrument 2" Gauge Clamp Thumb Nuts Aluminium Small 8mm x6. I am reasonably certain that it is a metric thread as an 8-32 screw will thread in loose at first, then bind up on the last thread or so in the nut. Now, can any one confirm the thread size or explain the difference between an 3BA Small 8mm x2 and a 8mm x6. I have never seen any threads specified this way, One of the usuals has what they refer to as a Smiths Instrument 2" Gauge Clamp Thumb Nut, but they do not state the thread size. I could buy one of each from the suppliers hoping to get one to fit (rather costly) or just force an 8-32 nut over the stud and call it done! Any help or comments?
Not sure on the threading. It’s likely a British thread though. Have you tried writing to one of the gauge specialists like Nisonger? Likely a more official source than an Ebay seller.
I’m trying to imagine where it possibly could have disappeared to. It’s not like the interior has a sump. Did you try pulling out the lower dash casing panels?
Google is your friend on this one.
https://maccmodels.co.uk/ba-size-chart/
Do you by any chance have a thread gauge?
If not, take a known screw, like an 8-32 and use it to compare against the stud you have.
I could only wish that something like this was the largest problem in my life. I really don’t have many, but even if my house burned down, it would only merit a shrug of the shoulder. Obliviousness should be more common.
FWIW there was a metric infusion into Smiths when they purchased the British branch of the French company Jaeger, which made chronometric speedometers. IIRC some Smiths instruments do contain metric components traceable to this late 1920s acquisition. I don’t know if these thumb nuts are among them.
Hrm if true this would bring the number of fastener standards on these cars to what like 5?
Rather than try to figure out a screw size that matches the knob, why don’t you take one of the remaining knurled knobs to a hardware store or Home Depot and check it against their thread displays? Then you’d be sure what you’re looking for.
Sorry, the 3bA a thread size you will not find in Home Depot. You’ll find metric and SAE but British Association… not so much.
Hi Russell,
I’ve not actually measured the thread, but if a 8-32 female starts loose and then jams, the thread will most likely be a 3BA. The Major Diameter of the 8-32 is 0.0026" larger than the 3BA. For a thread that has a Major Diameter of 0.1614" (3BA), 0.0026 is a significant percentage.
The difference in the Lead of the thread between 3BA and 8-32 is 0.0025"; also a significant percentage when the Lead of the 3BA thread is only 0.02874".That and the difference in the angle of the Thread Form (47.5 compared to 55 deg - BA. UNC respectively) makes forcing a 8-32 Thumb Nut on is not a good solution in my opinion.
If you can’t buy the correct part, buying a 3BA tap and making your own would be a better solution than forcing an 8-32 nut on. If you don’t have access to a lathe, discs can be cut by chucking a close match OD material in a Pistol Drill to spin the material and a hacksaw to part off the disk. Thumb Nuts made in this manner won’t have a great finish on the faces, but a better solution than forcing on an 8-32.
Brent
True, but since there was questioning if it was metric or not, I figured this would resolve that question.
I’ve seen these advertised on eBay ……
Absolutely nothing metric in these cars. Most fasteners are SAE, but Smiths used British standards. The mounting nuts would be a BA size, likely BA3. Just for reference, the speedometer fittings are BSB, but are often confused for metric threads, which have similar pitch.
I’m sure the usuals carry the correct thumb nuts, otherwise try an instrument rebuilder.
3BA refers to a #3 British Association thread.
They are a magnificently weird thread, as they are based on percentages (9/10 of the previous) of a base thread that is 6mm 1.0.
So 0BA is 6mm 1.0, 1BA is 90 percent of that, 2BA 81% etc etc.
I read your ad as a thumb nut that is 8mm in diameter, 2mm thick with a 3BA thread. This is correct for the instrument studs.
I have a full set of BA taps and would happily make you the part if you can’t get it. The only problem is that I am in Australia.
I don’t have the clock back yet so I don’t have the threaded stud as a reference, although the other gauges are still there. I did take the one nut that I have left to a hardware store near me that had the thread gauges used for checking thread sizes. They had an 8-32 threaded rod that I tried the nut on that started but then bound up, so it has to be something different. I do believe now that it is a 3BA thread. Moss does have two. one designated as “NUT, thumb” for $2.79 and another as “NUT, speedo & tach” for $2.99. Neither one listed for Jaguar in their “where used” and neither gives any thread or any other specifications. Moss has a $10 min. order, so that is what I would end up paying + shipping and not knowing what I am getting. (sadly just placed an order with them a few days ago) I normally stay away from ebay but so far have not had a problem with them. My nut’s OD is .380" and thickness is .160". It does not match with either ebay listings. I will email Moss and hopefully get some better information. BTW. I was putting the parts in the ash tray so as not to misplace them and then forget where I put them. The nut that made its escape is likely under one of the seats. Tried fishing it out with a telescoping magnet not knowing they were aluminum. Not going to remove a seat when it will be under the other seat than the one I first remove. Since this seems to be an unclear topic, I will follow up with what my final results are.
For the record, the studs on the back of the Smiths CET and CTE clock have 4BA threads. I’ll include a replacement nut when I return the clock.
Available on eBay
Item number 284773458400
Except the spark plugs and tire widths.
I think this is what you need:
If it is, PM me your mailing address and I’ll send it to you.
Oops, I see Mike has you covered.
As for Erica’s wondering where it could go – I dropped one when the shift boot was off and found it months (and miles) later perched on the little ledge of the gearbox cover. Good thing that ledge was greasy.
Makes perfect sense!
Hey George ……they were built in Britain 
….actually in Oz we are quite conversant with metric,ba, bsf, npt, and bsp etc…… I’ve now got to the point of being able to do 0.001” conversions to metric in my head …comes in handy on the lathe or mill…… which I’m quite new to precision machining