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MG TD TF 1500 - SA Bolt and Nut on a TD?

I am just about finished with my fastener list for the TD. Today I came across a bit of an oddity. I went to measure the nut that holds the pull started switch to the firewall bracket. The nut head is 9/16 SAE. I figured this was not correct, so I checked the threads of the starter switch. They are SAE.

This switch is a Lucas switch and is dated 8-58. So I figured it was originally for the MGA.

Well, I also have a switch dated 8-49. This is the TC type that has two bolts insteas of studs. The bolts are 9/16 SAE head and 5/16 -24 thread. I tried comprable whitworth and metric bolts and neither will go into the holes.

Here is a link to the pictures.

https://photos.app.goo.gl/Rnhxq5CC5CCrdCSg9
Bruce Cunha

Whitworth and BSF officially became obsolete in 1948 following an agreement between the UK, US, and Canada to standardise on the American UNC and UNF fasteners for future use.

I don't know but I guess the first couple of years production of TCs up to this date would have had BSF threads from suppliers like Lucas then changed to UNF

BSF is 22tpi and has a larger head size
5/16" x 24 is as you say 5/16" (SAE-fine) -UNF
The spanner size of 9/16 is the measurement over two oposite sides of the head of the bolt--Metric spanners use this method as well but Whitworth spanner size goes by the diameter of the bolt
So a 5/16" Whit bolt has a 5/16" shank and a head about 15mm accross and a 15mm spanner will fit but a proper Whit spanner will be marked 5/16" Whit
The closest metric bolt would be 8x1mm pitch but that's 25.4 tpi
Whitworth (or UNC) is SAEcourse so won't fit either

willy
William Revit

We were still using Whitworth and BSF at least until the back end of the 1950s, hence they are dominant on the TF. Fortunately sense prevailed after a few years with UNF and UNC when we moved to metric sizes. You will only find the older generation using anything based on imperial dimensions in the UK. The big exception is probably kPa and MPa which are a lot less user friendly that psi and Bar.
Dave H
Dave Hill

Just to clarify, since this is a T-Series forum, I suspect when you say TC you mean mga twin cam? The MG TC uses a completely different starter and switch than TD or MGA.
Steve Simmons

Down here we get it all from everywhere and soon become thread experts out of necesity
we had SAE fine and course from the US
BSF etc from Eng and of course the SAE stuff from Can. with Whit. head sizes
It all sort of settled down a bit with UNF-UNC, then we have metric--
People get confused with metric calling it fine and course metric but it's easy really and variable to suit the job, it's just measure and check what you've got, an example could be say an 8mm bolt but that could be .75-1-1.25-1.5--mm pitch, it's just a matter of checking what you've got
'Most' manufacturers have standardised now which is good but there are still traps
Toyota for example seem to run a finer pitch bolt than most other Jap. makers.
And then there are head sizes
All theJapanese makers use 10-12-14-16-17-19mm heads while Eng. seems to stick to the older White sizes roughly at 11-13-15-19mm--------------why
The other thing that peeees me off is when someone like Kia has a gread row of bolts around a bellhousing ,all the same length and diameter and do the top half a different pitch to the bottom half------------again why
If you work in a multi vehicle franchise it's a real pain
willy
William Revit

Thanks Steve. I was not aware the MG TC used a different starter. So as this is dated 8 49. It could be a variety of British cars. Perhaps the MG Y?

Anyone have a lucas part list that can identify this starter switch? It has bolts to affix the wires instead of studs, but otherwise is exactly like the early TD starter.

Willy. I tested both the bolt and the socket it threads into with metric, whitworth and SAE taps and dies. Only the SAE fit.

So lucas was making at lease some of their parts with SAE threads. The thread on the switch that affixes it to the bracket is is 3/8 and the nut fits a 9/16 wrench
Bruce Cunha

This thread was discussed between 25/08/2019 and 26/08/2019

MG TD TF 1500 index

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