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MG TD TF 1500 - Figuring BA screw size

I am trying to figure out the size of the screws that hold the cover on the fuel sending unit.

The top screw terminal has an 8 BA nut on it, but the screws that hold the plate on appear to be a different thread.

Whats the correct way of determining the size of a small machine screw? I don't have taps that go that small.
Bruce Cunha

I don't know an easy way to check the sizes of BA fasteners.

However, I believe the fasteners that you need are 3BA. See the Archive thread:
Moss Fuel Sending Unit Screw Error

This link to the thread may work:
https://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&access=&mode=archiveth&subject=8&subjectar=8&thread=2014081714332016927

From the Frame Up and Moss sell a fastener set for "original tanks"

FTFU -
# FSAF316, "Sending unit fastener set Sending unit to fuel tank." (6 pieces) $14.65

Moss Motors -
#323-418, "SCREW SET, sending unit to tank (6 screws)", Original Tanks, $25.99

Moss also sells a 6-screw set for "Replacement Tanks", $2.79. Probably has 10-32 fasteners.

I bought the Moss ##323-418 set. The fasteners appear to fit correctly. I haven't installed the unit or mounted the tank yet.

Lonnie
TF7211
LM Cook

I posted the wrong link to the thread: "Moss Fuel Sending Unit Screw Error"

Here is the correct link:
https://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&access=&mode=archiveth&subject=8&subjectar=8&thread=2012060301280032230

Lonnie
TF7211
LM Cook

Thanks Lonnie, but not that screw. I have those on my list and picked up a set from Doug at FTFU.

I am talking about the one on the metal cover. Originals are brass, but I don't know the size. They are smaller than the screws that hold the unit to the tank, but appear larger than the screw used for the electrical connection. The nut on that is 8BA.

I don't need them, Just wanted to get the size correct to add to my TD fastener list.



Bruce Cunha

Micrometer and thread pitch gauge
(they could be metric)
J Stone

I have not seen the size of the four screws confirmed by a reputable source. You may be the first to define the size of the fasteners.

I just now measured the screws in my TF sending unit that match the screws in your photo. They are 1/4" long cheese head screws 0.1375" diameter. TPI is near 40.

Screw sizes near these dimensions:
5BA (0.1260 diam; 43tpi)
M3.5 ISO Metric (0.1378 diam; 42.3tpi)
6-40 UNF (0.1380 diam; 40tpi)
4BA (0.1417 diam; 38.5tpi)

=============

LINKS:

Standard Threads - Dave DuBois (PDF of original Excel)
https://www.dbraun99.com/Dave%20DuBois/Standard_Threads.pdf
[This link Dave DuBois' document may not work from the BBS. If not, then copy and paste to your browser.]

A Compilation of Thread Size Information (Excel)
https://journeymans-workshop.uk/images/downloads/thread.ods

A Compilation of Thread Size Information (PDF)
https://journeymans-workshop.uk/images/downloads/thread.pdf

Theoretical BA sizes (scroll to bottom)
https://www.ba-bolts.co.uk/faq.html

Lonnie
TF7211
LM Cook

Thanks Lonnie.

So close to UNF and Metric.
Bruce Cunha

I finally got enough time to go out and check a sender that I have, an OEM
1951 TD #6554
The screws are brass, and cheese head

.OD= .1385 (micrometer)
just under the head where the threads are not cut it is .1395

A 38 tpi thread gauge locks right on-but, as it is only 1/4" long it's possible that would not show up 38-1/4 tpi is pretty close to 38 (This is with 5x magnification)
So for my money for that one is #4BA (.140x38-1/2 tpi)

(A newer version of that sender could be different)

A #6 uns is OD .138 - 36 tpi, that definitely does not gauge correctly
Although you could probably get a 6x36 in that hole, hardly noticing it was incorrect.

None of the metric thread gauges I have work (.60, .65, .70 etc ) Pretty much rules out the 3.5 mm screw
J Stone

Thanks J. I got 3.51 mm. Yours converts to 3.543

I believe you have it at 4 BA.

I found a 4 BA nut today. I will try it on one of the screws to confirm.
Bruce Cunha

Well... I rounded up on the thread od, more accurately it was .1383" so we are getting close.
<snicker>
J Stone

This thread was discussed between 29/01/2022 and 01/02/2022

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