PART NO.  SPL.PAGE  DESCRIPTION                                                           
ARG923    16.D.3    Screw -tank unit to tank (6) -(fuel tank, 1600) -was ACA5127

Screw for fuel tank sending unit to tank. #3BA x 1/2 (34.8 TPI)

-Moss Motors USA number 323-030 -- may be
#8-32-UNC (common) or #8-36-UNF (more rare).

Began as ACA5127 Hex Head Screw for MGA 1500.
Changed to ARG923 Slotted Pan Head Screw for MGA 1600.
Superseded by 53K165 Phillips Round Head Screw (after end of production)
This number 53K165 does not appear in any of the MGA Service Parts Lists.

On 3/15/2024, Bruce Hopgood in Montreal, Quebec, Canada wrote:
"Fasteners are a peeve of mine, and I really like to have these correct, no matter how banal the application. I thought I might share this with you. --- I am sorting a fuel tank, not a MGA unit but a direct contemporary. The car uses a same/similar sender unit. The vehicle IPL calls up the screws to hold the sender unit to the tank as #ARG923. The screws are a Hex Head, slot measure 1/4" across flats, and 0.600” long. A #8-32 nut threads on but is ‘loose’. Could this thread be BA?

"Apparently, vendors are not as precise as we would like. I did order some replacement screws #ARG923 from a UK vendor. I received a correct length, thread into the tank fine however have a Phillips Pozidriv raised/round head. These replacement screws also thread into the tank well, and a 8-32 nut spins on fine. -- Curious how the original part number ARG reference has morphed". -- Bruce

Comments from Barney:
First, Phillips and Pozidriv are different slot designs requiring different screw driver tools (but that's not the issue here).

I will go out on a limb here with an educated guess. I am inclined to believe that the MGA fuel tank and sender unit may fall into a certain class with some "non-MG" part suppliers in early post-war period. Think Auster, Austin, Lucas, Smiths/Jaeger, and SU. These "other" companies may not have been so eager to jump on the Unified thread standard, and continued producing components with other threads, Whitworth, BA, BSF, BSPP. This applies in particular to products that may have pre-war heritage continuing with only minor design changes in post-war period.

This leads me to believe that these threads in the MGA fuel tank were originally 3BA threads, 0.161" major diameter, 34.8 threads per inch, and 47.5 degree thread angle. A #8-32-UNC (or #8-36-UNF) thread is 0.003" larger major diameter, with less deep 60 degree thread angle. The very slightly larger #8 nut, being relatively thin, will screw right onto a 3BA screw, just a little loose. A #8 screw will not easily thread into a 3BA tapped hole. But the fit is close enough that someone may think it is just a "dirty" thread. If you run a #8 tap through the 2BA hole, the female thread will clean up enough to accept the #8 screw.

Those not so well informed may never know the difference. These days some parts suppliers may be offering #8 screws for these applications without ever mentioning that they are not original spec parts. So you may well just "clean out" the female threads and use the #8 screws.

The problem here is that if you subsequently try to install original issue 3BA screws into holes that have been cleaned out to #8 female threads, the 3BA screws will fit more loosely and may be at some increased risk of leaking or stripping out with too much torque. So if a #8 screw works easily in your fuel tank, then continue to use #8 screws, do not go back to the 3BA screws.

Do be informed, let the user be aware, and chose appropriately.