fuel tank sender unit.

[Ray White]

I have just taken delivery of a new stainless steel petrol tank and am in the process of transferring the low level sender unit from my old tank. The switch has a bakelite thumb screw which looks as if it should just screw off but it seems to be resisting ... and I don't want to force anything....it may be slightly bent...yes I have now removed it. I think there should be a collar piece that it screws onto but that is missing or broken off.

The six flange screws I am told are 5/32 "Whitworth but they are corroded and just break off. Fortunately the new tank comes with new screws which I assume are metric equivalent.

The sender unit is original and therefore has just three screws attaching the switch cover. I don't know if these are 3 or 4 BA ? I am also unsure how petrol could leak from there?

Any advice would be appreciated.

Thank you,

Ray.

[Duncan M]

I have the original 3 screw cover unit also. Works just fine. You need a good seal system on that 3 screw cover, because the unit is designed to fill with fuel. I use a cork gasket coated all over with Hylomar.

[Ray White]

The three screws on the switch are round slotted and the six screws on the flange are also slotted but hex head. I am given to believe they have different threads. 3 BA perhaps?

[Duncan M]

The three brass slotted cheesehead screws thread into blind holes. Looks like I got by with using hylomar coated thick gasket paper. As these screws thread into blind holes, I used a light coat of thread dressing.

The six hex head slotted brass screws do not go into blind holes, so I cleaned them up and dressed the threads in hylomar. For the large round gasket I used cork, coated in hylomar.

[Steve Simmons]

There's an improved, modern rubber seal for the sender to tank junction. From The Frame Up sells them, and possibly others.

[Ray White]

The three brass slotted cheesehead screws thread into blind holes. Looks like I got by with using hylomar coated thick gasket paper. As these screws thread into blind holes, I used a light coat of thread dressing.

The six hex head slotted brass screws do not go into blind holes, so I cleaned them up and dressed the threads in hylomar. For the large round gasket I used cork, coated in hylomar.

Just about in the picture is the collar that I am missing. I will need something to tighten the supply wire onto. Not quite sure how to do it yet.

[Rob Reilly]

Be absolutely sure on your screw threads. The originals were 3BA on the flange and 4BA on the cover.

This one had a broken wire, so I unwrapped one turn and re-soldered it.

I don't trust reproduction parts, I always double check them for errors like metric or SAE threads.
A lot of people have never heard of the BA thread system.

There is no seal on the float axle, so the chamber fills up with fuel, which is ok since there is no air and thus no chance of combustion in there.
The original flange and cover gaskets may have been cork or thick paper.
In the past I've used both, but now I use fuel resistant rubber.

Of possible academic interest, here is one for a Jaguar, which has two terminals. The second terminal is an on/off switch which connects to a low level light on the fuel gauge.

[Duncan M]

The first picture Rob posted is an interesting reproduction part, as it has the three screw cover lid. In the original TC sending unit there is no wound resistance wire behind the cover. The resistance wire in the orignal setup is wound on the back of the fuel level dash light.

On the original TC units, the area behind the 3 screw cover does fill up with fuel, and is where electrical contact is made. The reason for the resistance wire windings in the circuit is so when the contact is made behind the cover (to turn on the light) only a very small amount of current is in the circuit. When removing the 3 cover lid on an original unit, you must be certain the fuel level in the tank is nearly empty, or fuel will spill out

[Rob Reilly]

Actually it is not a reproduction, it is the original from a 1938 SS Jaguar saloon. The coiled resistance wire is how the fuel level is sensed. I just included it to help the discussion along. I assumed, apparently incorrectly, that it was pretty much the same as that of the TC. I was not aware that there was a version without a coiled resistance wire.

So if I understand correctly, in a TC you just have a warning light, no level gauge to indicate the amount in the tank?

Well, could be worse, in the TA I will have only a dipstick.

The second sender with the 4 screw cover is the original from a 1950 Jaguar.

[Duncan M]

Rob-
The sender and float mechanism in a TC do tell you fuel level as it gets low. When the light starts blinking, you have around an hour of driving time. When the light stops blinking and is always on, you have less than an hour to find a fueling station.

For those obsessed with numbers, the warning light can be used in combination with the trip meter or odometer. The light is on with about 2 US gallons left in the tank, which means you can drive around 50 miles before re-fueling.

Because of the resistance windings on the fuel level warning light of a stock TC, the bulb used is the same as what is used in a 2-cell old fashioned flashlight. 2.5V 0.5W, which is the same as the ignition warning light bulb for the TC. The TA uses a 2.5V 0.2W bulb for the stock ignition warning light bulb.

https://ttypes.org/one-of-lifes-little-mysteries/

[JonnyP]

What should it look like inside? Mine is an original 3 screw unit, but it doesn’t work (the light is on permanently). I’d like to fix it ultimately, when I’ve worked a bit further down the list of jobs that need doing…. I’d rather repair it than fit a cheap copy version.

[Ray White]

I think we should take care with the screw threads as there is much confusion about what was originally used. Modern replacement tanks and sender units have threads which are probably not the same as original.

The six flange screws - as originally used - are 5/16 BSW (British Standard Whitworth) - NOT 2 BA as many people think. This is a scarce thread these days and as 2BA is almost identical it is often used as a replacement. The reason many people think it's 2BA is because the pitch is almost identical (0.794mm W compared to 0.81 BA).

Another thread that (mostly Americans) mistakenly believe it to be is 8/32 UNC which is even closer. In fact the thread is so close - only 0.02mm larger diameter, with the same pitch - you can screw an 8/32 UNC into the flange without much effort. These are the 'incorrect' screws that Moss used to supply.

It is suspected that the historical reason for the tank flange threads being 5/32" is that it would have been standard practice for British petrol tank manufacturers back then to be using mainly BSW, BSF and BSP threads.

The cover screws are indeed 4 BA; an entirely Smiths design.

Incidentally, anyone with a Mecano set will be familiar with 5/32" BSW.

Hope this helps.

Ray.

[Rob Reilly]

Ray, I was wondering why you said 5/16" Whitworth. You perhaps meant 5/32" BSW?
I was not aware that the BSW thread system charts included anything that small.
My BSW chart starts at 1/4"-20 BSW as the smallest.
My BSF chart shows a 3/16"-32 BSF and a 7/32"-28 BSF, but nothing smaller.
For anything smaller I have to look on the BA chart.

Jaguars have 3BA threads in the gas tank senders, so I thought MG might be the same, but apparently I was mistaken.

I found some pictures of TC dashboards on the web. I see now that you do not have a gauge with a needle. I somehow thought maybe there was a needle gauge on some cars.

[Ray White]

Yes Rob...it was a typo.

[Duncan M]

Johnny, maybe your sender has got stuck in the ON position from the float being full of fuel?

[JonnyP]

Is there a way of emptying/repairing the original float if it has failed?

[cdrolshagen]

Hi Jonny,
yes you can, I used the part,
https://www.snydersantiqueauto.com/prod ... &cat=41706

[JonnyP]

Brilliant, thanks.

[Rob Reilly]

Moss has this brass float.
360-646

[Ray White]

I found the original float had gone rusty and leaked where the arm joins it.

I made a solder repair rather than buying a new one. How long it will last with ethanol petrol is anyone's guess.