Fuel Tank - Sending Units

11 Oct 96, from Cleo Bay:

When I was rebuilding my XK140 and E-type, both of the metal floats on the gas tank sending units were leaky. The electrical parts were fine. I first tried on the XK140 a new reproduction sending unit. It leaked. Nothing seems worse than gas on a tailpipe (except for the possible fire!) I could only keep the thing from leaking about 3 months at a time and it never gave warning before it started. I got rather tired of this! I finally decided to try and fix the original float without soldering. (I was afraid of things going boom). Anyway, I had some leftover gas tank sealer which was used to coat the floats once I had the fluid out of the floats. It worked! The sealer has been keeping gasoline out of the float as well as keeping air in. The original unit body does not leak and has not for over four years. I have used this repair method on three other cars, all of which have not reported any problems.

As an after the fact note on the reproduction sending unit, I did manage to get the one on my crankcase oil pan on my XK120 to seal up properly. I tightened up the electrical fitting and sealed the small access cover with no. 1 Permatex. The reproduction units are definitely not bolt-up!

11 Feb 97, from Dick White:

The 150 fuel gauge sender has two functions - 1) to provide a variable resistance for the fuel gauge to indicate fuel level and 2) close a switch to light the fuel low light. There are only two wires are attached to the sender - one for the gauge and one for the light. The circuit for both functions is completed through chassis ground (not a good method in my opinion). Inside the sender is a copper wound coil providing the resistance. There are two wipers, one on each side of the coil, which are moved, in unison, by the float. The wipers are electrically and mechanically tied to the float. They are grounded by the bushing to the case of the unit. When the tank is full the wipers are at their highest resistance position (lowest physical position) providing approx. 100 ohms resistance. As the fuel level drops the wipers move closer to the 'ground' end of the coil lowering the resistance. As the wipers approach the top, one of the wipers strikes a tab which lifts it off the coil and completes the electrical circuit for the low fuel light.

My sender was showing an intermittent open as it traveled from top to bottom. I opened it up expecting to find crud on the coil and a need for cleaning. This wasn't the case. The electrical contact between the wipers and coil was good but the electrical connection between the wipers and the case was intermittent. I fixed it by applying WD-40 to the bushing and working the float lever up and down. While apart I also cleaned the wipers and coil with Radio Shack Tuner Cleaner/Lubricant and cleaned the tab which functions as switch for the light with a small burnishing tool.

21 Oct 97, from Don McClellan:

Hi Dick... another trick I have used dozens of times (auto restoration-electrical) is to use some extra flexible wire (fine strands) between the case of the sender unit and the float arm or shaft. If the case is aluminum (ie Jags), a connector terminal and machine screw can be used to secure the wire to the case, or if it's steel, just solder it. Then the wire should be bent in a couple curly-q's(to provide flexibility) and soldered to the float arm (near the shaft). Works great and prevents intermittent connections to ground PERMANENTLY! Regards, Don McLellan

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Hi All, I have removed the gas tank sending unit to replace gasket. I have a very slow weep. Anyway, while I have it off I want to clean up the tapped holes which attach the unit to the tank. The only problem is I cannot find a tap in my tap and die set which matches the thread of the set screws which secure the sending unit to the tank. They appear to be a non standard size. Does anyone know what size they are? - Thanks, Neville Laing

Neville: Spending a few minutes with the Parts Book reveals that the tank "element unit" securing screws are Part No. AS.303/4H. This number shows up in the Standard Parts section under "Setscrews" as having a "3 BA" Thread. It also indicates that there were only two other "cheese head" screws that same size used elsewhere in the car (8 total). The alloy cars used 20, 1/4" countersunk "3 BA" thread screws on the "windscreen tapping plate"? I hope someone else on the list can translate "3 BA" into a more recognizable tap size for us. Good luck, Dick Cavicke

Neville, There are six screws holding on the sender unit. The original thread is not available in the US, I believe it is 8x36x1/4". I use a stainless steel fillister head 8x32x1/4" as a substitute. All you have to do is run a 8x32 tap into the holes, and they will accept the new screws. If you can't get SS at the local hardware store, you might be able to get brass, both will allow you to take things apart later if you ever need to. - Regards, Wray Schelin

I believe the screws are 5-40 (#5 screws with 40 threads per inch). I never could find a flat head brass replacement, but did find an allen head screw which fit correctly. You may be able to find a correct tap at an industrial supply house, like (if you're in the US) Enco, Grainger, or McMaster Carr. - Good Luck - Jag150

Assuming that they are 3BA (and I have not checked this), 3BA is 34.84 tpi and 0.1630" diameter (talk about standardisation!!). This happens to be a thread pitch of 0.73mm and diameter of 4.01mm, a standard 4mm screw has a pitch of 0.7mm, so a standard 4mm tap should do more good than harm. I got these numbers from Newnes Engineer's Reference Book 1951. Check the screw pitch against your 4mm tap before you put it in. - regards, Mike Morrin

Wray, Neville, One of the exasperating or amusing "charms" of the Jaguars from the transition period is the several different thread systems we must contend with. My FHC sports at least four, and possibly five or more if you get pedantic about it. They include: Whitworth, BA, Unified, Lucas' own spiral shapes, and some of the odd choices made by respectively Girling, Lockheed, Trico and Skinners Union. Although most can be easily bought from the UK, for the single car owner it is rarely worth it nor is the need frequent enough to stock all of the taps and dies for each of the variations, so sometimes a compromise is indeed warranted. However, forcing an 8-32 into a 34 pitch hole is to me too much of a "beat it to fit, paint it to match" approach. If you do not want to buy a modest set of the most common taps, a 4MM thread comes much closer to fill the 3BA thread form over the length of engagement than a -32 and it might even provide a more durable seal. - Regards, Klaus Nielsen

Klaus, No forcing required, the diameters are the same(.160"), only the threads are insignificantly different. I think there's confusion here, with the four smaller screws that hold on the small cover that is on the sender unit. I think Neville asked about the sender retainer screws, not the cover screws, which are something completely different. - Regards, Wray Schelin

Wray, I agree that forcing the slight (32 to 34+) difference will not make one whit of near term difference; but I just cannot bring myself to do it when better ways are at all possible. After all, we are talking about repairing/restoring rapidly diminishing resources and both BA tools and spare screws and nuts are still abundantly available from England. Just one guy's view......... - Still, best regards Klaus Nielsen

I have found the threads to be very close if not the same as 6-32 on the several XK tanks I have done... - Lee Eggers

Thanks to all wo have responded to my question.I have a 8nc -32 and 6nc-32 tap in my set, and as stated before, neither of them are correct. The 6--32 is a rattle fit.. After starting this discussion I got to admit that I do not think I have serious damage to the threads, the bigger problem is an accumulation of dirt in the bottom of the hole. I suspect the screws are bottoming out against the slug of dirt before pulling up on the sending unit. A can of WD40 and a needle may be all I need. - Thanks again Neville Laing

If you have any questions or comments send e-mail to: ted@jag-lovers.org

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