If you've been following the '51 Chevy preservation saga, we've covered several projects from paint refurbishing to mechanics (see www.southernwheels.com, click Archives), and are now finishing up with a fuel gauge repair, completing our list to make this old Chevy a dependable survivor driver.
        The problem with the gas gauge was that when the ignition switch was on, the dash unit pointer went over past Full.  A new tank had been installed by the previous owner and we weren't sure how he installed it, so we wanted to take it off and check it out.  We thought the trouble might just be a bad ground, from the tank to the frame.  Sometimes this ground is overlooked when installing a new tank.  For a quick test, we used a spring clip jumper wire, putting one lead on the tank edge and grounding the other end to a clean place on the cars frame.  This didn't fix the problem, leading us to other tests to determine if the trouble was in the dash unit, the wiring, or the tank unit.
        With the car on the the lift, and the battery disconnected, we put a transmission jack under the tank with a movers blanket protecting it from being scratched up, and unhooked the gas line, wire, and unbolted the two straps holding up the tank, allowing us to lower it.  The gas neck filler hose just didnt look right to us, so we removed it for inspection.  It turned out to be a radiator hose  Over time, gasoline will disintegrate a radiator hose.  A fuel-specific hose must be used for the type of fuel that will be going through it.  Gas hose has different characteristics from diesel hose, etc.  The original '51 filler neck rubber hose was straight going from the tank to the filler neck, and replacements are readily available from Chevy parts suppliers such as The Filling Station*.  Unfortunately, we had a replacement tank with a neck that came out at a 30-degree angle from the tank, requiring a shaped  or flexed rubber gas hose.  Have you tried to find a flex gasfiller neck hose lately?  Its not easy!  After exhausting all of the parts stores, I had to order a 1 1/2x 30flex hose from Canada*!
        While the hose was being shipped, we removed the tank and sending unit, to test them to find out what the problem was.  THE AC FUEL GAUGE as used on GM cars:  The gauge consists of two units:  (1) The dash unit and (2) The tank unit which is in the fuel tank.  These two units are connected by a wire and each unit is grounded.  The dash unit consists of two coils, 90 degrees apart, with an armature and a pointer at the intersection of the coil axis.  A stabilizer dampener is provided on the armature to prevent vibration on rough roads.  The tank unit is a housing that encloses a rheostat or resistance unit with a brush that contacts the resistance unit.  The brush is actuated by the float arm whose movement is controlled by the level of fuel in the tank. Variations in the fuel level change the value of the unit coils, so that the dash pointer shows the fuel level in the tank.
        GAUGE SERVICE:        If the gauge is not accurate in showing the correct amount of fuel in the tank, check to find the trouble by using an extrasending unit in good working order as a tester.  Youll need a 5-foot colored test wire with spring clips on each end.  Connect one end of the wire to the binding post of the test sending unit (you'll hook the other end up later).  Next, lay out a black test wire with spring clips on each end to use as a grounding wire, and you're ready to make your test.
 1. Turn off the ignition switch.
2. Disconnect the negative battery cable
3. Disconnect the wire on the dash unit that leads to the tank unit.
4. With your spring clip, connect the other end of the colored test wire from the test sending  unit to the dash unit post where the wire was removed.
5. Connect the black wire to the tester base and to any ground
6. Turn on the ignition switch, reconnect the negative battery cable, move the tester up and down.  
If the dash unit is okay, the pointer will move from Empty to Full.  If the pointer doesn't move, or it moves only part way, the dash unit is bad and needs to be rebuilt or replaced.
1. Turn off the ignition switch
2. Disconnect the negative battery cable
3. Reconnect the wire to the dash unit.
4. Follow the wire from the tank unit to the bayonet connection or the terminal junction block.  (This is if you have a junction box.  Our '51 doesn't.  Our wire runs uninterrupted from the tank unit to the dash gauge.)  Disconnect  the tank wire at the junction box and clean the contacts.
5. Attach the colored wire from the test sending unit to the end of the wire that runs to the instrument panel.
6. Attach the black tester wire to the car frame for a ground.
7. Connect the battery cable and turn on the ignition switch.  Move the arm of the tester up and         down.  If the wire from the gauge to the junction box is okay, the dash units pointer will move from Empty to Full freely.  If the pointer does not move or only partially moves, the problem is with the wire that runs from the dash unit to the junction box.
8. If the pointer moves correctly, then the problem is in the tank unit or in the wire that runs from the bayonet connection or terminal junction block to the tank unit.  Check this to tankwire for continuity using a multimeter.   If the wire to the tank is good, with clean, tight connections, then the tank sending unit is bad and will have to be rebuilt or replaced. above is the mechanical way of testing.  It is also possible to check the units with a multimeter.)
Our test showed the problem to be in the sending unit.  We bought a new one and this time the tests showed the dash gauge pointer moving correctly from Empty to Full as we swung the new tank sending units arm up and down.  We carefully installed the sender back into the tank, being careful to position it so it wouldnt bind up inside of the tank as the arm moves up and down.  Another problem with the previous owners installation, was that he ran new gas line from the tank to the fuel pump using brake line instead of gas line.  Brake line has a smaller inside diameter than gas line, and can cause vapor lock.  We bought a 25-foot roll of 5/16" gas line and some gas line nuts from Napa, and ran a new line.  With the gas gauge working, we  were ready to put everything back together.  Our new gas filler  neck hose was 1 1/2"ID end to end.  This was a slight problem because the neck on the tank was 1 1/2" and the filler neck was 1 1/4" but, with a good set of stainless hose clamps,  the filler neck rubber hose clamped right down.  When buying stainless clamps, it is important to get an all-stainless clamp.  Some are sold with a stainless screw and steel roller.  These won't stand up to as much torque when tightening them down, as compared to the all-stainless ones.  To button everything up, we hooked up our dash wire at the dash and tank, putting a rubber cap over the tank post to keep it from shorting out on the trunk pan, connected the fuel line and gas filler neck, then raised our jack and put the tank straps in place, tightening them up.  We ran a #12 ground wire from one of the sending unit mounting screws to a clean place on the car's frame.  Now, to test, we filled the tank and watched the dash gauge pointer slowly go up and stop at Full, as it was designed to do!  The last thing before regularly driving the car was to check the gas cap.  The '51's had a restricted-vented gas cap and ours was correct.  
We are now regularly driving the old Fleetline, and its nice and comforting to see the gas gauge working well and giving an accurate reading, allowing us to enjoy the drive without worrying about running out of gas!
Enjoy your cars this holiday season, and keep 'em driving!
 *Station Chevy Parts, 800-841-6622, www.fillingstation.com
 *Flex Gas Hose available at 888-530-7378 or www.ineedparts.com