Restoring the BT-13 Type Fuel Quantity Gage

The Type 122 Fuel Gage installed on a Stearman model 75.

Stearman model 75 aircraft typically have one of two types of fuel gages installed, the most common being the straight wire with an indicator within a tube, and the remainder being the round-faced BT-13 type gage.

The Consolidated Vultee BT-13 manual calls this the Type 122-150-A fuel gage, or just the Type 122 gage. The Type 122 gage was found originally on the Vultee BT-13 aircraft, and was also installed originally on some Boeing Stearman model 76 aircraft.

I have restored at least one Type 122 gage which had the Vultee emblem embossed on one side of the gage face, while having the Stearman logo embossed on the reverse!

Typically, however, the Type 122 gage was installed as a conversion when the surplus BT-13 gages were cheap and plentiful.

How the Gage Works

The Type 122 gage has a floating cork that operates a lever, which in turn operates a magnetic shaft, by means of a gear and pinion. The magnetic shaft actuates the dial pointer on the dry side of a solid aluminum diaphragm.

Photo 1. The gage is disassembled, in an upside-down orientation. From the top:

- The lamp base
- Face retaining ring
- Dial face
- Pointer
- Body casting
- Mechanism head, with float wire and float attached.

Missing is the instrument glass.

Photo number 1 shows the gage in its disassembled component state after cleaning, but before the dial face is restored.

The conversion from BT-13 configuration requires a reduction in length of the float wire by a few inches, and a change to the dial markings for the Stearman application. The Stearman tank also has to have the appropriate flange mounting welded on the tank bottom in place of the smaller fitting for the tube-type gage. The gasket surface is shown in Photo number 2.

Photo 2: The mating flange area of the Type 122 Gage

The BT-13 gages come in left and right orientations (for their original 2-tank set up), and either can be used for the Stearman, the difference being whether the dial reads on the eastern or western half of the face.

The BT-13 orientation had the mounting flange of the gage in the vertical plane, while the Stearman requires the mounting flange to be in the horizontal plane.

I use the BT-13 Erection and Maintenance manual as a guide in restoring these gages. However, it's not nearly as straightforward now as it was 60 years ago when the units were new and relatively un-corroded. I've seen one gage where the cast aluminum that is normally about 3/16's of an inch thick at the bottom, corroded right through. There will be more on this corrosion later.

In addition to the photos of the restoration of one gage, I'll paraphrase the BT-13 manual's overhaul instructions, along with some of my own notes.

Removal and Disassembly

1. The gage is held on to the tank with six AN500, or AN501 drilled-head screws, with lock washers, and sealed with a gasket on the mating surface.
2. The body casting is removed from the mechanism head by unscrewing four 8-32 screws and lock washers.
3. The screws should be removed at average room temperature. If these screws are frozen, I use a Dremel-type cutoff wheel and remove the screw head. With a little luck the remaining portion of the screw can be removed when the head unit is off. However, I've had to drill out more than one screw, and drill the head unit and tap the body out to 10-32. In Photo number 3, if you look closely, you can see that two of the 4 inner holes are larger than the others. Helicoil thread repair inserts can also be used to repair damaged threads here.
4. If the head mechanism is damaged and you have spare parts, the geared mechanism can be removed by filing the flare from one end of the gear axle, and using a pilot punch to tap it out. If you have no spare parts, leave the gear assembly alone. If it is stuck, a soak in penetrating oil and some gentle exercise usually cleans and frees up the gear assembly. Just remember to not force any movement.
5. If the float arm is to be replaced, it can be removed from the gear sleeve by placing the sleeve in a vice, and tapping with a pilot punch. This should be done after a good long soak in penetrating oil, preferably warmed. The BT-13 arm is one straight piece.
6. The orientation of the magnet and indicator does not need to be altered from the BT-13 position for the Stearman. However, if you do need to repair the miter gear and magnet orientation, the manual says to proceed as in steps 7 - 8.
7. To remove the miter gear, break (heat and remove) the solder and loosen the setscrew in the collar of the miter gear, and withdraw the shaft sleeve from the gear.
8. Drive the taper pin out of the lower gear. Loosen the spline screw in the limit stop. Hold the bottom gear and withdraw the shaft from the top. To remove the miter gear from the top of the shaft, drive the taper pin out of the gear, and tap the gear from the shaft.
9. You can now remove the lamp assembly, if it still exists, at the base of the housing by removing the two little screws and lock washers. If you are lucky, the screws will not be frozen, and you can utilize the illumination feature. Many just ignore this part on restoration due to the small screws binding in the housing, and the lamp plate being long gone.
10. The dial retaining glass (if it's still there!) can be removed with a screwdriver wedged beneath the brass retaining ring located in the groove of the housing.
11. The pointer can now be lifted from its pivot pin, and the two screws holding the dial plate/pivot plate can be removed. Don't try to lift the dial by the pivot pin! If the dial face won't come up easily, try working a piece of safety wire behind it to break any corrosive seal that has set up. You don't want to ruin the flatness of the dial, or the pivot pin's integrity.

Cleaning and re-assembly.

Repairs should be left to replacing parts that have worn or broken, removing broken screws, re-tapping holes and so forth. If the housing has a hole from corrosion, or a crack, I think I'd reject the whole part.

Photo 3:The polished gage casing, showing the drain hole and lamp housing.

Some of the head castings came without drain holes. Photo number 3 shows where the 1/8 NPT threaded hole is on this unit. I added a Curtis drain valve here to permit fuel, water, and sediment to be drained at the same time as the other sump drains. This will help immeasurably in preventing the casting from corroding due to trapped water in the fuel.

Putting it all back together:

1. New glass. A local glass company cut one for me out of 1/8" thick material for $10.
2. Clean and re-spray the dial face using a flat or semi-gloss instrument black. Press-apply numbers were then used for the markings, and the whole thing clear-coated so the numbers won't come off. You can find the vinyl numbers and dash markings at art supply stores and hobby stores. Water-slide decals are also available from the same sources, as well as aviation suppliers for marking the outer glass on instruments. The clear coat should not be a gloss finish, but matt. If you are not using a decal system from one supplier, test the combination of paint-decal-coating on another surface for compatibility. There are kits available today that permit you to create your own water-slide decals using your inexpensive ink-jet printer to print the decal. One such system is the "Super Cal" system. These systems provide their own final spray coating. Remember, the face of the gage is dry, and does not need to be fuel-proof.
3. Using a nice flat slab of marble, I made sure the flange mating surface was flat with a good rub with emery cloth of increasing fineness.
4. I chose to polish the housing, but it could be painted.
5. I clean the mechanism by soaking in penetrating oil for several days. I then use a good wash in MEK, and generous brushing with a small stainless steel brush and compressed air to finish up. The removes old fuel varnish buildup. The whole mechanism gets some lubrication from living in avgas, so I just make sure there is no binding in the gears, and everything is free to move. Add no oil or lubricant on re-assembly.
6. Make sure the indicator spins freely on its pivot. I painted the needle in the photo at the top of this page, with AN Orange in polyurethane. This indicator must be scrupulously clean or the meager amount of magnetism involved won't budge it.
7. Seal up the dial chamber with a small bead of 3M Windshield Sealant behind the glass face, then place the glass and snap-ring on. This sealant is superior to the old asphalt, and available at most auto stores. Once it sets up, you can clean off any residue with acetone. Do NOT try cleaning excess sealant while it is un-cured.
8. Before final assembly, fit the gage in the tank to adjust the float wire so the gage reads correctly at the empty position. If you are working with an unmodified BT-13 gage, you can then measure the height from the bottom of the tank to the top, to determine the optimum float wire length. Make sure to do this before final assembly. Each tank varies a bit depending upon dents and dings gathered over the years, sometimes by as much as an inch or more. Adjustments can be made by bending the float wire using 2 pairs of pliers. Float wires can also be lengthened by splicing in a length of appropriate diameter aluminum tubing and staking each end to the existing wire.
9. Putting the parts together is the reverse of disassembly, and straightforward. I did however use some safety wire on the screws holding the mechanism to the body rather than rely on lock-washers.
10. I made a new gasket using some gasket material with adhesive on one side.
I made sure the fit on the screw holes was very tight, so that you actually have to screw through the gasket.

Finally all the screws are safety-ed up, and Photo number 4 of the finished item shows the completed gage. It actually works too!

The gage can be re-calibrated at any time by removing the dial glass, and marking the indicator plate after verifying the tank quantities on first fill-up.

Since the gage was standard fitment on some model 75's, you should need to field approval for the installation. Good luck with yours.

Photo 4:A completed gage in zinc chromate primer. Note the removed lamp housing.

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