Overview:

The SBD's fuel system is relatively complex, with each wing having two internal fuel tanks (main and auxiliary) and the option for a droppable fuel tank, as seen in the diagram below. Naturally there are numerous pipes, pumps, and selector valves linking these tanks to the fuel line feeding the engine. In addition to rebuilding major structural components of the SBD's airframe, Pioneer Aero has also worked on the fuel system too. One aspect of this has involved the rebuild of the fuel sump selector valves. These ingenious devices helped ensure that any fuel in the SBD's main tanks was always able to reach the engine, regardless of the aircraft's attitude.

A diagram describing the SBD's fuel system layout. The sump selector valves for the left and righthand main fuel tanks are identified by callout numbers 12 and 14 respectively. (p.431 SBD-6 Erection & Maintenance Manual)
A diagram describing the SBD's fuel system layout. The sump selector valves for the left and righthand main fuel tanks are identified by callout numbers 12 and 14 respectively. (p.431 SBD-6 Erection & Maintenance Manual)

Fuel Sump Selector Valve:

The SBD features two fuel sump selector valves, one below each main fuel tank located in the right and lefthand sides of the wing center section (as shown in the image above). When the pilot chooses to run the engine from one of these tanks, its sump selector valve automatically regulates which of its two sumps (fore or aft) fuel drains from (see the diagram below).

As page 75 of the SBD-5 pilot's operating handbook describes: "...the attitude of the airplane determines whether the front or rear sump is used. These valves [also] prevent air from entering the system through uncovered outlets. The auxiliary tanks are not equipped with sump selector valves since they have single outlets."

Essentially, these valves ensure that fuel is sucked from whichever port of a given main fuel tank is lowest, no matter whether the aircraft is diving or climbing. The valve's internal mechanism is deceptively simple and controlled purely by gravity. Each valve features two roughly 2" diameter solid steel balls, one for each of the two sump openings. The balls roll either fore or aft to their respective lowest available position in the valve as aircraft attitude changes. The uppermost ball closes the uppermost port, while the lower of the two opens the lower port; an interconnect pin sliding between them ensures that at least one port is always open.

As described on page 436 of the SBD-5 Erection & Maintenance Manual, the "...sump selector fitting is located approximately midway between the two sump outlet castings on the bottom of each main fuel tank. This modified T-shaped unit, together with the fuel lines leading to it, is below the bottom surface of the wing center section and is readily reached by unfastening the fairing which covers the surface of the wing at that point."

Note: the auxiliary tanks (mounted in the outer wing panels) do not feature these valves since they have only one outlet and are designed for level flight operations only.

A schematic outlining the SBD's main and auxiliary tanks fuel system. Note that each of the two main tanks has its own fuel sump selector valve. (p.432 SBD-6 Erection & Maintenance Manual)
A schematic outlining the SBD's main and auxiliary tanks fuel system. Note that each of the two main tanks has its own fuel sump selector valve. (p.432 SBD-6 Erection & Maintenance Manual)
A close up taken from a wartime image of an SBD-5 which shows the underside of its wing. The red arrow is pointing to the formed, sheet metal fairing which covers the fuel sump selector valve and its associated pipework leading from the starboard main fuel tank. An identical setup is featured on the port wing as well.
A close up taken from a wartime image of an SBD-5 which shows the underside of its wing. The red arrow is pointing to the formed, sheet metal fairing which covers the fuel sump selector valve and its associated pipework leading from the starboard main fuel tank. An identical setup is featured on the port wing as well.

Valve Restoration:

The fuel sump selector valves refurbished for our Dauntless came from a donor airframe, as B-22's originals were corroded beyond airworthy repair. Pioneer disassembled, cleaned and inspected these components in November 2024, but as there was no immediate rush they had time to await the arrival of replacement seals, steel balls and retainer rings before finalizing valve overhaul. Sourcing replacement seals, as it happens, proved impossible, so new examples had to be made, and this proved quite challenging.

One of the fuel sump selector valves from the donor supply of parts during its disassembly. While there is a fair amount of detritus in the valve, all of the metal parts, save the steel balls, were refurbishable. (image via Pioneer Aero Ltd.)
One of the fuel sump selector valves from the donor supply of parts during its disassembly. While there is a fair amount of detritus in the valve, all of the metal parts, save the steel balls, were refurbishable. (image via Pioneer Aero Ltd.)
One of the fuel sump selector valves from the donor supply of parts. At the time this image was captured, it had been disassembled, cleaned and bead blasted ready for inspection. (image via Pioneer Aero Ltd.)
One of the fuel sump selector valves from the donor supply of parts. At the time this image was captured, it had been disassembled, cleaned and bead blasted ready for inspection. (image via Pioneer Aero Ltd.)

Some of the serviceable parts from the fuel sump selector valves following disassembly, cleaning and inspection. They were ready for anodizing and repainting at the time this image was captured. (image via Pioneer Aero Ltd.)

The outlet port fitting for one of the fuel sump selector valves after bead blasting and inspection. It was ready for anodizing and painting at the time this image was captured. (image via Pioneer Aero Ltd.)

Replacing the Seals:

A third party supplied the fuel sump selector valves to Douglas Aircraft, so the set of available Dauntless drawings does not include any supporting their remanufacture. While this was not a problem for the primary metal components, restorable examples being on hand, it was a different story for the seals. However, with great care, Pioneer was able to recreate the necessary fabrication drawings using the original seals and valve component dimensions as a guide. A seal manufacturing specialist was then able to take these drawings and run off several sets of replacement seals for the fuel sump selector valves.

Examples of the two custom seals fabricated for the fuel sump selector valves. The valve is not a Douglas part, so no drawings describing their dimensions were available in the set of SBD drawings on hand. So new drawings had to be created for the two seals based upon the original examples recovered from the valves and the groove dimensions the valve’s metal parts. The lefthand seal is a "double lip" seal (note the two steps) which the steel ball squeezes up against to shut off fuel flow from a given port; two rings of contact against the rubber ensure a positive seal. The righthand seal in the image is a chevron-type seal which sits in the top of selector valve's body. The convex 'v' ridge on the internal fitting presses into the seal's matching concave 'v', creating a seal between the two. (image via Pioneer Aero Ltd.)
Examples of the two custom seals fabricated for the fuel sump selector valves. The valve is not a Douglas part, so no drawings describing their dimensions were available in the set of SBD drawings on hand. So new drawings had to be created for the two seals based upon the original examples recovered from the valves and the groove dimensions the valve’s metal parts. The lefthand seal is a "double lip" seal (note the two steps) which the steel ball squeezes up against to shut off fuel flow from a given port; two rings of contact against the rubber ensure a positive seal. The righthand seal in the image is a chevron-type seal which sits in the top of selector valve's body. The convex 'v' ridge on the internal fitting presses into the seal's matching concave 'v', creating a seal between the two. (image via Pioneer Aero Ltd.)
An image showing how the steel ball mates up against the 'double-lip' seal inside the fuel sump selector valve to shut off the fuel flow from one end of the main fuel tank. The seal’s twin points of contact ensure a positive seal which, in turn, ensures the fuel flow is shut off from the given port. Each valve has two of these setups, one for each of the sump ports. Pioneer had to purchase brand new steel balls for the valves, as the originals were corroded. Note that the knurled, bare-metal ring the seal and ball sit atop is for display purposes only, and not part of the valve. (image via Pioneer Aero Ltd.)
An image showing how the steel ball mates up against the 'double-lip' seal inside the fuel sump selector valve to shut off the fuel flow from one end of the main fuel tank. The seal’s twin points of contact ensure a positive seal which, in turn, ensures the fuel flow is shut off from the given port. Each valve has two of these setups, one for each of the sump ports. Pioneer had to purchase brand new steel balls for the valves, as the originals were corroded. Note that the knurled, bare-metal ring the seal and ball sit atop is for display purposes only, and not part of the valve. (image via Pioneer Aero Ltd.)

Reassembly:

With new seals in hand, and having anodized and painted the metal components where needed, it was then possible for Pioneer's Allen House to return the SBD's fuel sump selector valves to airworthy condition. The valves will undergo bench-testing to ensure they work properly before being installed in the SBD.

The complete set of components for the two fuel sump selector valves ready to be assembled. The metal items are all from donor aircraft, except for the o-ring anti-compression rings and the balls. The seals, of course, are all newly-fabricated. (image via Pioneer Aero Ltd.)
The complete set of components for the two fuel sump selector valves ready to be assembled. The metal items are all from donor aircraft, except for the o-ring anti-compression rings and the balls. The seals, of course, are all newly-fabricated. (image via Pioneer Aero Ltd.)
This image depicts the beginning of the reassembly process for one of the fuel sump selector valves. The green fitting sits on the inside of the SBD wing center section’s lower skin. While it is not part of the valve itself, it connects directly to it. Sitting into the circular groove on the valve body’s exit port is the newly-fabricated chevron seal that the unpainted 'v' face on the green fitting presses against. (image via Pioneer Aero Ltd.)
This image depicts the beginning of the reassembly process for one of the fuel sump selector valves. The green fitting sits on the inside of the SBD wing center section’s lower skin. While it is not part of the valve itself, it connects directly to it. Sitting into the circular groove on the valve body’s exit port is the newly-fabricated chevron seal that the unpainted 'v' face on the green fitting presses against. (image via Pioneer Aero Ltd.)
A side view showing how the green-painted fitting from the previous image sits against the valve’s exit port. Note the slight gap which exists between the mounting holes for each of these components. This makes space for the SBD’s lower wing skin panel, as the two components attach either side of it (the green fitting being the internal component). When the associated main fuel tank is selected by the pilot, fuel passes from either end of the valve, up into the green fitting, then on towards the engine. (image via Pioneer Aero Ltd.)
A side view showing how the green-painted fitting from the previous image sits against the valve’s exit port. Note the slight gap which exists between the mounting holes for each of these components. This makes space for the SBD’s lower wing skin panel, as the two components attach either side of it (the green fitting being the internal component). When the associated main fuel tank is selected by the pilot, fuel passes from either end of the valve, up into the green fitting, then on towards the engine. (image via Pioneer Aero Ltd.)

Looking inside one end of the fuel sump selector valve's body. The circular groove around the base of the main bore is where the double lip seal will sit. The four raised webs along the barrel keep the ball centered. The small shaft hole at the center of the structure is where the interconnect pin sits–the pin ensuring that only one ball can seal a port at any given time. When the port is open, fuel flows from the opening where the photo was taken, past the ball and seal, through the opening in the bottom of the main bore and on towards the engine. (image via Pioneer Aero Ltd.)

A view inside the fuel sump selector valve body after the double lip seal has been fitted into the groove around the base of the main bore–along with the bare metal retaining ring and circlip to hold the seal in place. (image via Pioneer Aero Ltd.)

Another view inside the fuel sump selector valve during its reassembly, this time following installation of the ball. Note how the raised webs along the barrel wall keep the ball centered, allowing fuel to flow past it when that port is open. (image via Pioneer Aero Ltd.)

The fuel sump selector valve after the ball retaining cap has been placed atop the ends of the raised webs. This cap is the limit stop for when the ball is in the open position. (image via Pioneer Aero Ltd.)

One of the inlet fittings for the fuel sump selector valve following the fitting of a new o-ring. Sitting next to it is another o-ring along with one of the newly-fabricated anti-compression rings. The anti-compression ring sits on the outside of the installed o-ring; it prevents the inlet fitting from being overtightened and damaging the o-ring. (image via Pioneer Aero Ltd.)

A view of the inlet fitting after its installation into one end of the fuel sump selector valve. This is the final assembly step for this end of the component, with the same process being repeated for the other end. Note: the shiny metal visible between the inlet fitting and the valve body is the anti-compression ring. (image via Pioneer Aero Ltd.)

A view through the outlet port on the lower side of one of the fuel sump selector valves. The structure inside the valve holds the interconnect pin. The shiny silver interconnect pin can be seen on the right side of the structure. The interconnect pin ensures at least one port is always open. The length of the pin means that if one ball is on its lip seal (closed position), then the pin has pushed the other ball off its lip seal (open position). The pin is fitted once the first end of the valve is assembled and before the second ball is fitted. (image via Pioneer Aero Ltd.)
A view through the outlet port on the lower side of one of the fuel sump selector valves. The structure inside the valve holds the interconnect pin. The shiny silver interconnect pin can be seen on the right side of the structure. The interconnect pin ensures at least one port is always open. The length of the pin means that if one ball is on its lip seal (closed position), then the pin has pushed the other ball off its lip seal (open position). The pin is fitted once the first end of the valve is assembled and before the second ball is fitted. (image via Pioneer Aero Ltd.)
One of the two fuel sump selector valves following its complete reassembly. It will undergo bench-testing before fitment onto the wing center section. It sits like this on the underside of the wing, with the bench surface representing the exterior surface of the skin panel. (image via Pioneer Aero Ltd.)
One of the two fuel sump selector valves following its complete reassembly. It will undergo bench-testing before fitment onto the wing center section. It sits like this on the underside of the wing, with the bench surface representing the exterior surface of the skin panel. (image via Pioneer Aero Ltd.)