Fuel Pumps and Flow Testing for Big Engines with Carburetors

Big Carb Engines, Fuel Pumps, and Flow Testing, part 1
Source: 2025 Q1 Beartracks, Jared Yates
Back in early January I was departing for a short flight back home, when I got a fuel pressure warning on the takeoff roll. The EMS is configured to alert when the pressure drops below 3 or so PSI, because I have found that if I forget to turn on the boost pump, that’s about what it drops to at very high power settings. Usually the alert reminds me to reach down and turn on the boost pump, but this time, the switch was already on. When I got home, it didn’t take much testing to find that the little electric Facet cube pump had decided to retire.
This seemed like an easy thing to fix, the only question was whether I’d have to order a replacement or whether I might get lucky and find one at a local parts store. Little did I know, I was about to step into a rabbit hole that three months later I’m still not quite out of.
Our plane has a carbureted O-540, and larger carbureted engines are the only ones that share this problem. For folks with a carbureted 360, including ourselves when we had one, 3/8” fuel lines are perfect. They can provide around 30-35 gph, which is 150% of the required flow rate of the less-thirsty engine. For folks with a fuel injected engine, you’ll be using higher pressure pumps anyway, so the 3/8” lines are plenty sufficient, and this rabbit hole is moot. Some builders have been able to reach acceptable flow rates with 3/8” systems and big carbureted engines, especially if they don’t use a fuel flow measuring sensor, though I was not. Also, I really prefer to have a fuel flow indication for several reasons. It’s a great real-time diagnostic, it helps with establishing throttle/prop/mixture settings for various phases of flight, and helps with flight plan monitoring. An electronic system can keep track of how much fuel has passed through the sensor and provide an indirect fuel quantity indication and warnings. But the fuel flow sensors are all designed with tiny little passages, that make for a big flow reduction– much more on that later.
When we first acquired our current plane, it had an engine-driven diaphragm pump and the electric boost pump, which is a Facet 40108. I did a flow test to see if we could remove the pumps, but did not have anywhere close to sufficient flow with the fuel flow sensor in place, so I carried on with the pumps in the plan. The 40108 pump is handy because it has 37-degree flared fittings integrated into the inlet and outlet. The internal passages are the same size as the 3/8 lines, and it has no additional check valve or positive flow shutoff when power is removed. When I went to purchase a replacement, I found out that the 40108 pump had been discontinued. This shouldn’t be a problem, Facet has a whole line of pumps. Unfortunately, almost all of the cube pumps have 1/8” pipe thread fittings. That’s not going to work. The Vans builders have changed to the 40135, which has the right fuel pressure for a carb (nominally 5-7 PSI), but uses the 1/8” passages. It does not have a check valve per the specifications chart, though all of these pumps do have a natural check valve function and prevent reverse flow. There is a 40109 which has 3/8 pipe thread ports, but it has an extra effective check valve function. I ordered a 40135 and a 40109, and holding them on the bench, I could tell that the 40109 was not going to work. The check valve provided so much restriction to forward flow that I don’t think the gravity pressure was going to open the valve at all. The 40135’s natural check valve function was a little more stiff than my old 40108, which is 20 years older, but tolerable. The problem was just the tiny ports.

So if we were going to continue to use pumps, we had a few options. One would be to run multiple 40135 pumps in parallel. I considered one on each 3/8” line just upstream of the fuel valve, but there would be some serious drawbacks. Basically there would be no cross-flow capability between the left and right tanks, because the pumps are natural check valves. We could run two pumps side by side with Y or T fittings, in hopes that the flow rate between the two would be sufficient with the electric pumps off. I wasn’t confident that this was achievable, but also didn’t test it. Another option was to attempt to modify the pump, such as drilling and tapping new threads to increase the inlet and outlet size, or maybe destroying the check valve function of the 40109. I wasn’t confident that either of these would be successful, and even if they were, replacing the next broken pump would require repeating the mods.
The next option was to consider a fuel system redesign that would allow us to eliminate all of the pumps. I spoke with Tyler Williams about his installation, which is a carbureted engine bigger than ours, with no pumps. He found that by increasing the line size to 1/2” from the fuel valve to the carb, he was able to get sufficient flow without any pumps, though he was not using a fuel flow sensor. It makes sense– using 3/8” lines for single tanks, but 1/2” once they are combined. This was the most appealing option, but sometimes even a small change ends up big.
The first order of business was the fuel valve. Our plane was built before the Newton SPRL valve was available, so it had an Andair valve. There have been a few iterations of the Newton valve, and it is better now than ever before. The current version is actually Bearhawk-specific and is only available from Bearhawk Aircraft. It is manufactured so that the outlet is on the aft end of the valve, so that the fuel can flow aft/downhill to the gascolator. The SPRL valve has its own non-standard interchangeable fittings, with several different options available. Bearhawks with 3/8” systems can use Bob’s gascolator, which has 1/4” NPT threaded ports. The valve can be configured with 3/8 inlets for each tank, and a male threaded 1/4 NPT outlet which can be directly attached to the gascolator. This saves fittings and makes for a very clean installation. In our case we set aside the 1/4” fitting and ordered a 1/2” flared fitting, which is part number 05-04453 at Aircraft Spruce, for a pack of 3. Now all we had to do was defuel the plane, remake the two lines that bring the fuel to the valve (because the old lines were an inch too short) and redo the mounting of the valve to the floorboard. Glad I didn’t have any other plans for January?
We also needed to up-size the gascolator. The only viable option that I could find was from Steve’s Gascolator. They make two units with 3/8” NPT ports, and two different bowls. Height is important in the Bearhawk, because we are trying to fit the gascolator between the floor and the belly. The shortest combination from Steve’s was the SA3-10-B, which has a 3-ounce bowl. I used one AN822-8D fitting on the inlet side. This fitting has 3/8” pipe threads on one side and 1/2” flare on the other side of a 90-degree elbow. On the outlet side I eventually used the AN823-8D, which is a 45-degree version, because it did a better job of pointing the line where it needed to eventually get. If you are following along and ordering parts, the gascolator also requires a 1/8” pipe thread plug at the top. I used an aluminum Aeroquip FCM3685 though I did carefully file it down to be almost flush with the top of the gascolator housing, so that the gascolator could sit as high as possible. I made a u-shaped bracket out of aluminum left over from the instrument panel, which attaches to the floorboard with 3/16 rivets.
Going forward to the firewall, I increased the size of the existing AN837 bulkhead fitting to the –8, and that also needed one AN924-8 nut. These larger fittings are much more expensive, I’m noticing. Then I’d also need a 1/2” flexible line to go from the firewall to the carb, and a new fitting for the carb itself. The carb has 1/4” pipe threads, so something like an AN822 would be nice, except that MS20822 doesn’t list a fitting that has a 1/2” flare fitting and a 1/4” pipe thread. I found one at Pegasus Auto Racing, part number 3253-04-08. This fitting is aluminum, and I would prefer steel, but the carb itself is aluminum so I was willing to compromise. A 45 or even straight adapter could also work here, depending on how your line is routed. I have found that using a 45-degree fitting at the firewall and approaching the carb parallel to the thrust line helps keep the flex line from rubbing on the cowl and clear of the exhaust, but there are any number of good ways to get this done.
I didn’t have the where-with-all to figure all of this out with a single order. A few weeks and around $1000 into this project, what was there to show for it? I’m glad you asked. First, let’s talk about the flow rates, which means talking about flow testing. In the US, testing is not required by regulation. It used to be required for store-bought planes but it seems that requirement became less specific in recent years. Canadian builders tell me that the flow test is regulatory for their homebuilts. I decided that the test was important enough to comply with. I wanted to get 150% of the engine’s maximum flow rate. Past flight experience with this engine and prop tell me that I’d like to see 37gph in the test, in order to remove the pumps. With the new lines, and no fuel flow sensor, I ran some preliminary flow tests with the wheels all on the ground. (A proper test would come later, with the main wheels elevated.) I was getting 47 gallons per hour on Both. This is great! But then, I installed my old Flowscan fuel flow sensor in the flex line close to the carb, and the rate dropped to 32 GPH. This was not great. I had ordered an EI FT-90 Gold Cube sensor to try, and it increased the flow to 34 GPH, meaning there was still some work to do. Tune in next quarter for part two, getting to a successful fuel flow measurement.

Auxiliary Tanks


Installing the aux tanks is exactly the same as installing the mains with two exceptions:
1. There are no tank bay skin stiffeners to worry about.
2. The tank opening in the bottom of the wing has not been left open, so you’ll have to cut that out and fabricate a cover. You’ll also have to drill out some spar cap rivets for the bolts.
The original Bearhawk uses aux tanks that hold 11 gallons each. The Model B uses tanks that hold 9 gallons each.
Cutting the Bay Open
First, make certain you start working in the right bay: In the original Bearhawk, it is the third bay in from the tip. In the Model B, it is the fourth bay from the tip. Also, there is a right and left tank: the tanks go in so the connections are inboard and the filler caps are outboard. You’re going to cut the skin out of that bay and trim it flush with the rib and spar flanges and leave about 1/8” edge at the rear so you aren’t working too close to the rear spar face (protect it with layers of masking tape anyway). Once you’ve done that, you’re going to make a .025” aluminum cover that overlaps the hole 3/4” on all four sides, drill holes so they fall midway between the existing skin rivets (to give room for nut plates, etc.), dimple the cover, countersink the flanges, install nut plates and you’re done.
Making and installing the straps to hang the tanks follows the exact same procedures we outlined while installing the main tanks. To cut the bay open, start by drilling a 1/4” hole in each corner of the bay from inside the wing. Ideally, when we’re done, we want to have a radius in each corner to reduce the stress concentrations, so drill those first holes far enough from the corners that you can file a radius into the corners by hand. We’ll want to enlarge those holes to 3/8” to get a file in them.
Yes, in theory, you could drill these guide holes right in the corners tangential to the flanges. That, however, requires a steady hand and a mistake can cause headaches. Keep the bit out away from the flanges just a little and file into the corners with a chain saw sharpening file. 5/16” and 3/8” round files are available from Bishop Company, www.bishco.com and are handy for a lot of other uses.
File the corners back so they barely touch the flanges. Then, turn the wings over and draw a guide line from the edge of one hole to the edge of the other. That indicates the edges of the flanges and is the absolute do-not-cross line. Mark another line 3/32”-1/8” in from that first line. This is your cut line. Don’t get the lines confused. Do not try to cut the metal out with snips as it will distort enough that you’ll work your tail off cleaning it up. If you have a steady hand, a die grinder with a cut-off wheel will work, but any mistake will be a big one, so it may not be worth the risk.

Finishing the Edges
You’re going to have 1/16”-1/8” left to trim back and it will work quite quickly with a vixen file, which is available at body shop supply houses. When working aluminum anywhere in the airplane, your goals should always be the same regardless of what you’re working on:

  • No scratches of any kind anywhere in the material. This will require you to be careful in how you handle the pieces and where you lay them.
  • The edges will be sanded down to at least a 320 grit finish.
  • No sharp edges or burrs remain. Sharp edges give fatigue a place to start cracks.

So, when finishing the edges of the hole you just cut in your wing, keep all the above in mind. If you can drag a nylon stocking down the edge and not have it hang up anywhere, you know you’ve done a good job.
The edges of the hole will be defined by the flanges around it. These flanges will receive the nutplates that will hold the cover in place. Complete the straps, cut the filler cap holes, install the tanks, etc., just as you did the mains.

  • The aux tank uses two straps on the bottom with tensioners and one over the top. Make sure the top straps don’t touch the skin.
  • When the tank bay is cut open, the metal should be smooth and flush with the rib/spar flanges.


Plumbing
The aux tanks are plumbed so that fuel is transferred via a pump from the aux tanks into the mains. The aux fuel then becomes main fuel and feeds to the engine as usual. The connection is run from the bottom boss at the inboard end of the aux tank, through the pump (Facet 40171) and tees into the line that runs between the main tank and the sight gauge.
It’s up to the builder to decide whether to run the flow to the bottom or the top of the sight gauge.
Running it into the top of the sight gauge, even if you don’t have a plastic ball in the sight gauge, when the pump is running and there is fuel in the aux, it fills the sight gauge. When the aux runs dry, bubbles show in the sight gauge. If you have the ball, it forces the ball to the bottom and the ball pops up the top, when the aux tank is empty. Plumbing the Aux tanks into the upper sight glass location is good for keeping the fuel in the main tank from siphoning back to the Aux tank when on slope or if pump check valve leaks.
If you route the line to the bottom of the sight gauge and route the line from the Aux tank to the Main tank high with an anti siphon bend above the upper sight glass port that will usually stop back flow. In this case bubbles will go up the sight glass when transferring fuel and be easily visible that transfer is occurring or stopped.
Place an orifice of .040 at the inlet of the sight glass and an orifice of .030 at the top of the sight glass going back into the tank. The two orifices in series will damp the sight glass much more effectively.

Plumbing Aux to Main
You’ll run an aluminum line off the bottom fitting on the end of the aux tank. The ideal line is 3/8″. Some builders have used 1/4, but the transfer time can be 30-45 minutes with 1/4″ line. To do this, it’ll have to come through a rib. Drill the hole with a stepped drill (Unibit) because it yields a round, clean hole. Make the hole big enough to accept a rubber grommet around the fuel line. Some builders put a reinforcing doubler around the hole using .032 and a number of AN3 rivets. To avoid using a ninety-degree fitting to make the turn aft (there’s no room for it), bring the line out in a large curve and back to an Adel clamp.

There are a few options for mounting the pump.
On the Model B, we recommend putting the transfer pump about mid rib bay opposite of where the tank outlet is in bay 4 from the tip. This is in the bay with the aileron bellcrank and other things.
Some have used a “hat section” of .063 aluminum to mount the transfer pump just opposite of where the aux tank outlet is, with the bottom flanges of the hat section are riveted to the bottom skin.
Another option is to mount the pump on a piece of .050 aluminum that you bolt to the rear spar just inboard of the aileron bellcrank. You’ll have access to it through the large access panel later. Drill out four of the spar attach rivets (use a No. 21 bit) and use AN3 bolts to secure the pump mounting plate. The fittings used on the line are coupling MS20823-4D and nut AN816-4D, 818-4D, MX20819-4D

To get the outgoing line over the aileron actuation rod, you’ll have to use a 45 degree fitting (which gives plenty of arch to clear everything) or bend the line into a gentle “S” turn. You don’t have to worry about supporting the tubing at that point. It will have more than enough strength to support itself.
The feed line comes out of the aux tank and into the pump via the bottom line in the aux tank. Doublers are added to the ribs where the material is removed.
The line will run down the back of the tank bay false spar and you’ll support it with a padded Adel clamp about every ten inches. Where it hits the rib that forms the end of the tank bay, the line needs to make a 90 degree turn forward. Use an AN833 that will allow a tight turn, and also hold the line in place. Use another padded Adel clamp to attach the line to the second rib in from the root. When routing fuel lines (or any line for that matter) it is important to note that everywhere a line goes through aluminum, it is either connected via a bulkhead fitting or, if the line itself goes through the metal, there is a grommet/snap ring pressed into the hole. Fuel line should not be allowed to contact other metal and will be held in position with Adel clamps. It also shouldn’t run long stretches in mid-air with no support. When in doubt, snuggle it up against some structure and throw an Adel clamp around it. If you can easily flex it with your fingers, it needs support. AC43.13 gives guidance about how often fuel lines must be supported.

Aux Tank Quick Drain
There’s a boss on the bottom of the aux tank on the inboard, rear corner. That’s to install a quick drain. Thread it for a 1/8” NPT thread quick drain (Wicks CAV 110). You’ll need to cut a hole in the bottom of the wing large enough for the quick drain. This is yet another place where using a Unibit (the stepped type drill bits) will give much cleaner, perfectly round holes with clean edges and zero distortion.

The aluminum (.025) for the main tank covers is provided, but it is not provided for the aux tanks. Use 2024-T3 aluminum.
The procedures are the same for the aux covers as they were for the main covers.

Model B Note:
Builder Dave points out the following:
The Model B wings have a different configuration and the aux fuel tank goes in the 4th bay rather than the third. This changes the way you need to configure the plumbing. Here’s what I did….

– remove the center rib stiffener on the inboard rib and trim the bottom section. Make a .032″ doubler to fit over the space. This is where you need to make the exit hole for the fuel line. Replace the shorter stiffener.
– make a .063″ doubler plate to take the pump and fit it on the next rib stiffener aft. I drilled out the center 3 rivets and then made extra rivet holes.
– use a 45 nipple on the exit from the pump and route the piping under the aileron bracket and the fuel line along the bottom aft of the ribs

Kit Fuel System

First, a word about fuel systems. Fuel delivery problems are a leading cause of homebuilt mishaps. Take the design of the fuel system seriously, and do not deviate from the system designed by Bob in the Beartracks. Also, consider the general fuel line guidelines here.

The important fuel system requirements are:
1. The lines should go downhill, with no low points (except per point number 2) from the tank to the engine.
2. Any low points (measured while in a three-point on the ground) should have quick drains to eliminate trapped water.
3. 3/8” lines are required for the 0-540 and, although overkill for a 180, they can’t hurt. Some builders with very large engines (over 250hp) have run ½” lines, but keep in mind that larger lines are only helpful from the fuel valve down, since the fuel valve is fed by two 3/8” lines. Builders who have used ½” lines have also reported difficulty in making provisions for larger fuel valves, gascolators, etc.
4. Use no Teflon tape to seal joints because it might get in the lines or the carb. As we describe in the general section, use pipe thread compound only on tapered pipe thread fittings, and use no compounds on flare fittings.
5. Use nothing but aluminum line for plumbing, with flared fittings on the ends. It is acceptable to use 3003 or 5052-O. Don’t use things like rubber hoses and clamps.

If you buy 3003 fuel line it will be shipped to you in a coil. It is very soft and easily bent with your fingers, but a tubing bender makes more precise, regular bends. Buy one at your local NAPA store. 5052 line is much harder and stiffer, and shipped straight. The 5052 has better fatigue and corrosion resisting properties, but is more difficult to install.
A problem you might run into with 3003 is that because the line is coiled, it’s tough to get back straight and have it look good. So, try this trick: Uncoil it as best you can and squeeze one end tightly in your vice, then hog down on the other end with a big pair of Vice Grips. Pull the tubing as straight as you can and smack the vice grips with a hammer to pull the tubing taut. That’ll pull the fuel line as straight as if it came out of a die and makes it much easier to work with.

There are two lines coming out of the wing root area: one coming out of the back of the tank and going down the rear door jamb and the other coming out of the front of the tank and going down the front jamb. The lines go continually downward, even in a climbing attitude, and join with the selector valve and gascolator under the front floor.

Fuel Selector
The plumbing from the main tank to the fuel selector will be the same with or without aux tanks, since the aux tanks are plumbed into the main tanks. Use valve with ports for Right-Both-Left-Off, noting that there is limited space between the floorboard and the bottom of the airplane. The Newton SPRL valve fits with a little room to spare, but most Andair valves do not. Unfortunately, the Newton valve is not set up for tailwheel operations, and will need to be relabeled. As you finalize the location of your fuel selector valve, consider possible interference with the flap handle, and work to minimize possible contact with the feet of the front seat occupants.
Install a gascolator downhill from the fuel valve, which in the case of a taildragger, will be aft of the fuel valve. Route the line from the gascolator to the firewall, where you’ll transition to a flexible line to connect to the carburetor.

Here is a diagram from one builder. Note that he used 1/2″ lines in some sections. For most applications, 1/2″ lines are a little larger than necessary.

In August 2020, Bob issued the following Operation Notice about fuel systems that use fuel pumps:
Operation Notice
The Bearhawk Fuel System as shown in the Bearhawk Book is designed for use without a fuel pump.
If a fuel pump is used, extra care in flying is required so that neither main tanks become unported, as a fuel pump would rather suck air than fuel.
If one tank is very low and the other is not very low, set the fuel selector on the fullest tank and fly the plan as not to unport that tank.
A header tank (3 gal) could be used to solve this problem, but fuel in the cockpit can be a safety issue. (Vent the header tank to both main tanks)

Making Fuel and Brake Lines – General Considerations

Install hard aluminum lines whenever the line does not need to flex in operation. In most Bearhawks, the only flexible fuel line required will be from the firewall to the fuel metering device. Brake lines will need to flex at the top of each gear leg, at each rudder pedal, and at the caliper. Some builders accomplish the flexibility at the caliper with a loop of hard line to distribute the motion, and that usually holds up for a thousand hours or so. When ordering your lines, you’ll have at least two choices. 5052 aluminum comes in straight pieces and it work hardens with the first bend. It has excellent fatigue and corrosion resistance, and though harder to work with, will produce the best quality finished product. 3003 aluminum is usually shipped in a coil. This coil can be straightened by clamping one end in a vice and stretching from the other end with a set of vice grips. It may help to create a mock-up of the fuel line in a flexible material, such as a 1/16” steel welding rod, or a soft piece of “armature” wire from the sculpture section of the art supply store. It may take a few tries to get a line just right, and those tries can get expensive if you are scrapping lots of aircraft grade materials. Study the fuel line routing specified by Bob in the Beartracks newsletter. Separate articles address the standard arrangement and the auxiliary tank arrangement.
There are excellent resources for how to flare the tubing ends in both the Bingelis books and Beartracks, but the most important parts are to be sure you use the aviation 37-degree tool, don’t forget to install the nut and sleeve before you flare, and if your finished flare is in the least bit questionable in quality, discard it and start over. Practice on scrap pieces until you are sure you have it right. When bending tubes, consider using a hand-held tubing bender, but in any case use some sort of die that will yield consistent radii and no kinks. If you are planning to use a large engine, you may need all the fuel flow that you can get, so avoid abrupt bends, especially from the fuel valve downstream. Follow the guidance in AC 43.13 for supporting fuel lines, which will include, generally, a support every 16 inches. We suggest padded Adel clamps.
Ensure that there are no possible moisture traps in the lines, except for the tank sump points and the gascolator. This includes any low spots when the airplane is in the 3-point position. In a tailwheel airplane, if the gascolator is to be the lowest point in the system (as we recommend it should be), it will need to be located aft of the fuel selector. This way, any moisture in the line between the engine and the gascolator will settle back down to the gascolator when the airplane is parked, allowing you to drain it during your preflight inspection. Fuel system problems are a leading cause of EAB mishaps, so if you plan to deviate even slightly from Bob’s system designs, we strongly suggest that you consider consulting with Bob or a very experienced A&P. A small change can lead to an unexpected problem later.

Builder Contribution:
Here are two conceptual drawings of the fuel system provided by a fellow builder. These drawings show the location of each type of fitting, and while it is based on the 4-Place, the concepts are very similar for all three airplanes. Note that this builder used 1/2″ lines in places, but for most applications the system will only include 3/8. That will change the sizes of a few of the fittings compared to his drawing.

Routing Fuel Lines from the Tank to the Wing Root

Remove the tank from the wing to start prepping it for the fuel lines.
The fuel feed lines and sight gauge lines must be run from the tank through the root ribs, ribs #1 and 2. This is why in the tank mounting step, we offset the tank to the outboard side of the tank bay to give provide the maximum room to work in.
Start by installing the tank fittings. The threads that go into the tank are NPT threads, which are slightly tapered. NPT threads require a sealer to prevent leakage, as detailed here. We recommend “Fuel Lube,” which is sold at Aircraft Spruce as “EZ Turn.” A small tube will likely provide all you’ll need for your project. This product is impervious to fuel and water, and also helps prevent thread galling. Use this only on the NPT threads, never on any part of a flared fitting. Use it sparingly and wipe off any globs of it once the fitting is secured in place.
Warning: Do NOT use Teflon tape for the fuel system connections. Little pieces could be cut off by the threads and contaminate the fuel system.

The inboard side of the tank receives the finger strainers and AN fittings pictured above. Front fuel feed fitting: AN816-6D. Sight gauge fittings: AN816-4D. Rear feed line fitting: AN824-6D.
One note of caution: The finger strainers (ACS PN 05-17700) are made of relatively soft brass, and they can collapse if they are tightened without support. Before you tighten them with a wrench, install the AN fitting inside the finger strainer to support it against the pressures of the wrench. Use the box end of a wrench instead of the open end.
With the fittings installed, you can position the fuel tank back in the tank bay. The next step will require tank removal again, so there is no need to tighten the straps, other than an optional test fit. Make sure the tank is offset to the outboard side of the tank bay to allow maximum room for the fittings.
The AN824-6D tank fitting at the aft end of the tank is a 45 degree elbow because its location lines up with the edge of a rib lightening hole. Notice how drawing #2 in the plans shows how the tank is offset to the outboard side, and how the aft fuel line is angled aft before finally passing through the root ribs.
The next step is to locate the fuel line clearance holes in ribs #1 and #2. With the tank in the bay, there is no room to get a drill motor in place to drill the holes. We’ll use reference lines and measurements instead, which will allow for removing the tank to drill the holes. Place a straight edge or ruler on the skin over the rivet line for the rib flange, and align it by eye with the centerline of the flared end of the aft fuel tank fitting. Draw a line with a Sharpie marker onto the wing skin that depicts this extended center line:

Next, measure the depth from the wing skin to the center of the fitting, measuring perpendicular to the skin. In this case, the center of the fitting was 1″ below the wing skin. Note the measurement somewhere. In the photos, the measurement is conveniently written next to the line that we drew in the last step.

Repeat this for all four tank fittings:

Remove the fuel tank and use a small square make vertical lines on the outboard side of the number 2 rib that correspond with the lines from the last step. Measure vertically on that line to mark where the center of the fuel line hole should go. Cut the holes in the rib with a step drill, using 1” holes for the feed lines and ¾” holes for the sight gauge lines. If you find that you need to remove more material than that, you’ll want to consider adding a doubler to the rib.
To mark the hole locations on rib #1, rest one edge of a square on the inboard side of the wing skin, and make span-wise lines that intersect the lines made in the last step. Use these marks to locate and drill the holes in rib #1. Use ¾” holes for the feed lines and ½” holes for the sight gauges. Use AN931-4-7 grommets for the sight gauges and AN931-6-10 grommets for the feed lines.

Here is a diagram provided by one builder of his fuel system in the wings:

Builder Bruce Case suggests the following edits to the above diagram for his Patrol, and also provided the fuel tank drawing:
Fuel Diagram With Errors
LEFT WING TANK ASSEMBLY
If you draw a diagram for your system, send it over and we’ll add it here too!

Pat’s Home Made Tool for Beading Tubes

Source: 2001 Beartracks, Pat Fagan

Hi Mike, This is a drawing of a tool I made to put a bead on the fuel lines where they will attach to hoses. It really works well, thought that you might want to include it in Bear-Tracks.

Pat Fagan – #232 – Pearblossom, CA

Thanks Pat – I’m sure that many builders can put such a tool to good use. ~ Mike

To use: Set collar for required depth. Place rod inside tool with end of taper under the ball bearing hole. Put the ball bearing in hole and slide tool inside fuel line tube. Screw on the end cap until snug. Turn nut another 1/4 turn by hand which will shove the rod under the ball bearing, forcing it outward. Turn the tool one turn and follow with another 1/4 turn on the nut. Repeat until you have the bead that you want.