Fuel Cap Dipstick (and Always Chock Your Wheels)

Why fumble around with a loose dipstick if you can add one to your fuel cap?

Before I delve into this month’s project, here’s the backstory on where it came from: I started my flight training with 10 hours in a Piper Tomahawk at Brackett Field in La Verne, California. Later (25 years to be exact), in 2007 I resumed my training at Fullerton Municipal Airport, where I took lessons in both a Flight Design CTS and a Remos GX. Unfortunately, this was during the economic downturn of 2008 and after a handful of flights, the flight school folded.

Not long after, I read about the Foothill Flying Club at Cable Airport in Upland and the opportunity to train in a brand-new Gobosh G700S (the USA S-LSA variant of the Polish-built AT-4). With instructor Rick Strack, I completed my flight training and fulfilled my dream of becoming a licensed pilot.

Using the shopmade freehand dipstick (left). The notches on the side are for checking the Funkist. The four notches engraved on the lower face (right) correspond to the level in the wing tanks on the Jabiru. The dark segment shows the tank to be just over 1/4 full. The humongous handle is so you never have to go fishing for it inside the tank.

I continued to rent and fly the club’s Gobosh for the better part of the next two years. All in all, it was a pretty cool aircraft. One magazine reviewer said at the time, “The Gobosh excels in clever details (such as) the dipstick on the fuel cap for easy reading.”

I always thought the fuel cap dipstick was a great idea. The fuel tank on the Gobosh was between the cockpit and the engine. While having one central tank was convenient for filling up (versus two wing tanks), it was not so easy to see inside the tank to confirm the fuel level. The factory-attached aluminum dipstick—it was maybe 10 inches long by two inches wide—more or less solved the problem. (A Google search of “Gobosh excels in clever details” will pop up an image of what I’m talking about.)

The first step was to measure the diameter of the stem, the screw for the grounding lead and the depth of the tank (left). The OD of the stem was, coincidently, a perfect match for the ID of a piece of ½-inch-diameter 0.049-wall 6061 aluminum “hobby” tubing sold in one-foot sections at my local hardware store. The dipstick design I settled on looks like a long fingernail (right). The three small holes represent ¼, ½ and ¾ levels. The length is deliberately shy of the bottom of the tank by an eighth of an inch. I wanted to avoid any possible chance of the dipstick burnishing a hole in the tank and causing a fuel leak.

The Gobosh dipstick had one flaw: It was almost impossible to read in direct sunlight. If you could get it under some shade you could read it, but you had to move fast before the fuel evaporated. If you didn’t mind the smell of gasoline, you could feel where the fuel was with your finger. But the lingering smell of gasoline can be discomforting (…is that a fuel leak or my smelly finger?).

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To secure the tube in the mill for the machining operations, I drilled a ½-inch hole into a wood block and then sawed it in half to form a clamp. After locating the center, the first operation was to drill the mounting hole. The tubing was left long to allow secure clamping for the machining operations. Cutting to length was the last step in the machining process.
A four-flute end mill was slowly plunged straight down to mill the notch.

Fast forward to 2021 and I am doing a preflight on my Jabiru. I remove the fuel caps on each tank and use my shop-made dipstick to check the fuel level. I made the dipstick in 2011 for my first airplane, the Victor Stanley-built Funkist (a Teenie-Two variant). The Funkist fuel tank is between the engine and cockpit and is a relatively deep tank, so you need a dipstick to confirm the exact fuel level. When I made the dipstick, I sandblasted it with the idea that fuel clinging to a matte surface would be easier to read. It works great, even in bright sunlight. Later, when I got the Jabiru, I added a second set of graduation marks on it to correspond to the shallower profile of the Jabiru’s wing tanks. As I was putting the dipstick away, it flashed into my head: Why not add dipsticks to my fuel caps, like the Gobosh? Yes, the Jabiru tanks are shallow wing tanks and the fuel cap is completely different, but the caps have a stem/extension for attaching a wire grounding tether, which proved to be ready-made for attaching a dipstick. It’s so simple, I don’t know why I didn’t think of it before!

The photo sequence with top-view illustrations shows how the “halving” operation was done on the mini mill using a two-inch-diameter fine-tooth slitting saw. The first cut was made by plunging the slitting saw just above the centerline of the notch and traversing to the end.
With the blade repositioned to the original starting point, the saw was advanced through the previous kerf until it came through the opposite side and then traversed to the end (left). To finish, carefully “climb-cut” the remaining section up to the notch (right).
After halving, notice how the flats were slightly proud of the notch (left). A few careful strokes with a hand file brought the flats down to the notch depth (right).
Some 180-grit sandpaper wrapped around a drill was used to smooth out the file marks and blend the flats to the notch (left). The dipstick was cut to length on the lathe using a parting tool (right).
The fingernail profile was made using a non-woven deburring wheel (left). The same wheel was used to deburr the milled edges. From tubing to the sandblasted final part (right).
After attaching the dipstick to the fuel cap stem with the grounding tether screw, a final check was made to verify the gap between the dipstick and the bottom of the tank.


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