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L21B Original liquid fuel gauge assembly questions

cubnut93

Registered User
I am into the final stretch of getting this airplane finished and I have some questions about the assembly of the original liquid glass tube fuel gauges. I have seen several other aircraft that must have had similar problems, so maybe it is just a common problem with the Cubs.

Any way, I noticed that when I started to install these gauges into the airplane nothing fits the way you think it should! The gauges don't line up with the holes in the new interior panels and they don't line up with the fittings that come out of the fuel tanks! I purchased new cork gaskets from Univair that go at each end of the glass tube as well as a new red indicator ball that float inside of the tube. I reused the bottom 90* fittings that screw into the gauge assembly and I purchased new AN fittings for the top of the gauges. I realize that you can screw these fittings into the gauge more to get the proper distance so that the holes in the interior panels will line up. But I had to really crank on those fittings to even get close---of course after doing that the cork gaskets were toast (smashed to the point that they would never seal correctly). Plus the glass tube now has a crack at one end.

What are my options???? This is a certified airplane so I am somewhat limited as to what I can do!
 
File the holes in the interior panel to fit. The extra opening area will be hidden under the fuel gauge out of sight.
 
Dakota Cub makes replacement certified gauges of a much improved design. Check out their web site.
 
Thanks for the input---

Does anyone know the distance measurement between the two holes in the root panel where the fuel gauges go?
 
Thanks for the input---

Does anyone know the distance measurement between the two holes in the root panel where the fuel gauges go?
Haha just a little shorter than the distance between the two elbows on the site glass! Go with the Dakota Cub guages, way nicer than the originals and not prone to leaking.
 
Mine were leaking about a year ago. Took it appart, squared up the ends of the glass tube (very carefully) on a disc sander. Insterted new Univair ball, applied a small amount of fuel lube to the gaskets and the AN threads. Tightened them carefully not to over overtighten. No leaks, but the ball sank after about 6 months. The 38 year old Piper ball still works fine. Mine leaked due to shrunken gaskets and very uneven glass tube ends.
 
Looking at the original gauges it seems as though the glass tubes need to be a little shorter (I have no idea if the glass tubes are original or replacements). How much can I grind them down before the glass cracks? Also--I am thinking that the thick cork gaskets are not the best type for this application. They squeeze down when the fitting at the top is tightened! Anyone ever used a different type of gasket on the glass tubes?

I looked at the Dakota Cub gauges online and I will call them for some other info that I need about their units.
 
I called Dakota Cub and asked about the measurement on the fuel gauges from the top and bottom outlet center line and they said that their gauges are 7 inches. I trimmed a little bit off the glass tube with my dremel and I installed thinner gaskets that were fuel proof. That gave me 7-1/8 inches--sooooo I elongated the two holes in the panels just a little. It is a tight fit but they made it and they are passable.
 
I'm replacing the left tank in a stock L21/PA18 and noticed that the fuel gauge was marked (engraved) TOP but was installed on the bottom. The TOP fitting has the bushing in it and the gauge was always wet when full. The parts manual shows the bushing on the bottom (?). I understand that the housing will allow a little more distance if installed upside down but it seems as though it will be prone to leaking with the straight threads of the bushing on the bottom. Is this a jump ball or ? Any help ------------ ss
 
I have never seen them installed upside down. I would install right side up with new cork gaskets if it were me. Too big a pain in the butt if it leaks again.
 
I'd like to point out a couple of things here. First, The 90 degree fittings in the ends of the gauge were NOT originally AN fittings. So when you install the newer fittings the lengths will be slightly different. If you install new gauges and the elongated holes are unsightly to you, fab another panel accordingly. Not a big deal.

Secondly some of you are trying to re engineer a gauge that has been in service since the late forties. There's not a lot of re design left here. Cork gaskets, installed properly, will last twenty years or more without leaking. Debating whether the one end should be up verse the other? Grinding glass? When the tank is full, both ends are submerged in fuel without leaking!

Just assemble it as per the book (there is one. If you have parts of your airplane disassembled and do not have a book, you are completely wrong) if you have any questions. Assemble it with fuel lube on all threads. And DO NOT over torque the fittings/bushings. The glass tube WILL break and/or the aluminum housing WILL crack.

And all of this without a 337 or anything!

If you don't like a glass tube as per original, just get another design that makes you happy.

Web

P.S. I apologize for not sticking to electrical stuff.
 
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