cessna fuel sending unit gasket

L

LSK

Filing Flight Plan
Joined
Aug 23, 2020
Messages
26
Display Name
Display name:
Rowdy
Anyone know if there is a torque spec for the screws on the Cessna fuel sending unit -

If it matters the 1962-67 era standard tanks
 
And a quick look at the 172 and 182 parts manuals thru 73-74 models shows AN520-10R10 screws used for the senders. Confirm for yourself.
 
The mating surface is probably not flat anymore and the thick rubber replacement gasket is not soft enough to properly conform to the undulating surface. I had this problem last year on my '66 fuel tank. Someone had mentioned that they flattened out the mating surface with some tool (?hammer, pliers, something?). I ended up having to use a small amount of Aviation fuel resistant gasket sealer (*certainly not purchased from NAPA at the request of my AP*) to fully complete the seal, but I did not know that it was possible to flatten out the mating surface at the time.
 
Capt. Geoffrey Thorpe said:
Over torqued. You can't reef on those soft gaskets like other applications that use the same size screw. But, I am wrong because I'm not an A&P.
Click to expand...
Yeah, and no telling who did it on a plane that is 18 years older than I am and still had its original fuel senders until last year. Don’t know why it was like that, but i agree that it was probably flat from the factory.

Do you have a good procedure for flattening it back out? Apart from just buying a new fuel tank. I’ll ask my AP about doing it at next annual or next time the sender has to come out. Good thing is at least the sender is at the extreme top of the tank so it can only possibly leak when the tanks are 100% filled. That is how we discovered it - a few days later when a fuel leak developed after I filled the tank up to the brim for a long flight and had some drips come from the wing.
 
The old gaskets were cork and they conformed to the surface very well. But they'd also get old and hard and shrink some and start leaking, so people would tighten the screws to stop the leakage. Enough of that would distort the tank, pulling the areas around the anchor nuts upward and leaving a wavy surface that doesn't seal at all, mostly. Then Cessna and others went to "modern" synthetic gasket materials that are harder than the old cork and the problems just got worse. Using a soft rubber, along with a sealant, means that the gasket would squeeze out under pressure and some serious leaks developed.

So now you have to do something about that distorted, wavy flange around that hole in the tank. The aluminum is soft and can be set straight, but bend it enough, or often enough, and you can crack it. You need a small wooden block maybe an inch and a half long and a half-inch wide and half-inch thick. You cut a shallow arch into one side, like a little bridge. Set that on the tank with the ends resting on two adjacent screw holes, arch down. With a small pair of water-pump pliers, or better yet, small Knipex pliers, and a small wooden block taped to the end of the outside jaw of the pliers, you reach the outside end into the tank and put the inside jaw on the middle of the little bridge and carefully squeeze. Bring the metal up just a bit above level and let go; it will spring back down level. Some fooling will get it right. You need that little block on the outside jaw to get clear of the bent-down flange around the hole.

Knipex plier jaws stay parallel so the hole doesn't end up with other types of distortion.

Fuel Lube (EZ-Turn) seals well but it also lets that gasket squeeze out too easily. I used to wish cork gaskets were still available. You can buy cork gasket sheet, made with a fuel-proof synthetic matrix. I made new gaskets for the fitting on my boat fuel tanks with it after the synthetic rubber sheet stuff gave me fits. I also found that Hi-Tack gasket sealant, applied on the fitting plate only and the fitting screwed down really lightly for a few hours, would let the sealant set up and then I could torque the screws without the gasket crawling out. The sealant on one side only lets the fitting be easily removed again later on.

Use Fuel Lube on the screw threads. They extend into the tank and fuel crawls up them and out under the screw head. Cessna used tiny cork washers and aluminum backup washers but the fuel still seeped past them. It doesn't seep past a tiny bit of Fuel Lube.
 
You must log in or register to reply here.
Back
Top