1985 K100 fuel leak at sending unit- replaced the rubber gasket and still leaks

[daveson]

If the tank is vertical when you install the sender,  gravity might cause it to be off centre before you tighten it.

[Laitch]

I may also try doubling up the O ring.

It's a gasket, not an O-ring. An O-ring has a round cross section.

[Wheezyryder]

Laitch- it leaks between the retaining nut and the cylindrical body of the sending unit (where I'm pointing in the second picture and the beginning of this thread).

Sorry to hear you spent time in the ICU. I wouldn't wish it on my worst enemy. Especially now with all the policy changes and lack of staff/resources due to Covid.

I had been tightening it with a giant adjustable wrench. I finally went today and bought a 36mm socket to minimize the change of damaging the nut. When I get the new Oring in I'll try again, but it sure makes me nervous knowing it's unobtanium if I break it. Im cool as a cucumber when someone's heart stops at work, but torquing that plastic nut makes me sweat

Daveson- I think I was holding the sending unit perpendicular as I tightened, but I'll make sure to pay attention to it when I get a chance to work on it again. I would love for it to be something simple like that!

[Laitch]

The ICU was like being in a berth of a train's sleeping car passing through the perpetual twilight of a foreign country. It was interesting way to get out of town without leaving. I think I'd wish on my worst enemy the inability to make it to an ICU, at least the one where I was. Maybe I'll recommend yours to him. 

[The Mighty Gryphon]

 I wonder if you have a crack somewhere in the root of the threads on the tank spigot.  It's possible that there is a crack someplace in the threads that the pull of the nut opens when torque is applied. 

If the gasket is working properly between the flange on the sending unit and the end of the spigot there is no way fuel can get to the threads.  With both of these surfaces clean and undamaged the gasket should have no trouble making a tight seal.

You will need a good magnifier to look at those threads.

Another possibility is that your new gasket is 30 years old and has hardened too much to make a good seal.  You might want to let the gasket soak in some Automatic Transmission Fluid for a couple days while you are busy with other things.  ATF has additives that will help soften rubber parts and might just help the gasket work. 

[The Mighty Gryphon]

Another possibility is that the nut is bottoming against the tank before getting enough pressure on the gasket to give a good seal.  In that case, adding a second gasket on top of the first one might fix the problem.

In the photos I think I counted eight threads on the nut above the gasket, and eight threads on the spigot.  That doesn't seem to provide any additional room for tightening. 

[Laitch]

In my last whack at this obsession, the attached photo shows the "arrow" pointing toward the rear fuel port to its right. I don't understand what the gasket surface of the nut bears against on the sender but I've seen this orientation a couple of times in my search. I've sent a query out to somebody familiar with this arcane part.

Buenas noches.

[daveson]

I'm thinking the gasket bears against the end of the threaded hole in the tank, on the photo in the post "tank hole" If the gasket is a bit damaged from the multiple attempts, I'd try spinning it around.

In the first photo the arrow is in line with a protrusion in the electrical socket, so it's a safe bet that a slot in the electrical plug lines up with the arrow before pressing the plug in.

[Laitch]

I'm thinking the gasket bears against the end of the threaded hole in the tank,

I'm with you on that one from the beginning of this confab. Your second observation is lost on me though. The only alignment I see is the "arrow" with an M on the nut. One of the problems with this thing is that it won't seat firmly on top of the mounting nut, and according to the owner, that's the place it's leaking. If the gasket were also bearing on the shoulder of the device that would help to seal the passage of fuel. The device doesn't appear to have been pressed in. What would be helpful is a photo like the one in Reply #13 showing if there is an alignment tab on the other bearing surface of the nut.

[rbm]

I'm not familiar with the construction details of the sender unit, but does it have a moulded shoulder against which the gasket bears and then compresses between that shoulder and the tank when the nut is tightened?  Or does tightening the nut cause the gasket to deform between the nut and tank which then seals by tightening around the circumference of the sender?

[Laitch]

I'm not familiar with the construction details of the sender unit . . .

Nobody yet seems familiar with its construction including the moto's owner, but there's hope on the horizon, as usual.    If we knew how it was constructed the fun here would stall.

[daveson]

I'm thinking the second one, so maybe a bit of Vaseline would prevent damage to the seal.

And I'm thinking, actually hoping, the sender is not at 90 degrees to the tank base,  creating the uneven gap, and preventing the nut from fully tightening. One of the good things about bricks, you can get your hand in the tank, wiggle it straight maybe, while tightening.

In the centre of the nut, where the electrical plug goes, there are four pins. There is a thingy there in line with the outside of the plug and the arrow to prevent the plug from being installed in a wrong orientation. 

[Laitch]

There is a thingy there . . .

The bump in the plug cavity and the arrow are parts of the same molding. The electrical plug connecting to the sender doesn't seem to be part of this problem, although anything seems possible under widespread contagion delirium. At least my hand sanitizer pump isn't leaking. 

[daveson]

That's it, the bump. So there will be a slot in the electrical plug that needs to line up with it. You can't see the bump when you put the plug there,  so the arrow lines up with the slot. A bit like putting a trailer plug in (hopefully it's the same in the US of A.

Yep, I also think the arrow has nothing to do with the problem.

[Wheezyryder]

"In the first photo the arrow is in line with a protrusion in the electri wecal socket, so it's a safe bet that a slot in the electrical plug lines up with the arrow before pressing the plug in."

Daveson- Aha! You're right about this! I was thinking that the arrow had some part to play in orienting the sending unit. But you're right... It lines up with the notch in the electrical connection that prevents plugging in the wires in the wrong orientation. I hadn't noticed that. It's likely just there as a visual reference when plugging in the wires. Therefore... The arrow likely has nothing to do with my leak or why the sending unit doesn't sit flush to the retaining nut.

[Laitch]

Daveson- Aha! You're right about this!

Hey! Aren't you supposed to be somewhere saving lives? You'd best get back to it and leave the futile deliberations to us.

[rbm]

I guess an easy way to tell if there is a shoulder or not is to try and slide the nut off the sender body.  If there is a shoulder, it will not be possible to remove the nut.  If there is no shoulder, the nut should slide off.  I'm guessing that there is no shoulder.

If there is no shoulder, all the sealing is banking on proper deformation of the gasket, which depends on properly shaped surfaces and a pliable gasket.
 

[daveson]

Yep if all checks good, do the nut up finger tight,  then put your hand in the tank and wiggle the sender straighter. Then you should be able to get it a bit more finger tight. Do it a few times till it's straight, then when it's straight, use a spanner. Hopefully that will do it.

[Laitch]

I guess an easy way to tell if there is a shoulder or not is to try and slide the nut off the sender body. 

Yep if all checks good, do the nut up finger tight,  then put your hand in the tank and wiggle the sender straighter. Then you should be able to get it a bit more finger tight. Do it a few times till it's straight, then when it's straight, use a spanner.

It should be a snap for someone who has inserted a nasogastric tube. 

[Wheezyryder]

Laitch- they do occasionally give us breaks in between our life saving duties. And of course when I'm on break I'm thinking about the important things in life... Like why my motorbike won't stop leaking fuel?!? And yes I've done plenty of nasogastric tubes. If fact, if you can think of a hole in the human body, I've stuck a tube in it.

Rbm- it is not possible to remove the nut. I can get the sending unit to sit flush with the nut when it's not screwed in. But when I screw it into the tank it gets crooked.

I've tried reaching into the tank to steady the sending unit while I tighten the nut. Couldn't figure out why but it kept getting crooked. I'll try again tonight after work.

[rbm]

In that case, the gasket is being sandwiched between the shoulder and the face of the tank opening.  If the sending unit cants in the same way every time, then there is possibly some distortion in the metal in the opening that is causing the sending unit to hang up.  Check the uniformity of the tank opening to make sure it is perfectly circular, and smooth.  Is there any evidence on the plastic of scoring caused by metal shards?  Another thing to try would be to remove the gasket and see if the sending unit can be inserted into the tank opening to the point where the shoulder will mate perfectly in the opening.

[Wheezyryder]

Alright everyone...after thinking about this for the last couple days I went out today and looked at the problem again. And I realized that the sending unit stays perpendicular and flush to the retaining nut most of the way while screwing it down. It's only in the last 1/8 inch or so of the retaining nut's travel that the sending unit gets crooked. I'm still not sure why it get's crooked. When it gets crooked, the sending unit flange would no longer put even pressure to compress the gasket against the spigot on the tank. So I'm going to try using 2 gaskets to shim it out a bit in the hope that it would tighten down without getting crooked. Already ordered new gaskets.

Then I removed the sending unit/ retaining nut assembly and looked at the inside of it while making it crooked. I realized that even when it is crooked, it seems that the inside diameter of the gasket maintains a tight seal around the cylindrical shaft of the sending unit. But the outside diameter of the gasket pulls away from the threads of the  retaining nut. So I thought that a thread sealer tape might help to fix any imperfections in that seal. I ordered a product called Petro Tape that is basically a fuel proof teflon tape. If the double gasket doesn't work, I'll give that a shot.

[daveson]

Might there be a little rock or something like that which might come out with compressed air, both sides of the nut, after removing the seal?

So now I'm thinking there is a shoulder for the sender over the tank outlet like RBM asked way back, is that right?

[Wheezyryder]

Daveson- I tried your suggestion and blew compressed air in from both sides. Didn't see anything come out and it still gets crooked when you screw it on the tank. And yes there is a shoulder or flange on the sending unit that sits over the tank outlet (I think I understand your question correctly). I'm gonna post some pics but it's difficult to get a good pic to show the inside.

[Wheezyryder]

That first pic shows the whole sending unit/retaining nut assembly with the gasket pulled back over the cylindrical body of the unit. The next 2 show the retaining nut end of the assembly from both sides. The nut is captured by a flange on both sides. The gasket is compressed between the inner flange and the mouth of the tank outlet.