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916 Fuel Flange, about to pull my hair out

3K views 25 replies 12 participants last post by  BobbyBoy 
#1 ·
Hi All,

I'm at my wit's end trying to get the fuel flange reinstalled on my 916. Yesterday I went and bought a used tank since the mating surface was a bit distorted. Before anyone asks, yes I replaced the o-ring. I've now tried reinstalling the flange 4 times, each time I get a steady drip.

I've tried applying light silicone grease and pushing it in.

I've tried permatex #2 gasket maker directly on the ring, and around the perimeter of the flange.

I've tried permatex gasket dressing (blue, always tacky).

I'm begining to think this is not somthing possible to fix. Has anyone had one of these things not be a nightmare?
 
#4 ·
Are you sure it isn't the fuel SENDER that's leaking? The originals had a plastic nut. It is prone to cracking, or it could also be the viton o-ring above it.

I have an extra unit, if needed. And I think I also have an aluminum replacement nut. The new assembly versions have switched to the aluminum nut as well.
 
#6 ·
First time I had my 998 back home after buying it, I smelled gas. Sure enough it was that stupid plastic nut. I feel like there is a decent chance this might be it; my experience with the giant o-ring and flange is that it's pretty difficult to nick it if it's lubed up nice and good. And if you're getting the same leak on a second tank (if I read the post right!) I'd lean heavily towards taking the sender nut off and giving it a good look-see.
 
#7 ·
You could always try to double up the o-rings at the flange. I'm not sure why there's a difference (other than the MV Agusta tank being plastic) but the MV parts and service manuals show an o-ring in the same position as the Ducati, but an additional one at the base of the flange.

MV Brutale pump on the left in my pic, Duc 916 on the right. The other pic is from the net showing both o-rings in place.
 

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#10 ·
lots of other things to leak - quick release fittings, sender nut, sender wires, breather or cap recess drain outlets.

often, if there's fuel in the tank when it's upside down it gets into the cap vent circuit and then comes out the bottom when it's turned over. sometimes it pisses out all over you.
 
#11 ·
Thanks for the reply all. I'm on my second tank now, youd think if it were the tank that would solve it. I haven't tried doubling up, do the other bikes you mentioned use the same flanges part? Would the second o-ring go above, closer to the flsnge portion of the player or below (deeper in the tank)?
I've check the fuel sender but, and to be sure applied some sealant there (fuel resistant), so I've eliminated that. This is coming after I replaced the quick releases, and I looked at them closely, I dont think the release is there. My dad and I sat and watched fuel drip for about 10 minutes yesterday, I'm really thinking it's coming from the o ring. I tried 4 times now to install, each time with a fresh o ring.
Last night I applied a layer of permatex gasket maker at the base of the flsnge to try and over kill the leak. Next step will be buying a new flange.
I'll try and include pics and a video when I get home.
 
#18 ·
@Namor sorry for the lack of clarity. The leak looks like it's going past the o-ring, at least best I can tell. I'm usually a fairly competent mechanic, so I'm a bit stumped that after 2 fuel tanks, and 4 rings it's still leaking. Only thing I can think is the baseplate (flange) itself is warped, or the second fuel tank is also slightly warped. I've been using silicon spray to get it in, maybe I should try a different lubricant?
 
#19 ·
I believe I've got a spare flange somewhere in the attic, it had a broken breather post on the inside tank portion but that got JB welded back together and seems to hold. I don't mind forwarding it on to you to see if it works out. It's missing the quick disconnectors; it was a 748E flange but it's been threaded for the 1/4 NPT threads of the QDs. If you're interested I can probably get it in the mail Wednesday or Thursday (if I still have it, pretty sure I do but I'll need to check in the morning!)
 
#20 ·
Found it; it's not in the best of shape and has been repaired. It was working when I pulled it out of the bike 4 1/2 years ago and have stored it since. Here are some pictures of it; it's yours if you want it.
 

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#22 ·
I have had a similar issue and it was the tank that was the cause. The inner lip had gotten bent out of being concentric. No idea how - might have been when the pump got pulled at an angle at some point. Previous owner had put a lot of gasket sealant on it to stop it. I was able to bend it back to being round using teaspoon handles and levering them against the bolt shrouds. As soon as it was true again, the leak stopped.
 
#23 ·
@theapp I appreciate the offer. I'll keep it in mind. Right now I'm trying the "boat load of freakin gasketmaker" option. Taking a close look, the new tank is ever so slightly out of round on one lip, so frustrating! One other thought, I'm using a black gasket from gotham cycles instead of the offical Ducati ones, I read elsewhere that they're better. Wonder if they aren't as good as advertised?
 
#24 ·
@lukemillar, looking at mine I think I have a similar issue. The original tank was defnitley bad, the new one seemed better but has a spot a little off. I keep thinking it should be good because it came off a wreck that must have been running in order to wreck. Seems if the tank was leaking like this when it was running, you'd just have a little hiroshima reenactment (and no tank to part out).
 
#25 ·
Fixed

Hey guys,

I'm please to report I've fixed it, and wanted to capture the fix here. I've got the tank sitting half full with no drips as of this morning, and tested while sitting on the bike.

The problem it seems was the mating surface. I spent several hours this weekend sanding. I started by using a micrometer to make sure the tank side of the flange was close as I could get to true and round, within a half MM at all points. I spent a bit of time sanding down that surface, but had some marks I just couldn't remove. To improve that surface, I used a compound called qik steel (fuel resistant variant), that's like a putty. It's specifically mentioned as being useful for fixing fuel tanks. After allowing that stuff to cure, I sanded it down so it was just a thin layer over the original surface. I used progressively finer grade sand paper starting at 80 grit to knock down the thickest portion, and moving down until I got to 1000 grit. The surface was almost mirror smooth.

I also sanded the flange plate. I did not use the qik steel stuff on the flange plate because it was far cleaner. With the flange I started with 150 grit and again moved down to 1000 grit. I could see a reflection by the time I was done.

To install the o-ring I used the well established method of slightly longer bolts to draw the plate in. I lubricated with silicone grease applying inside the plate groove, then installing the ring and applying grease to the outside of the ring. I also lubricated the tank. In previous attempts I'd tried using silicone spray and I think it bound too much. The grease I used was actually labeled dielectric grease (I verified it was silicone based) which came in a little tube and was separate from the other lubrication products. This grease was thicker, and a little more persistent. I used no additional sealant on the o-ring.

While drawing the plate in I used longer bolts as I said above, and drew nuts in with 3/4 turns each until it was all the way down.

Parts wise, I reused my original tank since it's flange was about the same as the new (used) one I bought, and it was cosmetically way better off. I also used the flange that came with the bike. For an o-ring I used a green OEM ducati one.

Thanks to everyone who chipped in, especially Mike Schwitzer who gave me a call and talked me through the process.

Hopefully sometime this week I can finally ride the bike I bought in November. There is a slight leak coming from one of the fuel line I replaced, but I'm comfortable saying that's just a loose clamp. Next steps are to perform a valve inspection & belt change, and rebuild a fork tube. Wish me luck.
 
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