fuel cap rattle

[Air4Life]

I replaced the two seals and now it sits to high off the tank.  It seems to be sealing good, but now it has this annoying rattle accompanying the ride.

I order parts 17 and 6.  What am I doing wrong here?

[Air4Life]

I will add that in the diagram 17 appears to fit inside 6; however, in my situation (note pic) this is not the case.  Wrong size??

[Bob_Roller]

The rubber seal is on the bottom, so it contacts the mating surface on the tank filler neck to properly seal .

The rubber seal does not go on top of the plastic cover .

The diagram is misleading, not uncommon in the BMW parts illustrations .

[Air4Life]

Wait a second, Bob.  Are you suggesting they way I have the large seal (#6) is not supposed to fit within the groove?  

I had to use some effort to get it like that.  I thought I read somewhere that it fits in the "groove".  In fact that's how some people access the cap if their pawl wears, that is, by pulling it over the groove and exposing the small holes that allow for access to temporarily LOCK up the free spinning  cap.

[Bob_Roller]

In the picture you have posted, the rubber seal should be just above the hole in the threaded area of the cap .

It looks like you have it installed correctly .

[Mike V]

Air,
In my experience with the replacement cap gasket (not the larger upper rain seal) is; that it is a universal replacement gasket and does not conform to the original gasket that came with the cap from the factory. It sits lower on the cap and even though it seals the tank well, the cap will rest a little higher when screwed on the tank. I have replacement cap gaskets on both my bikes and I find the cap does not sit down on the tank as it did with the original gasket.  Another observation I've made with the larger upper rain seal is the material seems to be made of a much stiffer material rather than the original subtle and softer OEM rain seal.

From what i understand these are the only replacements available for both of these seals.  They work, but do not fit as the originals.  I'm not sure why you are getting rattling from the cap. It may be the lip of the slightly raised rain seal is contacting the upper limits of the tank during operation.  Or might be coming from the tank and the frame?  Be sure the rubber connection bits under the tank are in good shape and proper placement.

I try not to secure the cap to the point of it clicking.  I stop when the cap o-ring contacts the tank filler neck and leave it at that.

[Barry]

Fuel caps will rattle if the seal #6 is too thick as it doesn't allow the cap to screw down far enough. The two part cap relies on some tension between the top half and the bottom half to stop it rattling. If the seal is too thick the top skirt will not make sufficient contact with with the tank to provide this tension.  Have you compared the thickness of the new seal with the old.

[Mike V]

Barry,

The replacement seal and original seal are completely different.  The original seal fit in the recessed area just above his new seal in his photo. They are different in thickness and diameter.  The replacement seal slips around the threaded portion of the cap while the original seal actually seemed to be poured or molded into the cap flange or lip.  

[Mike V]

This photo shows the replacement gasket in place. Just at the end of my phillips screwdriver is where the original gasket or seal was positioned.   The original gasket allowed the cap to be secured deeper into the filler neck while the new replacement gasket is thicker and does not allow the same cap depth into the tank.

[Air4Life]

Thanks Bob, Mike, and Barry.  Here's the kicker.  The original seal disintegrated - ohhh, back in the September I'd say, and I just got around to replacing it.  I decided to do right by the machine... Oh boy.

I think Mike hit it on the head.  From what I remember of the old seal, it was different.  So now, do I find a large enough O ring to put under the Rain skirt so that it takes up the difference and gets rid of the annoying growl/rattle, or do I chuck the smaller seal altogether.  Hadn't had an issue for over six thousand miles.

...gunna have to meditate on this one for a while.  

[Barry]

Mike,

Yes I see what you mean.  That new seal is nothing like the original which didn't have the lower lip. The original was more of a L section. I suspect that the lower lip on the new seal is intended to sit inside the tank aperture but is too large in diameter to fit.


Air,

I know you spent good money on what you thought was the correct seal but one thing that I used for a while which worked perfectly well was an ordinary Nitrile rubber O ring of something like 38 - 40 mm diameter and 4mm in section. I remember the 4mm section was important as a thicker O ring caused the cap to sit too high and rattle. The diameter of the O ring should be such that you have to stretch it a little over the cap so it sits reasonably tightly in the groove.  Sounds too easy but it really did work just fine. The cap tightened down nicely and it never leaked.

[Bob_Roller]

If you can find an o-ring that is the same diameter, but a thinner cross section and is fuel resistant, it will work just fine .

I just took a look at the seal on the fuel cap on my'81 R65, took it to work today, and the seal is falling apart, need to replace it .

For what it's worth, the original seal is nothing more than a large thin rubber gasket/washer .

I replaced the seals on the fuel caps of the the two LS's and the replacement seal is about twice the thickness of the original .

[Mike V]

...gunna have to meditate on this one for a while.

Kind of a nasty deal Air, I share in your frustration.  You may check with McMaster Carr for a proper o-ring or replacement. Don't know why your cap is rattling with the replacement seal, neither one of mine do so far.  But Barry may be correct - due to the higher profile of the cap when secured it's causing some noise. Try running it for a while and see if it compresses enough to stop the rattle.  Ever consider riding with a tank bag?

Let us know if you find something as a replacement.  And, double check your tank fitment onto the frame.  Just to be sure the noise is coming from the cap.  Good luck with it...

[Air4Life]

Mike,

No, its definitely the cap.  It just, I mean, JUST hovers over the  tank.  Place my hand on it and it goes away.

[Mike V]

Air, check my memory on this one...if I remember correctly; one side of the gasket has a smaller diameter lip than the other?  If so, flip the gasket over so the smaller diameter is towards the tank, or away from the top of the cap.

Or am I making things up?