Airhead (well, /7 at least) Gas Cap examination

Discussion in 'Airheads' started by word1, Nov 9, 2010.

  1. word1

    word1 I'm working on it...

    Jul 24, 2004
    Rome, GA
    If you haven't taken a look at your Gas Cap recently, maybe it's time.

    The cap contains two easily replaceable parts which might need replacement.

    The cap on my '78 RS was slightly sticky to remove the other day, and I took a closer look only to find that the bottom gasket had turned into some type of rapidly deteriorating, and brittle, waxy substance.

    As you probably know, the plastic ring you see on the outside of the cap doesn't seal gas. The bottom (inner) gasket is the actual gasoline seal.

    These part numbers and descriptions were shamelessly borrowed from MAX BMW Motorcycles' on-line parts fiche:

    51 25 1 453 148
    The old gas caps used to come with a red bottom gasket, which sealed gasoline. It was never available as a replacement part until about 1985 when BMW made a black sealing ring available. This is the gasket for the actual cap. It replaces the old red gasket that breaks down. This ring goes right over the threads to where the red ring was on the bottom of the cap.

    16 11 2 307 360 'Ring'
    This is the gasket that goes in between your fuel tank and the gas cap assembly. It does not seal gasoline; it prevents rain/snow/ dirt from getting to the overflow trough built into the tank.

    BTW, I would suggest evaluating the bottom gasket in a non-destructive (read: very careful) fashion. If it's red (ish) in color, you want the replacement parts 'in hand' before subjecting it to any probing.

    I also suggest replacing the outer ring, even if it looks OK. The new ring has a pronounced downward shape (long gone on my old one) and helps the cap fit properly on the tank.

    Both parts are under $15 US, and replacement took just a few minutes.

  2. mfp4073

    mfp4073 Long timer

    Jul 5, 2010
    Central Florida
    Thanks! my /6 has a /7 tank and cap. Didnt know there was a seal on there. Mine is completely missing!
  3. ausfahrt

    ausfahrt mach schnell

    Apr 17, 2008
    Fl & Vt
    Timely info for me, thanks.

    I seem to have a different issue though. The threaded part is disintegrating and I can't quite bring myself to spend about $150.00 for a new complete gas cap from my dealer. Any suggestions?
  4. boxerboy81

    boxerboy81 Stay Horizontal

    Jul 22, 2009
    Melbourne, Oz
    Yeowch ..

    they come up on ebay..keep a watch.



  5. CurlyMike

    CurlyMike Formerly SaddleSoar

    Feb 4, 2008
    east of Dallas
    OK, so this is kinda weird. I was just thinking about these caps tonight. I need like 4 of them...

    Think about it, almost all tanks that come up for sale on ebay or ibmwr have no caps. Someone or everyone is hoarding caps. There has to be a modern fit for these. I know it has to be vented and fit a weird area with a flared top. Someone somewhere has to have figured this mystery out. Someone needs to come clean and help the rest of us poor slobs out here.

    I mean if I can go to a local auto retailer and get a stinking air filter that fits my /2, surely there is options. Its a source of frustration if you cant tell...

    What would it take to machine them like these are. They are only $15.00-$25.00. Would it be that hard to make them so they vent properly?