Gas Cap Venting Fix

[Mike V]

I have a question for those of you that have performed the Gas Cap drilling fix for the pesky +78 gas caps that do not vent properly. Bum and Oak (airmail 5/07) describe this in their archives. The instructions are; from the bottom of the cap, drill a 3/32" hole 1/2" outward from the center through the shell past the void and continuing through a soft 1/8" thick layer then stopping. Then, obviously blowing as much debris out as possible. I have a question since this is a one-shot deal...

There are 4 existing holes in the bottom shell along the outer perimeter, while probing through these holes I can feel an inner shell about 1/2" or more from center. Is the new drill hole to be on the inside or outside of this inner shell???

[bruce_launceston]

Hi Mike, I have not experienced the venting problem but I have had my gas cap apart.
The '84 cap (without the BMW logo) can be pried apart, the wide top section push fits over the threaded steel section.
Perhaps if you can get yours apart you might find the blockage or at least understand where extra holes will help with venting?
The metal thread is bonded to a plastic inner with 'keyways' that locate the white plastic lock barrel. I guess they are saying to only drill the bonded metal and plastic part and not into the lock barrel itself.

If you get this far check the condition of the black lock tongue and the spring behind it because IMHO these are the cause of the 'spinning stuck gas cap' that we read about. My fix was to put a screw through the side of the lock barrel to create a fixed 'tongue' to permanently engage the lock.

The photos show my cap with the old screw that I put there 20 years ago, I replaced it with a shiny new one with square sides (and screwed all the way in).

Disclaimer: I have only pulled apart a '84 cap not an early cap with the BMW logo on top!

[montmil]

I drilled my cap to "fix" a venting concern. I've modified your photo to show where I drilled the hole 0.50" from the center. Vents now!   Monte

[Mike V]

Thanks Guys,
I followed the instructions by Bum and Oak; 1/2" outward from center and drilled the hole, then through the soft material.  Seems to work great now. Constant high volume of flow and no hissing at the cap from a vacuum pull. Hopefully this will cure my starvation problems at speed when switching to reserve.

Bruce, Monte...thanks for the photos.