Pin Hole Gas Caps Education Please.

[PaulCDF]

I have these two caps.
One missing center stem: air passes through either direction easily.
One with the center stem air passed from chromed top down with some effort but does not in opposite direction.
Have not run these before as know nothing about them, Any insights tips, Thanks In Advance.


Indian pin hole gas caps top.JPG




Indian pin hole gas caps 2.JPG

[Steve Slocombe]

Up to about 1929, the last of the JDs, Harley used gas caps with a top breather hole and a leather washer underneath. Later they put the breather hole in the side of the gas cap and omitted the leather washer, presumably to stop rainwater running through and pinholing the bottom of the tinplate tanks. Part number was unchanged, so don't block the later breather hole by fitting the leather washer. For 1935/6 the Harley tank caps were made larger to make them easier to turn, still with the breather hole in the side, then changed completely for the knuckleheads.Your larger gas caps seem to have extra holes underneath.

[pisten-bully]

Paul, those are original style caps and I don't know who made them. Guessing those caps were used the year after threaded caps were gone and then continued until the end...but I only say that because I've not seen any other style of original bayonet caps! They vent through that hole, the small pin you're missing on one should "float" and prevent gas from weeping out when your tank is full. For mine it's a once a year step to clean the venting, I'll poke a tip cleaner in from the top and then on the bottom side I'll squirt a bunch of WD-40 into the chamber with two holes, shake the cap a bit, drain it and then blow compressed air through the top and also through the bottom. I use cork gaskets, 2 per cap, purchased from Greers. That's all I got...except I think they're not very easy to find!

[ChiefTwoKicks]

Harry's on the right track. I used Kroil or PJ Blaster rather than WD-40. Just be patient and careful. I also used some hemostst forceps to hold the pin while I worked the pin as if I was lapping the seat on the tiny check valve. I have the heavy garlock like washer under the thin stainless(?) washer. You may be able to make a rubber gasket instead of the cork, garlock or whatever. You will need some sort of washer, you want to be sure the cap fits snug when you complete the repairs. Good Luck, C2K

[PaulCDF]

Thanks Fellas; I did not know if that stem was supposed to move in towards the gas (it doesn't) with forceps and Kroil rotates freely. Thanks Again.

[ChiefTwoKicks]

I'd like to mention a couple things about the venting of the gas caps. The early original caps for the bayonet bungs (not screw-on '35 or earlier) have the bell shape center piece on the inside of the cap. I suspect the tiny hole with the "pin" works alot like the same suck&blow vent as you have on the cam cover. The disc in the cam cover works just like pin, on the gas cap. Except the gas cap doesn't suck&blow as cam cover does. (rapidly, and with much more volume and pressure). The gas caps still suck&blow,, but the volume and forces are greatly diminished.
When the fuel in your tanks get warm, especially on a hot day, the gas in the tank expands and the vapors in the tank cause the tiny pin to raise up and release any pressure created. Thats why the cap should seal properly with the gasket. The design is very basic. The real reason for the gasket isn't only to keep the fluid from ruining a nice paint job, it designed to make the vent valve work correctly, and the lip on the gas tank bung is designed to keep from sloshing fuel up and around the opening. Of course, if your fuel tanks are only half full you won't see the tanks spilling out the bung hole. But, tanks only half full will also increase the volume of the vent gasses, and it could be possible the pin hole would look as if it was leaking badly. Ypu'd see varnish or a tiny hole in the cap that appeared very dirty. The tiny hole isn't designed to spew a huge amount of vapors, especially when working overtime, trying to keep up with such an increase in volume.

For a better understanding of how vacuum principals work go to; Bratannica.com, ARCHIMEDES; the principle of Vacuum and Volumes;. or VACUUMVACAERO.COM/information-resources for an in-depth review.

Sorry about the lengthy explanation. Sometimes, the most basic of principals are the most difficult to comprehend.

Good Luck, C2K

[pisten-bully]

Roger, I've wondered enough about how they work (or don't work as the case may be) that I've contemplated disassembling one to see what's inside! As you say, it's likely pretty simple and I've got working caps that are +/- 70 years old!