Transmissions need to be vented. If they don't have a vent or it's clogged, pressure builds up in the transmission housing, causing the seals to leak. On the 4-speed Big Twin models, Harley-Davidson solved the ventilation problem with a screw that had a thin blind bore the length of it, which met an equally thin cross bore in the screw head. The L-shaped bore allowed the inside of the gearbox to “breathe”. Anyone who has been riding old 4-speed Harleys for a while knows from their own experience that this breather screw doesn't stay clean for long and either gets clogged up by dirt particles or, when riding in the rain or washing, even lets water get into the inside of the gearbox.

This is not possible with the PanAm breather screw. With the help of a short nipple and a rubber hose, the original 90° bore is turned into a 180° turn, and no more dirt or water will pass through. The countersink of the screw head fits original ratchet tops from 1965-early 1979 or accessory ratchet tops. Earlier original tops had a steeper countersink angle. If you don't mind the countersink angle mismatch, you will of course be able to make use of the PanAm screw on earlier ratchet tops or on tank-shift tops.