A machined billet aluminium filler cap for fuel cells and custom tanks, replacing the pressed or plastic caps that come with budget cells. Billet construction means the sealing face is machined flat and stays that way, which matters on a car that gets thrown around a circuit with a full tank of fuel sloshing against the underside of the cap. It is a common upgrade during a fuel cell installation, and equally at home on a fabricated surge or swirl tank built from tube and weld-on domes. The machined finish also tidies up a boot-mounted cell installation considerably, which counts for something at a show or scrutineering.