Pin Index Safety System
The Pin Index Safety System (PISS) is a safety feature that prevents connecting the wrong gas cylinder to a regulator or medical device. It works by matching patterns of holes on the cylinder valve with pins on the yoke. There are two holes in specific positions on the valve, and each gas has its own pin pattern. The pins are 4 mm in diameter and 6 mm long, and they must line up with the holes for a proper, tight seal.
A Bodok seal keeps the connection gas-tight between the cylinder valve and the yoke or regulator. It’s a neoprene washer with a metal reinforcing ring that resists high pressures (about 2,000 psi) and won’t splay like older fiber washers. Bodok seals were introduced in the 1950s with PISS. When changing cylinders, be careful not to leave the Bodok seal sticking to the valve face, or it can be lost and cause leaks.
PISS can be bypassed if pins are removed or damaged, or if extra washers are used, or in some valve designs by inverting the cylinder. There have been occasional color-coding errors that can lead to confusion.
Larger cylinders (above size E, 679 liters) often use threaded connectors. Connector standards vary by region: CGA in the United States; BS, DIN, and AFNOR in Europe; and JIS in Japan.
Blanking plugs can be used to seal unused yokes to prevent leaks.
Bodok seals are also used in emergency oxygen kits for underwater diving, while most scuba regulators use traditional O-rings with DIN 477 or CGA 850 yoke connections.
This page was last edited on 2 February 2026, at 08:49 (CET).