O-ring for M35A3 and most M939A2 truck rims. P/N 12363606. NSN 5331013147598.
3/16 inch thick (0.19 inch). For 20 inch rim. Factory original OEM part, made in USA.
For M939 series using a thicker O-ring see P/N 52-49529-00.
Note: if the tire does not hold air this O-ring is likely not sealing correctly. Rim sealing surfaces must be free of rust and pitting for this O-ring to function correctly. O-ring should be replaced when the rim is disassembled for tire work.