Off the shelf radiator cap for a 66 250S - Trident / Supercheap / Repco equivalent?

Boonie - the 1966 W108 250S uses the same radiator cap fitment as the W108/W109 family through 1972. Pressure rating 1.0 bar (~14.5 psi), 39mm filler neck, lever-style with two locking tabs.

Off-the-shelf equivalents that fit and hold pressure:

• Tridon CC0006 - 14 psi, exact fitment, available at Repco for $18.50. This is the one I run on my project car when the OEM cap is out for service.
• Gates 31301 - 14 psi, also fits, $22 at Supercheap.
• Stant 10231 - 14 psi, Bursons sometimes has them at $26.

The Tridon is the cheapest and the most commonly stocked. The 250S system doesn't need a 1.1 or 1.2 bar cap - stay at 1.0 bar (14-14.5 psi). Higher pressure caps stress the rubber hoses and the heater core which on a 60-year-old car you do not want.

Online OEM cap from Niemoeller takes 8-12 weeks because they ship via standard post, that's why yours is taking forever. The Tridon CC0006 will hold you over for as long as you need.

Boonie I've still got the original Stant 10231 on my SEL from when I bought the car new and it holds pressure. They're a good cap. Get one of these.

Steve woulda just driven to Repco.