The petrol filler cap is one of the most visible items at the rear of a classic MG, typically mounted on the rear wing or rear deck and visible from the driver's mirror, both functional in sealing the petrol tank against vapour loss and contamination and decorative as a polished chromed surface complementing the rest of the car's brightwork. Replacement caps restore the visual appearance where the original has tarnished, been damaged, or lost, locking caps add security against the increasingly common opportunist petrol theft from cars parked in public locations, and competition-style caps add a sporting appearance for cars built to a period rally or fast-road character.
Period-Style & Standard Replacement Caps
The original caps fitted to the cars during production were typically non-locking polished chromed or stainless steel items, often carrying the MG octagon at the centre, a distinctive period item that contributed substantially to the car's visual character, and the standard non-locking replacement cap provides a straightforward direct fit for owners renewing the original. The cap style varies by application, the earlier roadsters typically using a lift-off pattern held by a chain or strap, while later cars used a hinged or screw-on pattern, and each style is stocked in the appropriate period-correct visual specification with the chromed finish maintaining the standard expected on a classic-MG application.
For owners restoring a car to strict factory-original specification, the period-correct non-locking cap is the appropriate choice, although a factory-fit locking cap was never standard equipment and locking caps were originally available only as an after-market accessory through BMC Service Limited.
Locking Caps for Security
Locking petrol caps add security against opportunist petrol theft, an increasingly practical consideration given the rising value of premium fuel and the common practice of leaving classic cars parked unattended at shows or in public car parks. The locking cap uses an internal key-operated mechanism that prevents the cap being removed from the filler neck without the key, securing the tank contents against drainage, with the visual appearance similar to the standard chromed cap from outside the car and the lock mechanism concealed inside the cap body. Locking caps are stocked in chrome finish, in a stainless-steel finish that closely matches the factory stainless cap and offers superior long-term corrosion resistance, in an RV8-style dome-faced design inspired by the original MG RV8 and the early 1960s factory accessory giving the car a subtly more distinguished appearance, and in a matt black finish for owners preferring a more discreet look or matching darker-coloured cars. All locking caps are typically supplied with two keys.
Quick-Release & Competition Caps
For modified or competition-inspired cars, quick-release filler caps in the Aston or Monza style provide a functional and visually striking upgrade, particularly well-suited to cars with Sebring bodywork or other period competition modifications, the cap designed for rapid refuelling during pit stops with a flip-up lever rather than a screw thread. A threaded collar is available separately for installations where the existing filler neck needs adapting to the quick-release cap, and for owners who prefer the look of a quick-release cap with the practicality of a locking mechanism, an imitation quick-release cap is offered combining the robust competition styling with a concealed standard locking cap within. The factory offered a Le Mans-type quick-release cap as an option for competition-prepared cars on some applications, more typically seen on works competition cars than on standard road cars.
Modern Petrol & Tank Venting
Modern UK petrol, particularly E10 unleaded as the standard forecourt fuel, has different vapour-recovery and venting characteristics than the petrol the original cars were designed around, and modern filler caps incorporate the appropriate sealing arrangement to suit modern petrol specifications, preventing the petrol-vapour issues that can develop with simpler older cap designs. It is essential that the cap maintains correct tank venting, as a non-vented cap on a vented tank system causes a vacuum to develop in the tank under sustained driving, resulting in fuel starvation as the pump can no longer draw fuel against the partial vacuum, so any replacement cap should be specified as vented for the application. The seal between the cap and the filler neck must provide a positive closure that prevents vapour escaping and fuel splashing during spirited driving or over rough surfaces, and the filler neck seal, filler pipe, and tank-to-filler hose should be inspected whenever the cap is attended to, as the seals deteriorate over time and can allow fumes to escape into the boot, often the first sign of degraded sealing being a fuel smell after a journey.