The Ace Mercury wheel disc was an original factory option for the MGA, and also for the ZA-ZB Magnettes, MGB and MG Midget. This was not exclusively an MG option, as the same style parts could be purchased as aftermarket parts for any model car. The difference here is that the MG parts carried an MG specific style center badge. Most had an MG octagon fitting installed. Some may have had an eared spinner similar to a dummy knockoff nut (but not standard option for MGA).

Construction is of bright anodized aluminum. The parts are light weight but not particularly strong, so care must be exercised to avoid damage during installation or removal. There are small louvers that serve as air scoops to pull in air and push it inward through any openings in the wheel to help cool the brakes. These covers are therefore right and left handed with two of each in a set of four. The covers should be installed with the open edge of the louvers facing forward at top of wheel, so it scoops in air with forward rotation.

The design changed slightly to accommodate the change of steel wheel design. It was part number 97H675/97H676 for most for MGA 1500 production, then changed to part number BHA4165/BHA4166 Com.(C)63577 in January 1959. Rimbellishers were discontinued at the same time because they did not fit the new wheels.

These wheel covers could be ordered from the MG factory as optional parts during original assembly of the car. They were often installed or sold over a dealer's parts counter as a dealer option. MGA and Magnette covers are for 15-inch wheels. MGB covers are for 14" wheels. MG Midget covers are for 13-inch wheels. The aftermarket parts were available for various wheel sizes and with various center emblems to suite many different makes and models of cars.