The brake-caliper piston is one of the components most prone to corrosion damage on a classic-MG brake system. The piston works in a sealed environment behind the dust boot, but the inside face of the piston is exposed to the brake fluid (which is hygroscopic and absorbs moisture over time), and the outside face can suffer water ingress past an aged dust boot. The combination produces pitting and corrosion on the piston's working surface, which then damages the piston seal during normal operation and produces a slow brake-fluid leak that compromises the entire system.
High-Quality Stainless Steel Construction
The MGOC Stainless Caliper Pistons are made from high-quality stainless steel and will not rust like the original mild-steel pistons fitted to classic-MG calipers from the factory. Stainless construction means the piston working face stays smooth and corrosion-free across years of service, even on cars stored over winter or used infrequently, the conditions where mild-steel pistons typically develop the surface pitting that destroys seals and causes the brake-caliper leaks that take a perfectly serviceable car off the road.
Why Stainless
Stainless-steel pistons solve the corrosion problem at source. The stainless construction does not pit or corrode under brake-fluid exposure, does not suffer the moisture-driven surface deterioration that aged plated-mild-steel pistons show, and presents a smooth working surface to the piston seal throughout the working life of the caliper. The fit and finish are matched to the original-equipment specification, the same diameter, the same length, the same dust-boot groove geometry, so the stainless piston drops into the original caliper bore without any modification to the caliper body or to the supporting hardware. The cost premium over a standard plated mild-steel piston is modest, and given that brake calipers are typically rebuilt once or twice in a car's working life, the stainless piston is typically the right choice at the point of rebuild.
Application Range
Stainless caliper pistons are stocked in the diameters and lengths used across the classic-MG range. The standard MGB front caliper takes a specific piston dimension; the MGB GT V8 (with larger front discs and a different caliper specification) takes a different piston; the Midget takes a smaller piston suited to its lighter braking demand; the MGA, MGC and the wider range each have their own piston dimensions. The pistons are matched to the specific caliper application at the point of order, ensuring the right part arrives for the rebuild being undertaken.
For owners with modified or non-standard brake setups, uprated calipers from a later application, or aftermarket competition brakes, the technical team is available to confirm the right piston specification.
Installation and Supporting Items
Caliper rebuilding is a standard workshop operation that is well within the scope of a mechanically-confident owner. The old piston is removed (typically by applying air pressure through the brake-line connection to push the piston out of the bore, a fluid-displaced removal, with the appropriate care to catch the piston as it leaves the bore), the piston seal is renewed in the bore groove, the new stainless piston is inserted with the appropriate lubrication, and the dust boot is fitted over the piston and into its retaining groove. The work uses the standard brake-rebuild seal kits available separately, with the stainless piston substituting for the lower-grade piston that would otherwise be supplied as part of the rebuild kit. The caliper is then refitted to the car, the brake line is reconnected, and the system is bled in the usual way.