The MGA With An Attitude
FITTING NEW PISTONS to MGA Disc Brake Caliper - BT-104A

This does not apply to MGA 1500 or Twin Cam or Deluxe models. This only applies to the standard MGA 1600 model.

You may have (should have) read in the previous article that MGB pistons can be substituted into the MGA calipers by removing the center guide pin from the caliper. This is a perfectly functional modification which saves some money (when MGA pistions are much more expensive). In the MGA caliper the center pin is a tight press fit in the cylinder. To remove it, PULL VERY HARD. If that doesn't work, cut the pin off nearly flush with the back of the bore using a thin abrasive wheel in hand grinder (Dremel tool).

But if you already have new MGA pistions in hand, or you want to retain originality for future reference, then there are a few tricks to installing the new parts.

In recent times replacement MGA type pistons have been supplied without the friction stop and sleeve. To use the new pistons in original fashion you need to transfer the small piston stop and sleeve from the old pistons to the new ones. These parts are held in place by a crimp in the narrow flange around the center hole in the piston. See arows in the picture for location of the crimp. To remove the small parts from the old piston you will need to remove the crimp. The easiest way to do this is to use a hand grinder to grind away the part of the flange which is deformed, after which the small parts should pull out easily (knock on wood).

Install the small parts in the new piston and crimp the flange (similar to the original part) to hold them in place. When installed the pistonn stop (the smaller tube part) should have a small amount of end float inside the sleeve (about .010"). This is to allow the piston to withdraw slightly on release due to the residual flexing of the piston sealing ring (see notes in prior article).

If you are replacing the pistons because they were pitted from corrosion, you might consider looking for chrome plated pistons. Or you can buy stainless steel pistons, maybe for less cost than stock parts from the common British car parts suppliers.
Also see:
Notice Style E for MGA, Part Number: 100529 U.S. #: 17H7730
Notice Style D for MGB, Part Number: 114712 U.S. #: 19-1169

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