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MG MGA - Master Cylinder Piston Variation

In the process of converting my ’59 steel wheel roadster from drum to disk brakes I discovered that the brake side of the MC was sticking and as a result the calipers were not releasing. It looks like the same issue that is documented on Barney’s site (BT101b). The MC was resleeved in brass about four years ago and I installed a Moss rebuild kit. This afternoon when I tore down the MC I also pulled out a spare pair of MC pistons to compare to those in the Lockheed MC. As you can see in the attached photo, my spare set of pistons have a dome at the primary cup end and my existing pair has a flat end. Overall the dimensions between the two are virtually the same. The only measurable difference is that the flat end type is 2.05” long and the dome type is 2.0”

I have no way of knowing for sure if the domed pistons are original MGA/Lockheed types, I picked them up at a jumble along with the MC top, front plate, springs and slow release valve. All of these bits are similar to those in my Lockheed MC except that the springs have metal caps were my springs have blue plastic caps. Anyone have any thoughts on the variation? Could this be the same piston from a TD or TF? Maybe an early production MGA?

John



jbackman

I haven't seen the domed end piston before. I will take a wild guess and say it might work without the wave washer.

The one that looks like a flat end may actually have a flat step about 0.005-inch high, like a thin pilot diameter in the center. I think it is visible in the picture (unless my eyes are playing tricks on me again).
The lower flat around the edge by the holes accommodates a thin brass wave washer. The wave washer (like a spring) helps push the rubber cup up a little to allow fluid to flow through the small holes on the return stroke, when the fluid will then flow past the lip of the primary pressure cup. As said, with the domed end this may work without the wave washer. The flat end piston is normal for the MGA Lockheed master cylinder.
Barney Gaylord

This thread was discussed between 03/06/2012 and 05/06/2012

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This thread is from the archive. The Live MG MGA BBS is active now.