At 03:35 PM 8/28/04 -0400, Tony Buscaglia wrote:
"The first seal in the kit, that fits over the end of the piston, were too large for the piston that was in my MC."

This is for Moss Motors part number 180-970 Rebuild Kit for the 7/8" bore dual master cylinder, MGA, Sprite/Midget (not 1098cc), TR2-4A. Problem is that the front main cup (with the donut hole) is too wide. Groove in the spool is 1/4" wide and the center hub of the cup seal is 3/8" wide, so the cup seal doesn't fit on the spool.


The Moss seal is apparently correct for OEM application. Mr. Buscaglia seems to have a non-original rebuild type piston in his master cylinder. See addendum below for some possible piston variations you might encounter.

ADDENDUM - Oct 4, 2004:
Moss research into this situation reveals that their seal kits DO contain the correct seal for the MGA OEM assembly. For the past 10 years Moss has been supplying AP Lockheed (now Delphi-Lockheed) master cylinders and rebuild kits, the design has not changed and is still the same as OEM. Moss has been getting only about two returns per year noted "not correct for application".

The picture above shows a different piston, apparently some aftermarket part, probably installed during a past rebuild. Some time ago Roadstar-Nelson, and Beck Arnley used to sell rebuilt master cylinders for this application which contained non-OE type pistons and seals with the narrower groove and seal as shown in the picture at the top of this page. These replacement pistons are also larger diameter. Today rebuilders may opt to replace these non-OE pistons with original type parts, so the original type seals can be used, but that would require sleeving of the cylinders.

Notice: Illustrations in the "MGA Workshop Manual" and the "MGA Service Parts List" may not look exactly like the original spec piston (but they are close).

See figure at right. The aftermarket type piston has all ridges with about the same OD, and the narrower seal fits between the first two ridges. The OE type piston has a smaller ridge a little distance in from the end, and the wider EO seal fits over that ridge.

Moss Motors has noted their intention to post an illustration in the next printing of their MGA parts catalog to call attention to this variation in some aftermarket pistons and seals.

Before you ask, no, I do not know where you can buy original type pistons. Apparently some rebuilders make their own original type pistons for replacements during restoration services.

Addendum 3/17/07:
At 09:33 PM 3/16/2007 -0600, Mark Powning wrote:
"I worked at Roadstar Nelson for a long time. I know for a fact that the piston used is .025" bigger than OE. I would not suggest you replace the rebuilt piston with a stock piston. The clearance is too great and could easily loose all pressure from a catastrophic seal failure."

Okay then. Apparently the odd pistons were used to accommodate a machined oversized bore to avoid the expense of sleeving the cylinders. If you have a master cylinder with these odd replacement pistons, and you want to change back to standard pistons and standard rebuild kits, you will need to have the cylinder sleeved to get back to standard bore size.