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MG MGB Technical - Clutch MC Parts Question

master cylinder
danny jacob

The washer you speak of is made of very thin
shim stock spring stainless steel, and has a slight
curvature in it's shape. I mic'd one belonging to
an original "cup seal" design M/C at 0.005" thick,
having a curve of 1/16" (crown height).

It sort of acts like a one-way valve in that when
you depress the clutch pedal, the washer flattens
itself and covers the tiny fluid feed holes on the
face of the M/C piston. When the pedal is released,
the washer returns to it's curved shape and exposes
the feed holes and allows fluid to enter the bore.

Without this washer, you'll have one heck of a time
trying to bleed the clutch system (as though this
weren't already a dicey enough job when everything
is 100% correct).

On the earlier (Original) "cup seal" master cylinder,
there is no provision to keep this washer centered
within the bore-line of the M/C and it will often slide
down between the piston face and cup seal and scrape
it's edge along the bottom surface of the bore and
gradually become mis-shapened.

The later (Modified Design) "ring seal" master cylinder
has a piston with an extended nose that keeps everything
centered within the bore-line.

This washer is/was included in the OEM Lockheed brand
M/C rebuild kit. Some aftermarket or reproduction kits
may - or may not - include this washer, depending on the
particular manufacturer of the kit.

Your best bet is to get a hold of a rebuild kit that includes
this washer. This can be confirmed by asking your parts
supplier before ordering.

Having this washer will save you from (some) hair-pulling
when it comes time to bleed the clutch system.

If yours is an "Original" cup-seal type M/C (see
Moss catalog to confirm this) - then I happen to
have a spare washer (one, only - used, but in
perfect shape) and can send it to you if you
email me your address.

Safety Fast.

Daniel Wong

This thread was discussed on 24/04/2006

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