At some point during Twin Cam production there was a design change in the Dunlop master cylinder to add a second seal on the piston. The Girling clutch master cylinder is very similar with internal parts, but has its own set of change notes. In fact the pistons and rebuild kits appear to be interchangeable (although not exactly identical). The first picture (cropped from the Service Parts List) shows the early configuration with a single "O" ring seal (10) near the pushrod end of the piston (4).

Picture below left shows the early piston. Picture below right shows the later piston. The illustrations give the dimensions. The early piston can be machined to accept the later seal set.

The old original main spring (Item No. 10, Illustration No. 9 in the parts book) inside the master cylinder was 88 mm long and had an outside diameter of 20 mm. The replacement main spring (Item 33, no illustration) for the replacement service kit 8G8496 with the new piston with two seals is 100 mm in length, with the same outside diameter and wire thickness. It is thus definitely stronger when compressed. The wire diameter is 1,62 mm for both springs. The later, longer, stronger spring should be used with the later piston with two seals.

In retrospect it appears that the newer design is (was) intended to superseded the earlier design. This could be viewed as a safety issue as well. This means that dealer service parts would only be available for the later type master cylinder. If/when an early type part might fail, it should be replaced by the later type part rather than being repaired. These days that means you may have to machine the early type piston to accept the later type seals, as the early type o-ring is no longer available.

As a point of reference, it is so far known that car number 801 still had the early style pistons.

This is a Dunlop master cylinder for Jaguar, much different housing but same internal parts.

Here is a drawing with some dimensions. The red circle is location of the output port for the Twin Cam. The light blue circle is location of the output port on a slightly odd part, garnered from an eBay ad (see next page). If you need to use a grease gun to force the piston out, you need to plug the internal port between the pressure chamber and reservoir. Klaus Junker suggests a small plug of hardwood backed up by a wedge as shown in the left end of this drawing.

Mark Hester used a plastic coat hanger with the end sharpened in a pencil sharpener. Perhaps a golf tee could serve the same purpose.

The Zerk fitting shown here is tapped and screwed into the end of a brake line flare nut fitting. A 3/8-24-UNF bolt with an axial drilled hole works as well.

Addendum November 2010:
On 11/18/2010, Frank Netterberg wrote:
"My Dunlop cylinder has a plastic strap (now pink in colour) around the cylinder marked VBO 4843 MOD KIT on it. I understand that this is the Dunlop part no. for the later kit incorporating the double seal piston and stronger spring listed as BMC part no. 8G8219 and, apparently as PBR K7167x (seals only?).

"The whole assembly looks very similar to that used on a Toyota Hilux, but I have not tried it".