Thanks Chris for the drawings and notes.
In search of some piston seal the pistons had to come out.
Rather than take all the valve gear apart I took off the front cylinder covers
unscrewed the grub screws from the crossheads and using two holes drilled into the piston faces I was
able to unscrew the piston rods from the crossheads , As expected the O rings were, shall we say, not of a circular
cross section . Now the choice is either just replace the O rings or make new pistons and fit proper rings .
Brian