Hi Deval, the piston dimensions are pretty much stock with a few exceptions.
The overall fit of the stock piston in the group head bore (which is 2.017" bore diameter) is very sloppy at a piston diameter of 1.928", so I increased the piston diameter to 1.980" for my piston, to stop it from rocking in the bore and upsetting the seals. The ring lands are located in the stock locations (up and down), but the dimensions are slightly changed. The small diameter of the bottom of the seals on a stock piston are 1.613", mine are 1.620", just slightly bigger which pushes the seals against the wall very slightly harder. And, the width of the ring grooves has been tightened up... the stock piston groove is .315" (or about 8mm) but the seals from both Cafelat and Sorrento are both 7mm, around .276" so I tightened up my groove width to .295" so the seals would be held more 'upright' without deflection up and down as you move the piston. The seals I am using currently are the Sorrento stock seals, which are a V type seal, not U. I added 6 very small holes in the circumference of the face of the piston so that some pressure would transfer under the lip of the bottom-most, critical pressure seal and 'puff out' the seal lip into the bore wall, as had been noted by others (Paul Pratt) to be a common finding in other lever machines but was absent in the Londinium models. But, the brass I used does have around 2% lead in it, which I discovered only after machining it... so I had to send it out to be cerakoted, a ceramic coating which will seal it so it is inert and not leach lead into my drinks. Lastly, I omitted the 2 large holes in the face of the piston (used to torque the piston to remove it) and instead put a small hole in the side of the piston for a small pin wrench; this leaves the working face much cleaner.
I finally got the job batch back from the powder coater after numerous calls (he was on vacation):
I will be installing it today and should be up and running by tonight...