PD series  Petter engines air cooled, with 3-1/2" bore and 4-1/3" stroke. Anybody seen one?
  When looking through the Petter specs. Many share the common 4-1/3" stroke. Noting that the PH use a 3-7/16 bore and the PJ has a much larger 3-13/16 bore. With the PD being a little larger than the PH with 3-1/2".
  The line of thinking is.........When in there anyways and boring. The PH sleeve while being thick walled won't take a 3-13/16" bore up from the 3-7/16". There is however plenty of sleeve wall thickness left if bored 3-1/2" and a PD piston used.
  Any ideas if the PD engines shared the same piston pin and deck height as the AV,PH & PJ engines? Odds are they do as commonality reduces production costs.
  I wish I could have found a PJ1W engine but the PH1W is all I have to work with. The generator project leaves the factory PH1W a couple of HP short. I'm hoping a little tuning and mild tinkering by cleaning up the ports and a little more displacement. This engine would be able to make 10+HP at 1800rpm without excessive smoke.
  Of interest the PD engine with the 3-1/2" bore makes as much power as the PJ engines with the 3-13/16" bore? The PD is only 1/8" more bore than the 8HP PH series?
  Did the PD engines have better flowing cylinder heads or where did the extra HP come from?     

There's the big PDV, but they don't appear to have made a small single or twin cylinder version in that bore/stroke range.

That's got larger bore & stroke though.

http://www.stationary-engine.co.uk/PetterData/PetterPDV1.htm

There were a few PD's made, but 1956 or so, and not an engine that seems to crop up much at all these days.

http://www.stationary-engine.co.uk/PetterData/petter2.htm

Peter