If we exclude the Mk1 T35 (which has the high dome pistons), original Douglas postwar pistons came in 3 types. Pistons for the Mk3-5 machines were solid skirt and had 2 compression rings (1/16" wide) above the gudgeon pin and a 3/32" stepped scraper at the bottom of the skirt. 'Plus' pistons looked identical but were made of 'Y' alloy and were much lighter. For the Dragonfly, Douglas changed to a split skirt design (presumably to cut down on mechanical noise and reduce the chance of pistons nipping up). Whilst this was probably true, these pistons did suffer from the crowns collapsing when worked hard - the usual cure being to revert back to using 'Mark' pistons.
Back in the 1980's, original pistons were getting rather scarce, so the LDMCC had a couple of batches of pistons made by Peter Hepworth (PHC). These had the scraper rings located just below the gudgeon pin (a practice that we were ensured was better than the original set up). Experience showed that oil consumption varied dramatically from bike to bike. These pistons were then returned to be modified by drilling the oil drain holes below the scraper ring (which improved matters but was not a complete cure). The PHC pistons were also different in that they had wider metric width rings which didn't help the oil consumption as they would have taken longer to bed in.
In the 1990's, JP pistons announced that they could make pistons for the postwar Douglas, and following good reports, the LDMCC started supplying them through the spares scheme. All was well until the early 2000's when the quality control seemed to drop off - rumour had it that the rings were being 'bought in' rather than made 'in house' - the pistons were OK, just the rings needed careful checking.
Around 2010, following some adverse comments, the LDMCC looked elseware for pistons, and are now getting them made in the UK.
So that is the saga of the pistons for postwar machines - there are, of course, pistons from other machines that can be made to fit (usually Honda), but the above is just about the complete list of pistons that were available via the factory or the LDMCC.
Regards,
Eddie.
