Douglas fitted a flywheel clutch to the DT and marketed it as the SW5 in 1930-31; so it will fit.
The Douglas flywheel clutch was introduced on the 1923 Works racing RA models, and was available to the public on the 1924 OB. From then on a flywheel clutch was used on most models to 1938 (excluding the 2-speed 2-3/4hp and the 2-stoke Bantam of 1934-35). The design constantly evolved until the early to mid-thirties, by which point it settled down to two basic designs: one for the light weights and one for the heavy weights. So you have to be careful as to which one you look for. Ideally, you would want one from a SW5, or any ohv model after. This will have the large taper, and the double sided friction plate. The large side valves continued to use a single sided friction plate (often cork), that Douglas deemed not sufficient for the extra hp of the ohv.
From about 1933 onward, Douglas added drive pegs to the pressure plate that protrude slightly through the flywheel. First with two pegs, then three. Those were used on the ohv models until they were dropped from the catalog after 1935. Given the scarcity of ohv flywheel clutches, the last few folk that needed one just made new ones. They are not that complicated, the only challenging part is the carrier sleeve because it has to be case hardened and ground. I have drawings for a three-peg replica, with some redesigning of the carrier sleeve to make it a little easier to grind and improve the roller bearing by adding a cage. I have made them, and I know other have used the drawings to make their own.
A few minor things will need to be done to the DT engine to fit a clutch. The main bearing hub on the flywheel side of the crankcase will need a steel ring shrunk onto it with three pegs for the cam operation of the clutch throw out lever. If you are lucky, sometimes the factory put the ring on there, or machined the outside of the boss even though a clutch was not installed. Other times it is just left as cast and needs to be machined.
Initially (even on the SW5) these three clutch cam pegs were just solid, round pegs. Starting with, I think, the S6 600 side-valve, they started adding roller sleeves to the outside of the pegs. This is a much better design so you might as well copy that. The solid pegs, even though hardened, quickly develop flats.
You will need a clutch operating arm. Douglas only had a few different styles, so likely any one you find could be adapted. I have even made them by turning the hub and welding on an arm and sculpting it to loop like the original drop forging.
Then you will need a anchorage for the clutch cable. This is similar and bolts on the same place as the cable anchorage for the existing valve decompression cable. The later brackets simply have tapping for two cables instead of the one.
-Doug
