The Sports cams will fit in any of the post '48 motors - the only difference between the sports cam and the touring cam is the diameter of the base circle - the sports cam being about 30 thou smaller - giving about 15 thou more lift. Oddly, the wider timing gears in the Dragonfly give more problems than the narrower Mark gears. I put that down to them being over-oiled, and generating a hydraulic lock between the wider teeth (which eventually causes tooth breakage). When I first got my Dragonfly, the timing gears would last about 8,000 miles before the cam idler would shed teeth. During the second engine rebuild, I drilled 2 drain holes (5/16"dia) in the front wall of the crankcase so that the cam gear teeth only just dipped into the oil (by the depth of the teeth) - this seems to have been successful as my engine has now done another 35,000 miles without any further shedding of teeth. Also, if you want better, reliable performance, try running with double the tappet clearance. Mark series engines (with steel pushrods) had 5 thou tappet clearance, so it makes sense to run Dragonflies (with alloy pushrods) with a larger clearance - say 6 thou instead of the specified 3 thou. I am convinced Douglas only specified 3 thou because of critical comment regarding the mechanical clatter from the earlier engines. Likewise, with the recommended SAE 140 oil in the gearbox - the gearbox runs fine on straight 40 grade oil.
Eddie.
