The OC Owner’s manual gives three available ratios:
Solo:11.6-1
7.5-1
4.8-1
Sidecar:13.2-1
8.5-1
5.5-1
and in addition a 15-1 (presumably in 1st!) for hilly country.
With your choice of available gearbox output sprockets, the 16 tooth would give you the closest to 'stock' at 4.8-1 in top. I plan to run 10.4, 7.5, and 4.8 for the new gearbox on my SW; I reckon with paved roads I do not quite need such a low first of yesteryear. However with twin carbs, the DT/SW makes a few more hp than the OC, even though the later has a 100cc advantage.
I have never seen a spares list for the OC, so do not know what all was available in sprockets nor the original tooth count. In other lists they often helpfully called out the number of teeth. But as a general observation they did not offer alternate primary chain ratios, any changes were usually done via the secondary of final drive chain. The engine sprocket was not readily changed as it was part of the clutch. Rarely did they list an alternate gearbox input sprocket; however they do interchange readily from model to model (via the input shaft taper or just the sprocket alone via the cush-drive hub) so lacking other visual cues (wrong offset, wrong pitch chain) you can have a different sprocket than originally fitted and not know it. Here, checking with other OC owners might determine a consensus. As general chronology of transmission input sprockets, they went from skeleton-fretted sprockets, to lightening holes, to just plain plate sprockets from 1925 to 1938.
The OC uses could be fitted with a UG, SG, or TG prefix gearbox, and these are all identical, except for internal ratios, to the YG prefix boxes. However the ratios given above are close enough in proportion that they were probably achieved with just a sprocket change. Particularly with the YG box, used on the Dirt Track, there was a wide variety of gearbox output sprockets eventually available, and a few different input sprockets too! (From the 1929 Dirt Track spares list, 14t is the standard gearbox output, and 35t the standard input sprocket, 59t was the rear, but then, this was not intended for road use.) But this is to be expected from a competition machine. All these sprockets will fit the OC gearbox just fine and give opportunities to create many optional ratios.
14, 16 and 17 tooth gearbox output
And I’ve seen several 18t ones as well.
A 13t sprocket would give the 15-1 ratio mentioned above for hilly country, but since 14t is at the lower practical limit for size, this was probably achieved with a different rear sprocket fitted to the brake drum. The rear sprocket is more or less permanently fixed to the brake drum, but again the tapered spline is shared by other models which might offer alternate tooth counts by swapping the whole drum, or as inferred, the OC might have had an optional size available.
There are only a limited number of practical internal ratios possible with the SG, TG, UG, and YG gearboxes. The prefix denoted the particular internal ratio originally fitted (TG wide ratio, SG close ratio.) But in the intervening years, the gears get swapped about or damaged gears and gear cases replaced, so what is stamped on the outside of the box is not likely to bear any correlation to what is inside!
I guess a related question - anyone know what revs the OHV 600s go to?
Interestingly, the OC owners manual states the engine makes 17hp at 3800 rpm, one of the few occasions I can think of where Douglas gave the hp/rpm figures. What is the top end? When it goes ‘bang’ I suppose! The more substantial DT crankshaft and rods are regularly spun up to 5500 rpm, and probably further intermittently, though few will admit it. The late Bill Dent claimed to run his up DT sprinter to 7500 rpm, but then he had a wealth of spare parts for the inevitable rebuilds! It also might have been a cunning red herring to put his other Douglas sprinting competition permanently out of action!
If the rod bearings do not go, the next thing that demolishes a lower end is a fracture around the main bearing boss, on the drive side crankcase half. This is more of a fatigue issue, but can let go from overloading due to high rpm with heavy replacement pistons. Douglas did incrementally increase the casting thickness here as the years went by. New heat treated aluminum drive side castings will likely be available shortly, but in past some folks resorted to casting replacements in bronze for a bit of extra strength at the penalty of significant extra weight.
-Doug
