Martin,
Douglas used the 68x68mm dimensions on a number of OHV models. Their first commercial OHV, the S1 of 1921 was a 68x68mm engine (with a larger sibling the 733cc S2 of a very over-square 83x68mm.) This was followed by the successful RA of 1923-25, which also used 68x68mm. The OB of 1924 introduced the first of the 68x82mm ‘long-stroke’ 600cc motors, but the 350cc OW kept on with the old 68mm stroke, but with the cylinders bores suitably dropped down to 57mm, and shortly thereafter the OW itself was dropped.
When the DT came along in 1928, practically all the OHV models shared the same 82mm stroke crank- DT, OC, and TT/I.o.M replica. Bores were 62.25, 68, and 76mm for 500, 600, and special to order 750cc capacity. They did catalog a 350cc DT and TT/I.o.M. replica that used 68mm stroke crank, probably using old style forgings.
In 1931, Douglas decided to reintroduce the 68x68mm engine on a short-stroke DT. These had full lozenge shaped center webs and looked just like a shorter stroke version of the 82mm stroke crank. This is newsworthy as earlier 68mm stroke cranks were decidedly skimpy looking! Cylinders barrels were 4.156” long verses 4.825” of the long-stroke engine. The heads of the standard 600DT engine were then used with the 68mm bore for the 500cc short stroke, so that the combustion chamber size would match. A new connecting rod forging was introduced that was about 5.25” centers instead of 5.75” long. This went into a shorter and lower frame, with a 19” rear wheel to allow a shorter wheel base yet. (See Henry Body’s ‘Red Devil’ for a good example of a short-stroke DT.) But it seems to have been too little to late. They did not regain supremacy from Rudge on the cinders and the short-stroke DT never lived up to its promise. Those new rod forgings did not go to waste. They used them with the 82mm crankshaft in the F/G31 and OW/OW1 road OHV machines, and redesigned the side-valve engine heavyweight to use them as well (but not as extensively machined or polished.) They continued to use them till 1938 on the 600cc side valves.
Genuine short stoke DT engines have a prefix code OE, and those crankcases are few and far between. But the crankcase dimensionally is otherwise the same as the long-stroke, so there is nothing to stop one from placing the shorter barrels, rods, pushrods, and suitable heads to make a short-stroke motor. The EL prefix is that for the 62.25x82mm and 68x82mm 500DT and 600DT as Chris mentioned. Len Cole and Colin Clifford built a few short-stroke motors in the eighties and campaigned them with the long-stroke DT/SWs in the vintage sprints. Theory was they could rev higher, but like on the cinders, I do not think they ever showed any clear advantage over the long-stroke.
-Doug
Correction, OE is the crankcase prefix for the OB model. It seems that the short-stroke DTs used the EL prefix, same as any other DT model. 27Aug13 -Doug
