You can verify the major components in the identity tables located
here, as well as info on the model and yearly changes.
There is not much to go wrong with these models, indeed they have little horsepower to harm themselves. The engine is similar to the EW and A31 models (whence it evolved) but with much more substantial, and ‘modern’ side-valve cylinders and plate heads. In the 250cc, these are proportionally massive, making them trouble free but a fair bit of weight to haul around.
About the only flaw in the engine, shared with the other small Douglas twins, is the crankshaft lacks rigidity. Particularly so the center web, which tends to flex and allow the big end rollers to run at an angle. Being single-row, crowded roller, this does not seem to upset the crankpin or connecting rod all that much, but the rollers do tend to be driven sideways into the flanks of the center web, grooving them over time. You would think that being only a 250cc, and not capable of pulling the skin off the proverbial rice pudding, that it would be the least prone of the 350, and light 500cc models that also used this style crankshaft. But no, I’ve pulled down two 1935 250cc engine looking for a decent crank for my 350, and examined half a dozen other 250-350cc cranks, and they all seem to be afflicted with the same malady to a greater or lesser extent. I can only think that the owners revved the nuts off their 250s to try and get some power out of them, which would acerbate the flexing and also prevent the ‘spray-bar’ oiling to the connecting rod bearings from getting in there to combat wear. At least by the mid-thirties they got rid of the press in race for the conrod, which always worked loose, and went to a direct case-hardened rod.
I have heard said that conversions to twin carburetors mounted directly to the cylinders do wonders to allowing these engine to breath properly, and then they start to break the center web from excessive rpm!
The key seems to be to avoid high-rpm, and be of small stature and/or go on a diet!
-Doug
