Dave,
Sorry to here the crank is knackered, but not surprised. They are not very robust, and the center web is so flimsy they get up to all sorts of wear and will even break if you indulge in prolonged high RPM. Essentially the same crank was used through the 1936 250/350cc ‘gutless wonders’ with a simpler spray-tube and splash oiling verses the drilled and pressure system as used on the EW. You might be able to find one of those in good shape and drill it for the EW oiling system. However I took a pair 1935 250cc engines down searching for a crank for my 1931 350cc, and both had center webs that had been carved up by the big end rollers. To think they used much the same crankshaft in some light 500cc machines!
You would do better to find another whole crank assembly of matching components. Douglas assembled the cranks, pinned the crank pin joints, and ground the main bearing journals between centers to final size and truth. You may find just replacing the center section from another engine that the crankshaft does not run true enough. If it is just a matter of the alignment pins holding things out of skew, then temporarily leave the pins out. Knock the crank into alignment and bore the alignment holes out for slightly oversize pins. You will probably have to use a micro-boring bar, as I doubt a reamer is going to cut very accurately into such a miss-matched hole. Anyway, what I am getting at is they are not perfectly interchangeable components. They may be close enough, and if you can get the main journals to indicate within 0.002” then for an EW you are probably quite good enough. The bearing bores in the crankcase (and probably the joint face) were done in a (worn?) turret/capstan lathe and I have not found them to be precisely opposed. This is why Douglas likely used a narrow ball race on the timing side and a self-aligning double row ball on the drive side; so that it would have a little give!
Options for replacements are more limited here in the USA, where Dougies are not as common. So in the end I started a project to make a new crank for my 1931 350cc. This is not quite as hard as it first seems, though you do need a good size lathe. The hardest part is machining the center web. Even this is not so difficult if you make a fixture like that shown mounted to the lathe faceplate (see further on.) The fancy gold color was a failed experiment using titanium nitride plasma coating to increase the wear resistance. At the low processing temperature required to avoid drawing the temper out of the alloy steel, the adhesion was marginal.
-Doug
New turned blank from billet for the timing side. The flat is for a timed rotary breather not used on the earlier EW.
Ditto for the flywheel side.
The fixture for turning the center web.
From left to right, 350EW, A31, and new from billet. The yellow color is TiN plasma coating.
Old and the new drive side.
Original drive side, showing 'repair' with braze. Must have done wonders for the heat treatment. Note new crank is not drilled in the center of the main shaft to save weight like the original. Road license tax is no longer an issue!
More amateur repairs to the center web with braze. Also thread has been re-chased to a slightly smaller size.
Why folk braze the crank. The red arrows show where the big end rollers have worn into the face of the center web. The location of the arrow is about where you find the deepest wear, due to perhaps the way the crank flexes under load, and it is furthest away from a fresh supply of oil. With these later cranks, the oiling is by a spray bar over the crank, and the oil impacting and entering via the side of the connecting rod via the groove seen at the left-hand edge. The inner bearing race has already been removed.
The 'bits'. Note the alignment pin pattern is reversed, so that you can not install the center web backwards.
And the assembly, less crank pin races. This crank was destroyed attempting to use too aggressive a press fit for the crank pin races. So I will have to start over.
