Rollo,
Glad to hear that you have spent some time searching the forum, as there are many previous posts that I think you will find invaluable.
The three pins (dowels) you see are keys to index the throws to the center web. The other throw has the pattern reversed so that you can not assemble the center web backwards. These were drilled and pinned at assembly, so the cranks do not truly have interchangeable parts. People do get away with mix and matching of parts from different donor cranks, but the idea solution is to true the crank and then machine and fit new oversize dowels.
The dowels do not go completely through the throw as you can see in the next picture. So when you pull the throw off the crank pin, the dowels will come away with the throw. You may well find drilling swarf still at the bottom of the holes!
These pictures are of 350EW and A31 crank parts, but they are very similar to your 250 Aero.
To get the throw off, you need to make a tool that engages the shallow grooves in the sides of the throw. As you pointed out, there is no room between the connecting rod and throw to get a conventional gear type puller. The factory and repair depots probably had a tool like a gear or hub puller that engaged these slots, but the simple plate shown in the next few pictures is intended to be a throat plate so the crank can pressed apart in a conventional arbor press.
Once the throws are removed you will be able to access the connecting rods, rollers, and inner races. The inner races are a light press fit on the crank pins, but they can be stuck in place. There is little to grab on to, making them difficult to get off. The early races just have two small notches at the base, more so to let the oil out as on the engines like the EW, the oil was fed through the crank. This notch can just be seen in the next two pictures, the second with the race slightly pulled away. One method that I have found for grabbing the inner race is to clamp it in a lathe collet, (like a 5C style) which will grip it evenly all the way around the circumference with the maximum area of surface contact. Then a brass or aluminum rod or drift can be passed through the center to tap the crank pin out through the race. The race will slip in the collet, but it will move the crank pin too a little at a time. A single sleeve of paper around the race will increase the friction and grip in the collet. After re-chucking a few times, you eventually get the race clear.
From about the A31 onward, the oil was via a spray bar that drizzled oil into the path of the crank. Grooves in the face of the center web led the oil in to the connecting rod bearing. These races have larger cut-away(s) and there was probably a special puller that would engage the notches. The collet method described above works equally well here. This style (which should match your 250 Aero) is shown next, with the race inverted. These crankshafts are not drilled for central oil feed, obviously.
However, unless the races need renewing, nothing is to be gained by removing them.
What I have found is the weakness with these cranks is the center web flexes. The rollers then skew slightly and wear into the sides of the throw and the center web. The grooves made by the sides of the rollers can be seen in the last image above, marked by a felt tip pen X. While the crank throws and the center web are (usually!) heat treated, they are not case hardened. So they are prone to wear. There are no cages on the lightweight twins of this era, so it is the ends of the rollers trapped neatly between the throw and the center web that keep them tracking true. You will find the inner race just slightly longer than the rollers.
Unfortunately there is no good, easy fix to this sort of wear. I have seen the faces brazed, but obviously that is a soft material and in the process you destroy what little heat treatment the parts had. If the wear is only slight, most folk I am sure just re-assemble and figure it will last what ever remaining mileage they reckon to do. For more valuable OHV crankshaft, I have done localized tungsten inert gas bronze welding, but these had an aluminum cage that rubs against the center web. I suppose for the side valves one could do the same but substitute a hard facing filler metal.
On the later engines, there is no separate outer race in the connecting rod. The conrod races were found to be prone to work loose in the EW as the rod eye stretched, so they eliminated it and went to a direct case hard surface. They also decreased the roller diameter.
Another way to tackle the problem can be seen in this post:
https://www.douglasmotorcycles.net/index.php?topic=3040.msg11184#msg11184More notes on the roller sizes, connecting rods, and fitting of the same can be found here:
https://www.douglasmotorcycles.net/index.php?topic=109.msg259#msg259Good luck!
-Doug
