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New owner of 1930 Douglas G31

Started by alexd912, 26 Jun 2009 at 14:38

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Hi there,

My name is Alex and i have just acquired a Douglas G31 600cc.
I will be the first to admit that this is my first Douglas and any bike of this era and I would really appreciate some help and advice.

The bike appears to be in running order but as I have been told it has some deviations from originality, namely the leavers and handlebars, and no doubt a host of other parts.

I would very much like some facts about the engine and the prefixes I should be aware of when locating spares for the bike and more so the engine. My aim is to get the bike back as close as possible to original.

Is there anything I should be particularly careful with the operation of the engine or maintenance.

Also I would like to locate a owners manual and workshop manual for the bike as well.

Any support for a new Douglas owner would be very appreciated, oh and if anyone knows of any members in Finland then that would be great.

Some information about the bike:

Engine size   596 cc
Chassis No.   FD136
Engine No.   EU154

Here are some pictures of it.

Best regards


Best regards
1930 Douglas G31



The machine may have had an engine swap a some point. Chassis FD136 would have been originally allotted an ET prefix 500cc engine to make an F31 model. G31 models were frame prefix FG and engine prefix EU. I am not sure there was any difference between the frames other than the prefix stamp. The only difference between the engines were the cylinders and pistons. The cylinder head combustion chambers were enlarged slightly on the 600cc as well.

Gearbox prefixes could be AAG***(wide), ABG***(close), B/***(4-speed option 1932.) The primary chain case on these and the 1934-35 versions was a bit of a bodge. The mounting is poor and there is no a lot of chain clearance inside. I would not be surprised if it has cracks and weld repairs. You can make sheet steel replicas, but the advantage of the cast aluminum primary is when the chain breaks the bottom of the case gives way and lets the chain out! With the steel case the chain jams around the engine sprocket! I recommend using an all riven chain with no spring link. The later tends to get picked off by the clutch grease slinger; there is very little clearance. I have a 1934 version, and have been down this road before.

The engine is essentially the Dirt Track (DT) crankcase and lower end with a new timing cover/airbox. It is the long-stroke (82mm crank) but the engine used the short-stroke connecting rods and cylinders shorter than anything used up to that date to reduce the length of the engine. Same strengths and weaknesses as per the DT, except being a road model and not competition, the crank pins are usually in far better shape than you find the DT machines. There were slight changes to the crankcase to add a recirculating oil system, where as the Dirt Track was total loss. The top end was a new enclosed valve design, continued on through the 1935 OHV models (the frame and crankcases were redesigned in 1934.) Rocker arms arms shared with the DT, but the valves are slightly longer. I have new valves (see adverts.) 

I have never seen an owners manual or spare list for the F/G31 models. They were continued into 1932 and possibly 1933; though Douglas production in 1933 was very limited due to financial distress. Numbering would have started at 100 or 101.

As for non-standard parts, it looks to be minor. It would have been fitted with a BTH dyno-mag to support the electric lighting. Headlamp would be the BTH model for that year with the switch at the back of the shell. There would have been an accumulator carrier over the front cylinder.

The rear brake pedal is a replacement, and the brake rod and actuating arms would not be plated but painted black enamel. Perhaps some of these were plated for show models, but it is more likely a common case of over-restoration. Ditto the plated handlebar clamps. The recessed center of the flywheel should be infilled with black enamel. Catalog illustrations also show the brake backing plates, airbox cover, and the gearbox sprocket cover as black enamel, and not bare aluminum. Pushrod tubes, kick start lever, petrol tank, inlet and exhaust pipes all should be plated. The exhaust pipes are probably replacements, or at least the front as the sweep(s) are not as generous as the originals. This is a common problem with modern tube benders, as they want to use the standard dies that come with their machines and are not concerned with matching the original shape accurately.

The saddle has been raised, and normally would sit just a little lower. I have seen others raised, so perhaps the original position was uncomfortable. A cast aluminum toolbox and a rear parcel carrier were intended to be offered, but not shown on the catalog illustrations nor can I think of seeing one so fitted.

The front forks should be essentially DT, but with the ability to mount mudguards and a headlamp. I can not see enough of yours to be entirely sure, but suspect they are from a slightly later Douglas road model of 1934-35. The fork links support this theory, as they should be the DT forged style. Also the front brake should be on the right-hand side!

So all in all I would say the front forks are the most serious originality issue, but at least they do look like Douglas components of 1934-35. The other points are minor fitments and matters of finish. You should find it a very strong runner on the road, even with the single carburetor. So far as I know they also used the same camshaft as the DT models, that gave a lot of torque in the mid-range.

Hope this helps and best of luck with it!



Welcome MR Alex
this is not only  forum but
best ever family  to all of us we are all together
thanks for showing us the nice bike well maintened
and of course there are great members specialy MR Doug always helpfull to each and everyone
regards keep it up