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fitting a larger engine to a 250 Aero frame

Started by JonathanJoy, 06 Jan 2014 at 11:21

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I'm looking into the possibility of fitting a larger engine to a 250 Aero frame.  I know that at this time there was some interchange of parts between different models.  Has any one done this ? any information would be much appreciated.  Kind regards :question: 


My understanding is the 250, 350, 500, and 600cc Aero frames were identical. The primary chain line for the 250/350 and 500/600 are different, so you have to swap the gearbox too. There were some other differences, but not of a mechanical nature.



Thanks Doug,  So I need to look into which earlier engines would fit, am I correct assuming that Aero engines and parts could have come from earlier Douglas models (ie using up old stock) ?

Thanks for the reply


I am afraid an earlier engine will not help. While a lot of the features were derived from earlier engines and some components will interchange, the entire engine will not. The 500/600cc Aero derived from the 1934 Blue Chief model. So that is all the further back that goes; and that is assuming the frame tubes are the same distance apart. Blue Cheif engines are rather more scarce than Aero engines. The lightweights (250/350cc) engine likewise also only goes back to 1935. Prior to that the engine crankcase had a flat bottom and mounted on corbels, held by four bolts (A31 era) or two bolts (EW era). You will need an engine that mounts on the two frame tubes for your Aero frame.



Thanks Doug,
its a blue chief engine I was looking at , but with no cranks , rods or pistons.  Not sure how difficult these would be to source.  So I might have to be satisfied with the lightweight engine.  What was the performance like of a Blue Chief as opposed to a 250 Areo ?



The Blue Chief crankshaft is different than the 500/600cc Aero, in that the Blue Chief crank is drilled for oil feed via the timing chest end. The Aero(s) use a spray bar that drips oil into the path of the connecting rods. I am not sure if there are significant dimensional differences too.



Hi Jonathan
    Doug is correct in saying that the Aero crankshaft is different from that of the Blue Chief but it is possible to convert. I have done it twice. The main differences are the oiling system and the length of the front shaft. To convert the Aero crank for Blue Chief/Endeavour it has to be stripped and oilways drilled through the front end of the shaft and very accurate and precise drillings at angles though the centre web with small cross drillings from these passage ways into the sides of the two big end bearings. The front end of the Aero shaft is also longer than that of the Blue Chief but can be turned down to the correct length and profile. Oil is end fed via a spring loaded quill housed in the large headed bolt screwed into the timing cover. Pistons are readily available for Blue Chief/Endeavour from the pre-war spares scheme of the LDMCC in a choice of either cast iron or aluminium alloy and in most if not all oversizes. These pistons are peculiar to this engine and are of no use for any other 500cc Douglas engines. In completing your proposed Blue Chief engine your biggest problem is likely to be finding conrods which are common to several engines including some overhead valve engines where vast quantities have been used in sprinting. Most 500/600cc Aero engines I have seen for sale in recent years have been missing the conrods.


               As Chris said, decent conrods are the hard to find part for the 500/600 Aero engines - but it is possible to recondition rods with knackered bigends. Chris supplied me with a pair of such rods several years ago when I was desperate for some to go into my sprint motor. I felt sure there must be a way of bringing them back to a usable condition - after much measuring and comparing with postwar rods, I figured it should be possible to liner the bigends, much as Alpha's used to do for the Dragonfly. Having searched through my scrap box, I found a couple of liners that had been removed from Dragonfly rods during modification to needle roller bigends. These liners were about 30 thou smaller than the bigend eye of the Aero rod, so would stand being reground. It was then just a case of determining the best diameter to bore/grind out the rods to give reasonable strength with regard to both the conrod and the liner. At this point, I also figured that using 6mm rollers instead of 1/4" would result in a stronger liner. When fitting these liners, it is imperative that the bigend bore in the conrod has a good finish and is perfectly round and parallel, so that it gives the liner the best possible support (this is where some of the commercially available reconditioned units fall down, resulting in the liner flexing and eventually breaking up!). These reconditioned rods have now done 4 seasons of sprinting and the conrods are as good as the day I fitted them - the only bigend problem the bike has suffered was a broken bigend cage.
    Hope this is of some help,


Thanks Guys for the input.
When I first read your replies , I thought wow too much "fettling"
But the more I think of the project , the more i like the idea, there's something about the engineering challenge that attracts me, so it looks like I may have a project on

Thanks again


 Hi  Jonathan, if you decide not to go ahead with a larger engine, I have got one and a half 250 engines. I was building a flat tank special but gave up when I bought a 1925 two speed Scott. now need the money to spend on that.