Author Topic: OHC Douglas motor  (Read 7198 times)

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Offline Dirt Track

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OHC Douglas motor
« on: 18 Sep 2006 at 12:10 »
G'day all
Can any member throw any light on the racing OHC motor in existence over in the UK....I have seen it and so has Eric (Daren's mate).....I have never seen any mention of it anywhere.
If memory serves me correct I was told it was a factory job.
Howard.

Offline Doug

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Re: OHC Douglas motor
« Reply #1 on: 20 Sep 2006 at 02:41 »
Which one? I have not yet seen a true OHC Douglas engine, but there are several possible contenders.

First two pictures are a engine I photographed at the 1988 Bristol Cavalcade. While this looks like an OHC engine, in reality I would call it more of a ‘cam in head’ as the camshaft is situated alongside the combustion chamber, and not ‘above’ the valves in the traditional sense along the axis of the cylinder. It is not a factory job though, rather a privateer’s conversion (rather cleverly designed and well executed) of a standard Dirt Track engine. Note how it retains standard cylinder barrels and heads, the cam box attaching to the existing rocker perches. The drive is via chain from the timing chest to the cam, then short tappets to the rocker gear. This has be rather ingeniously added by the addition of a new timing chest cover, to which the standard small airbox timing chest cover has been applied.


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The design may have been inspired by the Douglas Dryad engine. This too was a ‘cam in head’ design of about 1932 for the light aeroplane market. Pictures of a prototype engine exist, but it does not look like it was ever put into production. Unlike the previous engine which has the cam across the head, the Dryad had the camshaft directly on the end of the cam drive shaft. But again, there are short tappets interposed between the cam lobes and the rocker gear. The Dryad was 1200cc, compared to the Sprite (based on the pushrod motorcycle engine) of 750cc.


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The patent illustration below for the Dryad valve gear shows the principle applied to a simplified single cylinder concept engine.


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Lastly, I have this picture of a pair of unusual Douglas valve rockers, reputably from a OHC Douglas engine and seemingly unsued. At first glance they look like standard Dirt Track pushrod rockers, till you notice neither end has the socket for the pushrod! They do not appear to be of quite the right configuration to be the remains of the Dryad engine. The magazine photo of the Dryad looks to have a split block bearing for the rockers, where as these pivot on a shaft like the Dirt Track. Nothing further is known about the origin of these or what the rest of the engine might have looked like.


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-Doug

Offline Dirt Track

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Re: OHC Douglas motor
« Reply #2 on: 20 Sep 2006 at 11:09 »
G'day Doug
Good stuff!
To be honest i cannot tell you if the first of your pictured engines is the one I speak of or not.....it was 10 years ago that I had a fleeting look at the engine in an enthusiasts workshop in the south of England....the owner has  very much a one eyed view when it comes to old motorcycles and loves his Douglas machinery.
Eric had seen the motor and may have taken some photos of it when he did......Daren might be able to throw some light on that.
I seem to remember the owner saying it was a factory job.
Howard.

 

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