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Valve Gear

A website dedicated to the construction of an accurate 1/2 scale replica of a 1937 Aero Douglas Motorcycle

The valve gear is as close to the full size as possible in this model as it has had a 70+ year trial period and seems to work pretty well.

All components have been made from mild steel and will be case-hardened, except the axles which will remained unhardened silver-steel.  It is incredibly simple with only seven moving parts operating the valves - not really much to go wrong is there!  

Here you can see the finished push-rods and cam gears which run directly on a central crankshaft gear and rotate half engine speed.  The Valves and springs are totally contained in the cylinder blocks.  The hole in the block above is to accept a steel bush which will then support the halfspeed gear for the magneto which pushes in through the top of the crankcase.  The ends of the camgear axles are also supported in the crankcase by pressed in steel bushes as per full size. 
The central crankshaft gear has a larger gear forced onto the end of it which drives the dynamo.  The driving dog shown screws onto the cranshaft holding the gear in position.  It will be located by a key but as this will terminally effect the timing this will be left until the very last moment.  There is a stepped screw in the end which is 6BA left handed to prevent the dog-nut from unscrewing and bursting the engine cover!  The dog will rotate the matching dog on the oil pump which is attached to the engine cover. 




Valve Guides

The hardened steel Valve Guides are a taper fit in the aluminium crankcase and normally, producing a special size locking taper and matching holes in the crankcase would be a real head scratcher but in this case it was all rather simple.  Im sure when the words 'locking taper' are spoken the most common reply is Morse as it is found pretty much everywhere in the model and industrial engineering scene. After measuring the original components I discovered that the half scale sizes were extremely close to the large end of a No.1 Morse taper - what a result.  Some of the tapered holes in the crankcase do not have a through path but come upto an obstruction pretty smartly meaning that the taper reamer used had to be cut down in length considerably.  This worked perfectly as the largest diameter of the insert is slightly under the largers diameter of the reamer meaning that the reamer could be cut to just over the length of the insert. 
These tapered holes in the crankcase were produced after the inserts were made so that they could be used as gauge plugs. 




Cam Followers, Shafts & Gear Plate



more to come