1923 TT Douglas

[SteveW]

In the Milntown Collection in the Isle of Man is the 1923 TT winning Freddy Dixon banking sidecar outfit, although I understand the RA machine is not the actual winning machine. The outfit was last used about 15 years ago in a TT parade but unfortunately one of the pistons disintegrated and seized the big end on that side.

I have read with interest the details, and complexity of the RA crank, and refurbishment of the damaged crank seems all but impossible. Manufacture of a new, simpler 'pressed up' crank has been investigated, as the engine will never be run 'in anger' again, but to have it run is our aim.

However, my query is, did Douglas fit a crankshaft of the RA dimensions, albeit a bolt-up or pressed-up type, to any model?
I am one of number of volunteers looking after the Milntown bikes and cars and do have access to some precision engineering facilities on the island. Any advice would be much appreciated.

[Doug]

All the prewar ohv crankshafts were forged from a single piece, from the 1921 S1/S2 to the OW/OW1 models of 1935. There were some Works only/protoypes prior to that, but the ones I have seen any details on also are one-piece. Not much more rigid than a bent coat hanger, but were one-piece (until they broke!)

-Doug

[SteveW]

We have made some progress with the Milntown banking RA Douglas, and here is an update. 

Thanks to the generosity of one of our Milntown vehicle volunteers, a new crankshaft has been manufactured by Steve Moynihan here on the Isle of Man. Steve is a well respected ex TT competitor with many years experience on Yamaha TZ cranks. The new crank is a 'pressed-up' design and utilises KTM con rods and big end bearings. 

After much research suitable pistons were sourced for the Douglas barrels without any re-boring. Honda pistons. 

Initial trial assembly is underway and I'm currently working on the cam timing with the TT Model manual available from the LDMCC being of great assistance.

Some work is being done on the rolling chassis without removing the original 'patina', and I have attached a photograph showing the banking mechanism with the sidecar body removed.

[SteveW]

I am working on the two RA cylinder heads now, and find that the valve stems are slotted and fitted with parallel split cotters each with a 'tab' that locates in that slot (see photographs).

Unfortunately only one of the four valves has the split cotters in place, with the other three having various pieces of steel through the slot to retain the valve.

Any advice on whether the 'tabbed' cotter arrangement is the original fixing, were they fitted to other engines of the period, and are they another part that's as rare as 'hen's teeth'?

[Doug]

The RA did use slotted valve stems. Or at least initially. The RA handbook is not clear if they used a straight cotter or that 'E' shape type. Both have their advantages and disadvantages. They did go through about six designs of valve collar to go with the cotter and the entire industry was moving away from slotted to grooved valve stems. Grooved stems are basically a time bomb waiting to cut loose. I imagine towards the end of the RA era (c1925) they had adopted the tapered collet design seen on the Dirt Track models onward.

I have made new slotted valve stems for a client that wanted the original setup on his 1932 flat head BSA side valve sloper. It is possible to cut a slot in modern valve stem material with a small carbide end mill. In that particular case the original BSA slots had a full radius at the ends, rather than being square. The other saving grace was it did not rev that high and if the end of the stem did break off it was not going to drop a valve into the cylinder.

On my RA I was going to investigate (have not got around to it yet) if I could use DT valves and tapered collets. I have new ones of those valves, and the longer stems used on the 1932-35 ohv models.

-Doug

[SteveW]

I've removed the valve guides from the two cylinder heads. The inlet ones are bronze and the exhaust ones cast iron. Both the exhaust ones have been broken. All four guides are stopped from rotating by pegs located in the heads that locate with cut-aways in the guide shoulder. I assume this is to maintain the alignment of small (2mm) lubrication holes in the guides and matching holes in the head. However, these holes have no evidence of external oil feed fittings and are just 'free to air'. 

I had planned to make four new valve guides from phosphor bronze, but a friend has queried whether the exhaust guides will be adequate in bronze rather than cast iron. Any advice from those better qualified than I in metallurgy would be welcomed.

[Doug]

You do not want to use a bronze material for valve guides in a 'dry' application; inlet or exhaust. They are better suited for enclosed valve gear where there is a positive lubrication, like an oil mist. Douglas (and many others) use cast iron as it does have some inherent lubricity in between the infrequent applications of the oil can.

Some of the later racing models tapped oil vapor off the crankcase which coalesced in small oil lines that ran to the individual valve guides. They were drilled through the head/guides as you describe. I am not aware that the RAs would have this feature, but it could have been added later. Usually when encountered they are choked shut with carbonized oil.

-Doug