RA engine?

[desmobikes]

Can anyone help. I have a 500cc OHV engine in a dirt track frame, the engine number is FE 187. The Clew book tells me this is a 1923 RA engine, however I have been told that RA engines had the G/box attatched to the rear of the engine, and the gear shift was an extension of the timing cover. I have a few douglas engines and I,m certain the engine number is genuine. There is no visual evidence of any g/box mountings being removed from the engine. Bob

[cardan]

Hi Bob,
Other than the missing gearbox mount and gear change lug, the motor could easily be RA. FE 156, with its bolt-on gearbox attached to the lug missing on your motor, is shown below for comparison. Of course the extra bits of alloy can be removed from the crank case either (a) crudely by a home "mechanic" or (b) carefully, by an engineer proud of his work. In the former case the removal would be obvious, in the latter it could be undetectable. The cylinders and engine number suggest a reasonably early motor - perhaps 1924-ish - so an RA minus a couple of lumps of alloy (and the complete air box) is the most likely explanation. OB gearbox also hints at a bit of a "make do with whatever we can lay our hands on" approach.
Leon

[cardan]

Actually it's very hard to see the cylinder heads in your photos. Viewed from "above", the photo below shows an OB head (left) and an S1/RA style head (right). Note the "joggle" in the rocker perches on the OB head, compared with the rectangular layout on the S1/RA.

Even though cylinders are S1/RA style, maybe the heads are OB. You've gotta love a good mixture!

Leon

[Doug]

Bob,

Yes it does appear to be an RA crankcase that has been 'pruned'. Not an uncommon fate fr RA engines, with many examples abounding living a second life in later frames. You can see the evidence of where the gearbox platform was, once you reference Leon's photo. The mount for the gearbox lever can be filed away without leaving a trace.

The seven appears to have been added, as it does not match the other numerals very well. So I suspect it is engine FE18. RAs were one of the few Douglas models, perhaps only, where numbering did not commence at 101. At least not in 1923, while the factory was still the only one playing around with them, as there seem to be a few about with double digits. They might have started at 101 when the RA was sold over the counter starting in 1924.

-Doug

[desmobikes]

Thanks Doug and Leon, another difference is the timing cover, Leons pic has an oblong shape, mine has a pear shape where the oil pump is fitted and the crank case matches the cover. There is also a flat on the top of the timing cover at around 11 oclock where I presume the Gear change material has been removed . I,ll keep digging, Bob

[cardan]

Although the RA air box is oblong, the crankcase underneath is pear shaped: http://www.flickr.com/photos/29460210@N08/5768093880/

Leon

[desmobikes]

Leon,I.m starting to understand how it should be. Is it possible for you or Doug to give me 4 pics  of the engine from front, back, and 2 sides.I would like to explore the possibility of rebuilding to origional. Bob

[cardan]

Hi Bob,
There are a couple of problems with taking the engine back to original, even aside from the hard work. The RA used its own gearbox (mounted on the back of the motor) and these are pretty scarce, so replacing the gearbox lug would probably not be a very useful activity. Much the same situation with the gear change lug: the RA gear change is different from other models (because it mounts on the motor), whereas other frames have lugs and gear change quadrants that suit the frame-mounted gearbox...
I reckon a better plan would be to leave the bike much as it is: as an example of a well-raced motorcycle, customised by an enthusiastic owner long after it should have been retired!
Cheers
Leon

[Blackdodge1000]

Hi Bob,
I agree with Leon.  Maybe sort your OB out for club runs etc & keep that old slider as a mark of the past.

Cheers, Neal

[cardan]

Speaking of RAs: The cylinders on Bob's motor are typical of S1 and early RA - see a cleaner pic below. The 494cc / 3 1/2 h.p. cylinders are 68 mm bore. I've seen 350cc / 2 3/4 h.p. RA heads, but does anyone have a 350 RA cylinder or, better still, two? They are probably identical to look at, but the bore is only 57mm.

Thanks

Leon

[cardan]

There is mention in Douglas literature c1923-4 of a "long stroke 3 1/2 h.p." RA. 3 1/2 h.p. translates to 500cc, so if the OB 82-mm stroke crank was used, the bore would have to be about 62 mm.

So perhaps there are three different bore sizes (and perhaps two different lengths) in these S1/RA cylinders:

57 mm (with 68mm crank) to give 347cc - 2 3/4 h.p. RA (i.e. RW)
62 mm (with 82mm crank) to give 495cc - long stroke 3 1/2 h.p. RA [Edit: 60.9 x 85? See post below.)
68 mm (with 68mm crank) to give 494cc - square 3 1/2 h.p. (standard) RA
68 mm (with 82 mm crank) to give 596cc - 4 h.p. sidecar racing RA

[Edit:
83mm (with 68mm crank) to give 736cc - the earlier S2, but is this the same casting?]

It's likely that the cylinders used with the long-stroke crank are longer than the standard RA cylinder. Have a look in the shed and if you have a cylinder of this pattern measure it up and let us know!

Cheers

Leon

[cardan]

Appendix 2 (Engine Symbols: 1921-38) of "The Best Twin" contains the following curious entry:

HE/    3 1/2 h.p. 1923 T.T. engine ohv, 2,397 x 85mm

If we take 2,397 to mean 2.397 inches or 60.9mm, this engine has a displacement of 495cc. The interesting part is the stroke of 85mm - 3mm longer than the OB. So it looks like the "long stroke 3 1/2 h.p. RA" used a special 85mm crank, with bore the same as the road-going 2 3/4s of the day, at just under 61mm. (The 600 sidecar racing RA, engine prefix GE, is listed as 68 x 82, same stoke as the OB.)

Has an HE prefix motor survived? If you come across a crank that causes the pistons to pop out the top of your OB cylinders by 1.5mm don't throw it away!

Leon

[Doug]

Leon,

If you want to talk about variations on the RA bore and stroke, then probably should split this off as its own topic from Bob's post about his RA/OB/DT bike.

-Doug