OB engine

[Alan]

Looking at a stripped down OB engine belonging to a friend of mine ( found in parts from a deceased estate), it appears all there except for the big end assemblies so the engine rebuild has stalled.
We think that the big end bearing arrangement would comprise of rollers held in a cage and my questions are as follows;
What dia would the rollers be, how many and if the rollers are offset ???, how are they held in position ?
Details of the construction of the cage would also be appreciated.
With imperial sized bearing material becoming very hard to source here in Australia are there any alternatives ?

[eddie]

Alan,
        You should be able to work out the diameter of the rollers by measuring the conrod eye and crankpin, and then halve the difference. Imperial size rollers are still listed here in the UK - try www.simplybearings.co.uk - they still hold rollers in 3/16, 1/4 and 5/16" dia. With regard to the cages, quite a lot of these early ones consisted of 2 side plates with spacer pins between each roller - they also had small holes at the end of each roller (presumably for lubrication) - very fiddly to try to reproduce!!
     Regards,
                   Eddie.

[eddie]

Alan,
        I've just had a look at what is reputed to be an OB crank, and it has quite deeply shouldered crankpins (to locate the rollers?). If this is the case, then the bigends may have crowded rollers - hence, no cages with this engine! Also, the distance between the shoulders measures .438" (7/16").

      Eddie.

[Ian]

My OC has crowded rollers - no issues

[Doug]

Alan,

Douglas' ideas on OHV crankshaft and rod design seemed to be constantly evolving until the DT era (1928) when it settled down until the end of the prewar OHV motors in 1935. The OB probably had several different iterations, shared with its contemporary models. The 1924 OB likely shared the same cage assembly as the RA, which had a double row of rollers with a significant gap between the two rows. This is copied from the S1/S2 Sports models of 1921-23.

S1/S2 connecting rod assembly



The 1925 OB handbook does show the this style cage in use. The RA cage was complicated and not very strong. What seem to be long skinny rivets are in fact screws, which had the tail ends riveted over for good measure!

However most assemblies that seem to be attributed to the 1926 OB/OC/TT era used a different arrangement. A pair of plates are riveted to the sides of the connecting rod big end eye. Some with seven and later (judging by higher part numbers) with five rivets passing through the rod.

RA or early OB cage assemblies on the right, five rivet OB connecting rod bigend plates on left.


A double row of crowded rollers are fitted, presumably (though not yet seen) separated by a thin hard washer in the middle. This was common practice with the output bearings on transmissions, as one long roller could not be adequately guided by using the end faces. While they do run in a groove in the crank it is the plates that keep the rollers square to the crankpin.  The groove in the crankpin is wider than necessary, and possibly a holdover from the earlier RA cage design.

The S1 crank had a scallop each side of the crank cheeks to allow loading of the rollers. The counterweight seems to have simply spanned this void. By the time of the OB, and probably the RA, it was determined that stuffer blocks were required to support the counterweight; held in place by countersunk screws through the counterweight and the stuffer block. This laterally located the counterweight. A later design (circa 1926) shows in addition of two #27 dowel pin holds to also locate the stuffer block. It is not know if this was introduced early enough to see use on the OB, but it definitely was used on the OC/TT.

S1/S2 crank detail


OB crank detail


OB crank, seven rivet rod plates


OB crank and rod


OB crank, notches in plates are to clear camshaft lobes


-Doug

[Alan]

Gentlemen....many thanks for your expert advice/help and lots to evaluate....the engine is ( we think) dated 1923 with an additional "X"
which I believe means an export model...
A quick look at the main bearing crank surface(s) which both display scuffed areas to one side would seem to indicate that someone "bodged it" in the distant past and possibly fitted only one roller per width, not two  side by side and this might explain why the engine "ceased to proceed" at some point...
Alan....

[David Lawrence]

Hi Alan,
Just a quick addition to the information you are gathering.
All the OB engines had the "X" as part of the engine number, if you care to publish the actual number we can confirm the date, we may possibly be able to date it more precisely.
If you prefer you can email the number directly to my email address.
I can also confirm Doug's points, it seemed there were various attempts at "improving" the cranks, in amongst my "past it" box I have a selection of differing cranks all supposedly from OB engines. Now thats a really helpful observation! :
Dave

[Alan]

Dave...no problem and will let you know the engine number after Xmas...even after nearly 30 yrs in Australia, the sight of Father Xmas
in full bearded attire but wearing red board shorts still makes me makes me need to look twice !!!

[David Lawrence]

Hi Alan,
Fair enough look forward to your info after Xmas, I do hope you enjoy your roast turkey whilst sitting on the beach with the temperature in the 30's? Meanwhile I'll attempt to swim down the lane so that we can load up my bike into the back of our transporter!!!!
Have a Great Christmas.
Dave

[Alan]

Dave..first of all the engine number is OE 249X so if you could date please..
Brief history...the complete bike was purchased in bits from a Mr Patterson in 1941 ( receipt attached) for 7 pounds ( do I hear sobbing !!) but the new owner was then posted overseas on war duties..His mum then moved and managed to lose all the bits except most of the engine parts (more sobbing) which then remained untouched ( except for some polishing by someone ??) until sold..

Back to the crank assembly...pic attached of the conrod (part no 7169) and as per Doug's pic, has no through holes so looks like a cage and outer retaining plates will be required.....
Questions...the conrod has a scalloped section on one side ( see pic)...just to make sure, is this scalloped side mounted facing inwards or outwards ?
.....the cage as detailed in Doug's pics contain 2 roller bearings side by side ( 5/16 dia by 1/4 in long will fit our engine) but the attached pic of our crank main bearing area shows a wear mark that appears to come from a single offset roller...one of Douglas's many experiments or simply incorrect ?
.....the 4 timing/magneto drive gears are missing and these will have to be made...if anyone could specify the number of teeth/dia per gear then this would be appreciated and we assume that the same tooth profile as later Duggies such as Mark's could be used.

Ta

[David Lawrence]

Hi Alan,
Regarding the OB engine the OE249X  puts it as a relatively early motor from 1924, we are fairly sure that Douglas started most of their engine production runs from 100, much of Doug K's fantastic detective work bears this out.
The records show that very few of the early motors survived, most that we know of being in the 7 and 800's, considering the short time span of the OB they must have produced a fairly large number. We would therefore suggest that your motor came from early 24. If more detailed information turns up I will let you know.
I have to admire the receipt, what a superb bit of history, pity the rest of the bike is not now in existance, its a great pity that the early production did not have matching frame and engine numbers so that it would be possible to discover if the frame made it also!!!
Dave

[cardan]

Hi Dave,

Regarding the OB numbers you comment "that the early production did not have matching frame and engine numbers". Does this mean that later OBs did have matching numbers? Matching, or close to matching? Jack's OB has frame HF835, engine OE836X, gearbox PG844 but no history is known so I'm wondering (a) if it is likely a "matching numbers" bike and (b) can you guess at the date?

Also, rather then there being a large number of OBs produced, I wonder is there a gap in the numbers - as you comment there are lots of survivors in the 7-8-900 range. Maybe 1925 started at 700? If it helps, I have sighted (in Australia) PG gearboxes 701, 747, 806, 844, 845, 861 and OE cases 271, 739, 836, 839, [edit also 892], 943.

Thanks

Leon

[Doug]

Leon,

What Dave means by early production numbering is (essentially) prewar verses postwar. Douglas did not start matching frame and engine numbers until the very end of the prewar era production, circa 1937-38.

According to the late Eddie Withers, who worked at Kingswood, engines for the OHV models were built up in the experimental department in small batches and then taken out to the assembly area for installation in a frame. Depending on the batch size and which engine was grabbed off the bench, would determine how close the engine number came to the frame number. For machines such as the 1934-35 OHV models where very few were ever made, the engine, frame, and gearbox numbers never stray very far from each other.

So no OB models would have matching frame and engine number except by coincidence or perhaps a special effort by the staff. As to if there were a gap in production numbering, or if Douglas rounded up to the next full hundred with the start of the new model production year, unknown. Similarly if Douglas ever 'reset the counter' from time to time is unknown.

Since the OB was only listed for 1924-25, it has to be one or the other!

-Doug


[correction: Jack Withers to Eddie Withers. 01Nov17 -Doug]

[cardan]

Thanks Doug. I've put up some photos of the OB at http://earlymotor.com/forsale It's a very prim and proper interpretation of an OB compared to the many period photos we see of the model on the dirt track in Australia!

Is there a big gap in the frame/engine/gb numbers of surviving OBs? For example, are there any numbers between say 350 and 700?

Leon