Identifying old Douglas motorcycles. 

Part 3: The Postwar Models, excluding Vespa scooter. 

© Doug Cross, Doug Kephart, October 2004 

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To help you navigate, the full set of files in this series is as follows:

Part 1: Models 2-3/4hp, 3-1/2, and 4hp, 1907 to 1926
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Part 2: Models of the 1920s and 1930s, excluding models in Part 1
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Part 3: Postwar Models
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Codes and numbers of the main components.

Location of the component codes and serial numbers of the Mark, Competition, and Plus series (1946-1955) are in the following locations: 

Frame: On the headstock, lug for left side front down tube. 

Engine: Normally on the crankcase above the left-hand cylinder, but on some early engines it is behind the right-hand cylinder base. 

Gearbox: On top of the box towards the right-hand side. 

On the Dragonfly (1956-1957), they are located: 

Frame: On the left side gusset plate for the swinging arm.  Just below pivot, in 1/8” tall characters, easily painted over. 

Engine: On the crankcase left hand cylinder base. 

Gearbox: On the top of the gearbox towards the left-hand side

Matching numbers?

Originally frame and engine numbers matched, but things have got a bit messy as folk keep the bikes running as best they can.  The early broken frame syndrome most prominent with the Mark 1s was cured during the Mark 3 series (well, mostly!), but as I've indicated below, un-numbered and quite legitimate frames do exist. 

For practical purposes, virtually all of the post-war engines and gearboxes are interchangeable, so deciding what model you have often means deciding what model you have got most of, and going for that.  If you look at the many photos of these bikes that appear on the net, you can have some fun working out just what combination the owner has got - and quite a few are definitely not what the owner thinks! 

The worst area for confusion is the gearbox - they change bikes like hot couriers!  Because all of the standard gearboxes are interchangeable - including Plus series boxes - for standard and Comp models I have not attempted to define what boxes go with which model.  It really is a case of "anything goes" - although there may well be a 'right' set for each model, perfectly matched engine/gearbox/frame sets are getting scarce nowadays.  Plus boxes have needle rollers in place of bronze bushes and Comp boxes were machined out inside to make space for the larger cogs for 1st and 2nd gears.  Three sets of gear ratios were available, (close, standard, and wide), and these can be used in any of the standard Mark boxes. 

A final note, most major components are wonderfully interchangeable between the various Mark and Plus models.  Cylinder jugs, the aforementioned gearboxes, swing-arms, sub-frames, front forks, wheels, the list goes on.  The cultural exchange even extends to the Dragonfly, with earlier engines being transplanted whole and vice versa.  Bear this in mind when you try to compare a post-war Douglas you are examining to the descriptions that follow and the table further below.  Oh, and good luck! 

Missing frame numbers - don't despair!

Early frames were liable to break, and were replaced by the factory, often with frames which would fit both the Mark 1s and Mark 3/4/5 models.  The Mark 1 and 3 had a rear sub-frame which attached half way up the back of the duplex frame rear down tubes, whereas later models, including the Plus bikes, had the rear sub-frame fastening much lower on a gusset at the base of the main frame rear down tubes.  Factory replacement frames may have provision for both types of rear sub-frame - and often no number, hence the occasional lack of a frame number in early post-war bikes.  This is something that the Registration Authorities and insurers tend to look a bit askance at - so now you can tell them why you can't quote a frame number! 

Also, just to add to the fun, not all numbers were allocated in a given series.  Large blocks, particularly in the Mark 1 and Mark 3 series were never assigned for unknown reasons. 

T35, Mark 1

Early (1946/47) models are known both as the T35 and subsequently the Mark 1 - they are the same thing. The engines of some early Mark 1s used the cylinders of the wartime stationary generating engines. Some Mark 1 cylinders can be distinguished from later models because they have a boss protruding between the cam followers at the base of the cylinder, which are square-ish and cannot rotate as later Mark cam followers do. These may well be ex-stationary engine parts (mine are - engine no 308, July 1947)

The rocker covers on Mark 1 engines all have 'Douglas' embossed in them. Very early generator covers are oval-ish, whilst later ones are more square-ish with the top and bottom edges parallel. Some of the rocker covers have the 'Douglas' name in a rectangular background frame - these were made by BSA, who also made the generators from which these rocker covers come. Don't worry - if you have them they are still genuine Douglas bits and appropriate for the early Mark 1s. 

A few early Mark 1s have brass carburetors. This is because when production started after the war, there was a shortage of some bits, and so they used pre-war carburetors if they didn't have any new ones! I have only seen one with them fitted, but I'm told they are genuine enough. Also, some frame lugs were brass, not iron. The head-stock on quite a few frames is of brass (one of mine is), and sometimes the lower lugs - or maybe only one of a pair - were brass. Even into the early Mark 3 era. Again, if you have them they're genuine enough, although they look a bit odd if the metal is showing through.

They are finished all over in black. The petrol tank has abbreviated blue panels to the fore of the knee grips and on the top as well, separated by a chromium plated swath. Early models had the chain guard attached to the sub-frame, subsequently they were attached more conventionally to the swing-arm. The front mudguard is rigidly mounted to the Radiadraulic (leading link) front fork- well up and clear of the wheel rebound- so a bit of an unsightly gap between tyre and ‘guard. 

2234 produced over twelve months.

A brief digression on the ever popular topic of interchangeability. Mark 1 cylinder head studs are 5/16" BSF, and the inlet valve passage from the carburetors are angled slightly inwards towards the rider’s toes. Later models from the Mark 3 onwards have round cam followers with no boss between them on the cylinder base, the head studs are larger (3/8" BSF) and the carburetors splay slightly outwards (away from the rider’s toes), giving less change in direction for the incoming fuel/air charge. Not a vast change in design, but actually quite a bit more efficient. However - these cylinders and heads are NOT interchangeable between the Mark 1 and the Mark 3-5 engines- if you want to put early cylinders on a Mark 3 or later, or vice versa, then you will have to use the corresponding cylinder heads as well.

T35 brochures and early service manuals mention a rigid rear version; this never went into production though some prototypes were made. The unused frame lugs were subsequently utilized in the rigid Competition (Trials) models.

DV60 or Mark 2

The DV60 was a further development of an abandoned wartime proposal, 74mm x 70mm for 602cc side valve.  This was dusted off after the war and a special rigid frame and a set of Radiadraulic front forks from the current production models applied to vie for a postwar Ministry of Supply contract.  It was not successful in trials.  Whether there were ever plans to put it into production for the civilian market had it been accepted by the military is unknown, but a stationary engine of 1000cc was subsequently based on the engine design.  These days it is given the honorary title of Mark 2 but this is a misnomer.  On paper at least the factory was working on another 350cc design, N.E. and N.E.2 (with shaft-drive), that appears to be the Mark 2 heir intent.  Only three complete DV60 prototypes were built plus a few spare engines (at least nine in total.)  A replica has been recently completed with one of the spare engines and an early prototype T35 rigid frame. 

Mark 3, Mark 3 Sports

Early Mark 3 bikes continued with the T35/S/ prefix to the serial number (but not always!); this was completely dropped before the end of the Mark 3 production run.  To cure engine roughness under certain conditions, the combustion chamber was redesigned by the venerable Freddie Dixon.  This necessitated changes to the cylinder head casting, and the cylinder barrel changed slightly at the same time.  This redesign entailed inclining the spark plug outwards to reposition to a more centralized location between the valves, resulting in the distinctive ‘notch’ or dip in the top edge of the rocker box covers.  The changes were available for road testing in April of 1948, and advertised for sale in June, so the Mark 3 is considered a mid-season change.  The name “Douglas” cast into the cover disappeared at the same time.   The blue side panels to the petrol tank now extend behind the knee grips to the rear of the tank, giving the appearance of a lot more colour.  Combined with the black, metallic silver top panel, gold panel lining, and the chromium plate swath as before, it was quite eye-catching.  The standard model had side valances to the front mudguard. 

The Sports models were numbered in series with the standard models.  The Sports model was introduced with upswept high level exhausts rather than the much quieter woffle box under the gearbox.  This high level system obstructed access to the central toolbox over the transmission, so a pair of cast-alloy pannier toolboxes was substituted on the rear sub-frame, with appropriate alterations to the frame/sub-frame for mounting.   The mudguards were sans-valence to give a sporty appearance and finished in metallic silver rather than black.  Engine-wise the Mark 3 Sports (and subsequent Mark 4 Sports) versions were virtually identical to standard specifications, except that they had some pretty hairy cams almost identical to the racing Plus cams.  These Sports cams have an 'S' stamped on them between the cam lobes - if you find some, then treasure them, as they have a full 60 degrees more opening time than standard cams - and with better breathing and carburetors the bike is transformed from a docile and reasonably swift plodder to a pretty interesting mount - for its day, anyway!  They were the fastest production 350cc at the time. 

1472 Mark 3, 842 Mark 3 Sports produced for a total of 2314 over eighteen months. 

Mark 4, Mark 4 Sports

The Mark 4 and Mark 4 Sports were very similar in appearance, more so than on the Mark 3 series.  All models had a floating front mudguard that followed the front wheel up and down till it fractured its mounts, at which point it tried to also follow the wheel round and round!  The standard continued with side valences on the frond mudguard which was an unfortunate bit of extra un-sprung mass.  While the frame was still black, the tank was in blue and silver panels with that chromium plated swath.  Optional was an overall application of polychromatic blue called “Bluebird”, including the petrol tank, but the later retained the swath of chromium plate as before.  New alloy pannier toolboxes of tear-drop shape took up residence on the rear sub-frame of both models.  These had an extension off the bottom to mount the passenger footrests, rather than being attached to the swing arm as previously. 

The Mark 4 Sports had down swept pipes ending in conventional tubular silences, rather than the woffle box of the standard model.  Flatter pattern handlebars, no valence on the front mudguard, and a sportier camshaft made the essential differences. 

1203 Mark 4, 554 Mark 4 Sports produced for a total of 1757 over ten months. 

Mark 5

The Sport model was dropped, likely as the Plus models (see below) were available for those with a penchant for more speed.  They are pretty much a continuation of the Mark 4.  However black is dropped for the now standard polychromatic blue; optional is a much less commonly seen polychromatic green.  The front mount of the petrol tank is adjustable to three positions to alter the knee grip height from low, too low, and lower yet!  Catalog info indicates it could still be had with a woffle box but the low level tubular silencer predominates.  Later on, Ferridax dual seat saddles became optional, as well as an external Vokes cartridge oil filter behind the left-hand cylinder. 

3061 produced over three years and nine months.  563 were in polychromatic green (also used on the Douglas Vespa scooter.)  The last two years production was very low, 141 in all of 1953 and 126 in the first six months of 1954, when Mark 5 production ceased. 

Ambiguous Mark frame number suffixes. 

One point of caution about frame numbers, particularly with the Sports models.  The final /S can look like /5, so you may get confused as to whether you have a  Mark 3 Sports frame or a Mark 5 frame.  If it has the prefix T35/S/ with that ambiguous suffix, it is a Mark 3 Sports.  (Standard Mark 3s had a /3 suffix and the Mark 1 had no suffix.)  The high mounting point of the rear sub-frame will also preclude it being a Mark 5.  If there is no T35/S/ prefix with that suffix, and/or the rear sub-frame mount is low, it could either be a later Mark 3 Sports or a Mark 5 - in which case look for the position of the rear passenger footrest mounts - if they are on the rear swinging arm, it's a Mark 3 Sports. 

With the Mark 4 Sports you are less liable to confuse the /4 and /S suffix.  Like the Mark 5 they never have the T35/S/ frame number prefix, and the Sports versions have the frame number ****/S/4 or ****/S4 (or rarely ****/4S).  They have low rear sub-frame mounting points, and no footrest mounts on the rear swinging arm. 


The Competition model, in this case it means Trials, is easy to spot - it was a mainly black bike with a ridged rear end; no rear suspension.  The rear lugs were from an aborted T35 rigid rear model that did not go into production. The engine mounting brackets near the front end of the lower frame tubes were reversed to point upwards and so raise the engine for more ground clearance.  The transmissions had extensions at the rear mounting lug to do the same.  All Comp engines were marked with a 'C' suffix - if they haven't got it, then they are Mark transplants.  They were available with full electrics or as 'one day trim' machines - i.e. no dynamo, battery, horn or lights.  The 1st and 2nd gears were exceptionally low, and the gearbox is machined out inside to make room for the larger cogs.  The exhaust is upswept, and joins into a single high level pipe over the transmission on the right-hand side.  Some had alloy heads, which appear to be direct copies of the standard iron head.  The mudguards were minimal section polished aluminum.   If you are curious about this model, there is a test report and photos in 'Motor Cycling' April 8th, 1948. 

A very small number (perhaps three) Mark 1 Competition bikes were made by the factory.  These look like standard Mark 1s but have upswept pipes, alloy mudguards, and no lights.  I am told that I have one - verified by Eric Brockway, Managing Director of Douglas Sales and Service, some years ago.  If so, then it has a rather deceptively sweet engine that is quite happy at up to 7500 rpm, and 80+ mph on a 1947 350cc bike was quite unusual in them days!  They were used in the Land's End Trial, and may have been tentative test-beds for the Mark 3 Sports, although the cams have exactly the same profile as those used in the standard Mark 1s (I've checked!) 

148 produced, numbered in sequence with the Mark 3, 4, and 5 series.  Seven produced in Mark 3 series, eighty three in the Mark 4 series, and fifty six in the Mark 5 series.  First was 5481 in December 1948.  Then 8963 to 8998 were nearly all Comps.   Last was 9968 in January 1951. 

Plus models

The Plus models are easy - The colour schemes were maroon and chrome (Plus 80) or gold and chrome (Plus 90).  Plus cylinder heads are quite distinct, the fins are inclined at an angle and not vertical; and the exhaust pipe are held to the head with a threaded and finned gland nut rather than the pipes being simply pushed in like the other models.  An optional extra was to get them cast in aluminum alloy. 

Some Plus engines also had alloy cylinders, but these were a mixed blessing.  The cylinder studs on these models went right through from the head into the crankcase, or ‘through studs’.  But because the studs were steel, they had a lower expansion rate than the alloy cylinders and heads, and tightening them to what would normally be considered proper tension could result in the expanding cylinders pulling them out of the crankcase if the engine got a bit too warm! 

Most Plus barrels are held on with cylinder base flange studs, just like the Mark barrels.  There is a flat section high on the crankcase timing cover which carries the rev counter drive gearbox, and there's a curious forward-pointing funnel sticking out on the left of the engine, where you normally take a cover off to lubricate the clutch mechanism.  This is designed to get extra air into the clutch bell housing and keep the clutch cork inserts from baking on the Plus 90.  Typically the Plus 80 had the conventional Ferrodo clutch and normal road ratios in the gearbox. 

Plus front forks have four simple lugs at the front of the tubes for occasional headlamp mounting, whereas all other forks (except Comps) have permanent triangular headlamp brackets brazed to the top of the fork tubes.  The front brake was a 9" diameter monster, so the paired front brake reaction link mountings at the bottom of the fork tubes are attached slightly higher than on the standard Mark frames (which had 8” brakes) - if you see a Mark fitted with a Plus brake you'll see that the reaction link and the leading link for the wheel spindle are not parallel, whereas on the Plus forks they were.  The result of such a miss-match is that the front mudguard 'nods' back and forward every time the bike goes over a bump, and eventually the vertical mudguard stays tend to break, even if the later form of mounting fitted to the Mark 3/4/5 is fitted - damned annoying! (It is possible re-locate the mounting stud lower on the Plus brake plate to cure this - but only if you don't mind altering one of the only approximately 500 of these units ever made!)  The Plus forks also have two-way oil damping, whilst the Marks have only one-way damping. 

Plus models had a wide range of accessories, from larger petrol tanks (some in alloy), T.T carburetors, alloy rims, alternative ratios, racing magnetos, and choice of three camshafts to name a few.  The Plus 90 was quickly eclipsed by the 350cc BSA Goldstar, and to recoup the outlay many were converted to road trim.  Engines that failed to dyno at least 25hp were relegated to the Plus 80.  All the Plus accessories were available as well as those for the Mark 5 such as crash bars, rear carrier, etc.  Literally they can range in specification from essentially a maroon colored Mark 5 to an out and out racing machine. 

Finally, the Plus models had 21inch front wheels to accommodate the large brake unit - but standard 19" wheels as used on the Marks series will accept the Plus brake (only just, I know - I've got one!)  They are commonly fitted to any variety of Mark nowadays, and fetch a high second hand price, because they are rare, look very impressive, and even stop the bike rather more efficiently than the standard front brake - so grab one if you see it for sale! 

Many of the parts for the Plus model, though they look like the standard Mark items, are just a wee bit different.  Besides those mentioned above, a few other parts that look standard at first glance but are not: slightly shorter cylinders, slightly deeper rocker covers, slightly thicker rocker blocks, larger diameter torsion bars, left-side swing arm pivot narrower on frame to clear transmission sprockets, different sub-frame, and probably many more details.  The crankshaft is a combination ball bearing and bush at the front (the Mark is just a bush) and the crankpins are stepped and use different sized rollers.  Some say the list of similarities would be shorter!  So if contemplating one of these, it would behoove you to have an expert along to see that you had all the right bits and pieces, and not a mixture, as finding the correct parts could be a long and difficult process. 

Like the Competition series, the Plus models were also numbered in sequence to the Mark series.  279 Plus 80 models over thirteen months, all numbered within the Mark 5 series.  First was 9355 in October 1950, last was 11967 in November 1951.  The first Plus 90 was 7563 in March 1950 and the last 12063 in March 1952 for a total of 220 over two years.  Ninety nine produced during the Mark 4 series and 121 during the Mark 5 series.  BUT: a batch of Plus 90 machines were produced which did not conform to the Mark series number scheme, at least not the engine.  These had a prefix X90, the highest known being X90/102.  It is believed these were made to achieve ACU recognition as a production model on sale to the public for racing purposes.  These machines would pre-date the Mark series Plus 90 models.  The X90 machines do not appear in the company production ledgers.


There was a six month gap between the last Mark 5 and the first Dragonfly, probably occupied by the high volume of Douglas Vespa scooter production.  The Dragonfly engines are quite visibly different to the Mark series.  The spark is now provided by coil ignition, with a distributor above the engine where the magneto used to be and covered by an alloy casting, so no Magdyno unit prominently on view.  The generator is a flat AC unit mounted at the front of the crankshaft.  The rocker covers are oval, without the prominent depression to allow access to the spark plug that the Mark 3/4/5s have, or the “Douglas” name cast in situ like the Mark 1.  A single carburetor is mounted at the top of the flywheel housing, with passages cast into the same and short connecting chromium plated tubes out to the cylinder heads proper. 

The frame is completely different, with shrouded coil over shock dampers (not internally-mounted torsion bars) at the rear, and very distinctive Earles front forks again with the shrouded coil over shock dampers.  The tank is much larger, and is streamlined with the front lamp, which is therefore fixed, and does not turn with the forks.  Up to and including machine 1275/6 were fitted with Mark series hubs and brakes.  Once stocks were exhausted, much criticized “British Hub” hubs and brakes were fitted.  The colour scheme can be beige (called “Stone”) and green, or black and silver with chrome trim. The earliest machines (frame no 1001/6 to 1008/6) were the prototypes, originally badged 'Dart' until Daimler objected (they already had the Daimler 'Dart' sports car, beloved of a few privileged Police forces!) 

1570 machines produced over two years.  First was 1001 in June 1955, and last was sold ex-works in July 1957.  Pride and Clark took many, including the last fifty six.  They sold 748 ex-works, and a further ninety nine returned from dealers unsold after production ceased.   250 to 300 were in black and silver paint, the rest stone and green colour, except two in Malta Post Office red! 

Model T35/Mark 1 - 350 OHV, May 1947 to May 1948

Nominal Year(s)

Frame No.

Engine No.

Gearbox markings

Any distinguishing features


T35/S/101 to T35/S/2976

As frame

No clear pattern

Fixed front mudguard, upright sparkplug, “Douglas” on rocker box covers, central mounted toolbox. 

Model DV60/Mark 2 - 600cc SV, 1948






Telescopic sprung seat post, ridged rear, quickly detachable wheels, crinkle hubs with straight headed spokes, flathead.  Prototypes only.

Model Mark 3, Mark 3 Sports - 350cc OHV, June 1948 to November 1949


T35/S/4001/3 to 6589/3

As frame

No clear pattern

Suffix /3 Standard; /S Sports.  Outward splayed inlet tracts.  Sparkplug inclined at 45 degrees and dip in rocker cover to suit.  Standard- central mounted toolbox and woffle box silencer valance on front mudguard on standard.  Sports - hotter cams, upswept exhaust pipes and pannier toolboxes. 

 Model Competition - 350cc OHV, December 1948-1951




All originally were stamped “Comp”

Ridged rear frame, engine raised in frame for additional ground clearance, alloy guards, two into one high level exhaust.  While the suffix was special, they were numbered in sequence to the Mark series.

Model Mark 4, Mark 4 Sports - 350cc OHV, November 1949 to September 1950


7000/4 to 8999/4

As frame

No clear pattern

Suffix /4 Standard; /S/4, /S4, or /4S Sports.  Cast alloy toolboxes on sub-frame teardrop shape, pillion footrests attached.  Black with blue and chrome petrol tank, optional overall Polychromatic blue.  Front mudguard that now travels with wheel.  Standard- valance on front mudguard, woffle box exhaust.  Sports - hotter cams, low level exhaust pipes with tubular silencers, no valance. 

Model Mark 5 - 350cc OHV, October 1950 to June 1954


 9001/5 to 12540/5

As frame

No clear pattern

Polychromatic blue.  Polychromatic green introduced on machine 11963 in November of 1951, then all machines after 11978 till end of Mark 5 production.  Some export bikes in different colours.  Otherwise similar in appearance to Mark 4.  Later Ferridax dual seat and Vokes oil filter optional. 

Prototype - 500cc OHV, 1951





But there was only one anyway - you won't find one of these lurking around in some old boy's shed!  Intended for the 1952 season, it never went into production.  Still exists. 

Model Plus 80, Plus 90, Clubman’s/Racing - 350cc OHV, 1950 to 1953


As Marks, with some ****/80 or ****/90

Early engines X90/***, then ****/80 or ****/90

80/***, 90/***, or X90/***

Believed to be of an early batch made for ACU accreditation as a production model.  But exact numbering rational and dates not clearly understood. 


As above, then 8000 to 9500 with or without suffix

As frame

No clear pattern

Plus 80 - maroon frame, Plus 90 - gold frame. Head fins angled, rev counter from timing cover, 9" front brake.    Serial number range approximate.


9500 to at least 11967

As frame

No clear pattern

Air funnel to clutch housing.  Last year Plus 80 actually manufactured though cataloged till 1953.  11967 last Plus 80, may have been a few more Plus 90 made that year with higher numbers. 


11970 to 12500

As frame

No clear pattern

Serial number range approximate. 

Model Mark 6 Dragonfly - 350cc OHV, June 1955 to November 1956


1001/6 to1008/6

As frame


Pre-production “Dart”


1009/6 to 2573/6

 As frame


Enclosed top of engine, generator on front of crank, Earles front forks, twin rear shock absorbers, headlamp shroud flared into front of tank and does not turn with handlebars.  Mark series hubs and brakes up to 1276/6, British Hub assemblies after.  Late machines with minor modifications to cylinder heads and gearbox kick start boss stamped ****/6/2

Note: where you see in the tables a code containing three or more asterisks (e.g., *****/6) the number of asterisks indicates the number of digits in the code number. 


The information in these tables has been collected from various sources: factory records, registers of extant machines, Douglas publications, articles in the London Douglas Motor Cycle Club magazine the New ConRod, and personal knowledge.  While every effort has been made to provide the most accurate information available, the information provide above is offered as a guide only.  It is not recognized as official dating data by any licensing authority. 

© Doug Cross, Doug Kephart, October 2004