Douglas are well known for having used threads of their own design, presumably, so that owners were encouraged to buy spare parts from the company down to the last nut and bolt. On these early machines, the unique threads were limited to three sizes only, 3/16 x 27 threads per inch (tpi), 17/64 x 25 tpi, and 5/16 x 25 tpi. However, many of the other thread sizes used in the Cycle Engineers Institute (CEI) range re-designated as British Standard Cycle (BSC) threads in 1950 were also non-preferred options, usually 20 tpi. Examples will be found of 28 tpi. as well as the CEI/BSC standard for sizes from Ό to Ύ of 26 tpi. Many bolts and nuts are also of non-standard shape and dimensions. Finally the thread used top and bottom in the oil pump body is unusual to say the least. (See below under Petrol Tanks)
The lists that follow are an attempt to provide comprehensive guidance on the thread sizes originally used by Douglas Motors Ltd. Inevitably I will have missed some applications and I cannot guarantee that I have got it all totally correct. I have based the information below on observations from five 2Ύ hp models of different ages, 1911 model D, 1916 model U, 1923 TS, 1924 SW and 1925 CW. There were several variations in frames, two main fork designs and four designs of fuel tank over the period 1910 to 1926 and I cannot be certain that I have covered every variation. It should be noted that because of the uniqueness of Douglas threads and the difficulties experienced in the past by repairers/restorers unable to acquire the correct taps and dies, threads may have been altered on an individual machine (including the sample machines above).
Taps and dies are available from The London Douglas Motor Cycle Club for the 17/64 x 25 tpi and the 5/16 x 25 tpi. threads. Dies have never been procured for the 3/16 x 27 tpi. size used for securing the timing cover and some other applications. (see Additional Notes below). However, taps of this size are available and The Club had a chaser made to enable timing cover screws to be produced with both Instrument Head (domed slotted with taper under head to fit countersink) for the early models up to and including EW and with a hexagon head for the 1930s models. Thus, replacement screws are available but I regret that many timing chest threads have been tapped out to another thread form, inserts installed or tapped oversize. It may be assumed that all 20 tpi, 26 tpi (except where otherwise quoted) and 28 tpi threads are British Standard Cycle thread form. (60 degree included angle). British Standard Whitworth (BSW) and British Standard Fine (BSF) threads have a 55 degree included angle. It is believed that the special Douglas threads are to Whitworth form. There are also examples of British Standard Pipe (BSP) (see Additional Notes below) and British Association (BA) threads used on the machines. Carburettor manufacturers, including Douglas on their own designs, also used metric threads.
Valve lifter spring box. 7/16 x 26 tpi.
Cylinder Barrels to Crankcase * (thick nuts) Qty 4 5/16" x 25 tpi.
Crank case halves. * Qty 4 17/64" x 25 tpi.
Valve lifter cable adjuster. Ό x 26 tpi. BSF.
Tapped holes in crankcase for plugs, non-return valve, drains etc. 7/16 x 20 tpi.
Tapped holes in timing chest for tappet guides and pivot studs. 7/16 x 20 tpi.
Screws retaining pivot studs locking plates. Qty. 2 3/16 x 27 tpi
Timing cover screws . Qty. 8 (90 degree Raised Countersunk Head) 3/16 x 27 tpi.
Nuts retaining diamond frame on pivot studs. Qty. 4 17/64 x 25 tpi.
Nuts retaining valve lifter mechanism. Qty. 2 (small hex.) 17/64 x 25 tpi.
Cylinder barrel exhaust. (not 1910 or 1911models) 1Ό x 20 tpi.
Cylinder barrel inlet. (not 1910, 1911 or 1912 models) 1Ό x 20 tpi.
Inlet manifold bolts 1910, 1911 & 1912 models. Qty. 2 17/64 x 25 tpi.
Carburettor studs later models 17/64" x 25 tpi.
Cylinder barrel valve caps. 1.3/8 x 18 tpi.
Sprocket on flywheel. (LEFT HAND THREAD) 1.13/16 x 20 tpi.
Bolts clamping engine to frame and nuts. Qty. 4 * (thick Nuts) 5/16 x 25 tpi.
Sparking Plug threads in inlet valve caps. 18mm x 1.5mm
Magneto studs 17/64" x25 tpi.
A flywheel clutch and kickstart mechanism was fitted to the CW models of 1924 to 1926 and the SW model of 1924.Threads used on these are as follows:-
Top hat to front of flywheel body 3/16" x 24 tpi. (Whit) max. 3/16" long under round head slotted screws Qty 4.
Back plate to body 7/32" x 24 tpi (Whit) max.1/4" long under slotted countersunk head. Qty 8.
Kickstart pivot spindle 7/16" x 20 tpi. Right hand thread.
Ratchet nut 9/16" x 20 tpi. (LEFT HAND THREAD)
Gearbox 2 & 3 speed
Studs and nuts suspending gearbox from frame. Qty. 2 (large hex.) 3/8 x 28 tpi.
Gear case through studs. Qty. 4 and nuts Qty. 8 3/16 x 27 tpi.
Nut securing Sprocket . (LEFT HAND THREAD) 13/16 x 28 tpi
Nut securing pulley. (drive sprocket on CW). 7/16 x 20 tpi.
Drain plug and filler plug, cup lubricator or filler tube. 7/16 x 20 tpi.
Screws suspending tank from frame. Qty. 4 (Whitworth) 7/32 x 24 tpi. (90 degree Raised Countersunk Head Screws)
Petrol and drain taps. 1/8 BSP.
Filler caps oil and petrol. 1.7/8 x 20 tpi.
Nut at base of plunger. Ό x 26 tpi. BSF.
Oil feed drip regulator (cylinder type) screws. Qty 2 believed to be 3 BA.
Oil Pump body top and base. 1.075 x 28tpi approx. (Individual samples varied between 1.069 and 1.079 diameter. -- 27.5mm (1.083) dia.x 0.9mm (28.2 tpi.) pitch. would also approximate. These would also be very non-standard metric threads.
Note 1910/1911 tanks have no drip feed regulator, a smaller diameter oil pump body, smaller diameter oil and petrol filler caps and longer suspension screws.
Frame and Forks
Steering head nut, small. Veteran and some frames up to 1920. 1 x 26 tpi.
Steering head nut, large. Some from 1916 and all from 1920. 1.1/8 x 26 tpi.
Steering head clamp bolt and nut. * 5/16 x 25 tpi.
Studs on handlebars and nuts (small hex.) for hand levers. Qty. 4 17/64 x 25 tpi.
Nuts for petrol/air and adv./ret. controls. 17/64 x 25 tpi.
Front stand on frames with early forks, bolt and nut. * 5/16 x 25 tpi.
Spring clip for early fork front stand, bolt and nut 17/64 x 25 tpi.
Pivot bolts in forks for front stand on later frames. Qty. 2 5/16 x 25 tpi.
Shouldered nut securing front stand on mudguard. 5/16 x 25 tpi.
Screws securing cable clamps top and bottom. Qty. 2 3/16 x 27 tpi.
Tapped hole for fitting rear mudguard in rear of frame. 17/64 x 25 tpi.
Tapped hole in saddle tube for belt guard. 17/64 x 25 tpi.
Rear stand pivot bolts into fork lug from 1916. Qty. 2 3/8 x 20 tpi. BSF.
Upper through bolt for back stay and nut. * 3/8 x 20 tpi. BSF.
Bolts for back stay into rear fork lug and nuts. Qty. 2 3/8 x 20 tpi. BSF.
Carrier to back stay bolts and nuts. Qty. 2 17/64 x 25 tpi.
Carrier to fork lug bolts and nuts. Qty. 2 17/64 x 25 tpi.
Gear change quadrant screws. Qty. 2 (special domed head) 17/64 x 25 tpi.
Knob to tram handle gear change, (large round headed screw) Ό x 26 tpi. BSF.
Block on top tube for models with hooked front seat spring. 17/64 x 25 tpi.
Fork springs on early forks. Bolts in springs and Nuts. Qty. 4 # 5/16 x 26 tpi.
Fork spindles on early forks, Qty 4 & Nuts. Qty 8 (castle nuts) 5/16 x 25 tpi.
Headlamp bracket on early forks. Bolt. 17/64 x 25 tpi.
Forks spring anchors on later forks. Qty 2 fork end. 3/8 x 20 tpi. BSF.
Fork spring anchors outer end and special pivot nuts. Qty 4 3/8 x 28 tpi.
Fork spindles on later forks. LH side. (all four spindles) 3/8 x 28 tpi.
Fork spindles RH side and grease lubricators. (three spindles) 9/16 x 20 tpi.
(Fourth has 3/8 x 28 tpi. thread both sides with a 9/16 x 20 tpi. thread in between.
Mudguard stays front and rear 1910/1911 models 17/64 x 25 tpi.
Mudguard stays front and rear 1912 on. 5/16 x 25 tpi.
Front mudguard to late forks, bolts and nuts. Qty. 2 17/64 x 25 tpi
Front mudguard to early forks, bolt and nut. 17/64 x 25 tpi.
Wheel spindle nuts. Qty. 4 3/8 x 26 tpi.
Oiler cups on early frames and forks. (also hubs on 1910/1911). 3/16 x 32 tpi. BSF
Oiler cups on hubs on later machines. Ό x 26 tpi. BSF
1910/1911 models have Chater-Lea hubs. Later models have Douglas hubs.
Early forks are those constructed from oval tubing and associated with frames with the small steering head. Later forks are made from round tubing.
* Bolts designated thus have a special domed head with a small dowel let into the shank below the head to prevent rotation.
# Springs designated thus were used on other makes of motorcycle and were bought in by Douglas so threads are standard BSC with 26 tpi.
BSP threads also known as Gas threads can be very confusing as their diameter is much greater than their nominal size. Thus, 1/8 BSP. is 0.383 diameter, has 28 tpi. and has the same 55 degree thread form as Whitworth. Ό BSP is 0.518 diameter and has 19 tpi. These are commonly used on petrol taps etc.
A tip for cleaning the 3/16 x 27 tpi. internal threads in the timing chest and elsewhere is to use a hexagon head timing cover screw from the 1930s models which are longer than the instrument head version and to cut a couple of slots on a helix with a junior hacksaw . This can be used as a tap or the correct tap can be obtained from LDMCC Spares by Club members.