Douglas 2 3/4hp - Various 17/64"x25tpi Fasteners - Carb Inlet Studs, Nuts, Washers - Stainless Steel

Product no.: 0898 DG234_029

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This 'variable' item listing offers a number of 17/64" x 25tpi fasteners, as fitted to Douglas 2 3/4hp and 4hp motorcycles in Edwardian times.  This diameter and thread size was obscure - and specific to Douglas models of the Edwardian era, and has not been commonly available for many years.

As part of restoring a 1919(ish) Douglas 2 3/4hp, I have come across a number of fasteners and fittings which were originally 17/64" diameter with a 25tpi thread.  This is an obscure size, half way between 1/4" and 5/16" (which have always been the 'normal' imperial diameter size for British motorcycle fittings of this type), and to my knowledge, is exclusive to early Douglas motorcycles.

I gather the rationale for the Douglas motorcycle company adopting these different sizes, was primarily it would mean that customers had to come back to Douglas themselves when searching for spares!  However, as these thread and diameter types have never been adopted by any other company - it is now an issue for Douglas owners if trying to restore a motorcycle where those fasteners using this thread type are old, badly corroded or just plain worn out!

I found this myself, when sympathetically restoring the original induction pipe and Amac carb on my own project.  I did not want to replate the original Amac (Douglas type) carb - but could see the original long induction pipe studs which pass either side of the carb looked very badly corroded and worn.  Replacing these with new stainless steel studs (with 17/64" x 25 tpi threads both ends) has transformed the overall look of the carb.

As can be seen in the accompanyning photographs - amongst my own collectoin of Douglas 2 3/4hp parts I have 3 induction pipes - but the two shown both came to me looking unrestored and untouched from new (and actually both with what may be WW1 paint on them, indicating they may have been of Ex WD WW1 bikes).  The one with the Amac carb fitted had longer studs fitted, and small 'reduced head' hex nuts fitted, while the second induction pipe had a much shorter stud, but with a normal size nut on it - all these looked original and had 25tpi threads on the nuts.

I have used both of these pipes/fittings as templates to directly copy the dimensions of the fastenings listed here.  All parts are CNC machined in stainless steel and look very nice.  The 'reduced head' nuts are an obscure hex size and required milling from a larger diameter bar - which look particularly pretty (but sorry not cheap, due to machining time required).  The larger nuts are also correct imperial hex, and flats linished.  

Incidentally, I am indebted to to an article posted by 

Note: apologies these items are not cheap - but to make them has entailed purchasing custom taps/dies (as well as CNC thread inserts) and being an obscure diameter, the studs are turned down along their entire length from a larger diameter stainless steel bar.  Therefore a lot of setup effort for small batch's dictate a higher price than similar commercially available fastenings

Table of 17/64" Fastenings Available (which may be added to in the future:

Item Item Title Description/Size Other Notes

Douglas Amac carb - long stud set with Reduced Head Washers

Kit includes:

2 x 17/64" x 25tpi studs x 2.125" long

2 x 0.375" AF (i.e. 1 BA/3/8 AF spanner size) 'Reduced Head' Milled nuts

2 x spring washers (SS)

Patterns from very corroded but original fastenings from Douglas 2 3/4hp induction pipe, fitted with Amac Douglas carb - as in photographs.

This carb/pipe unit came to me unrestored with a project bike, and had the same 'reduced head' nuts - the smaller hex ensuring the nuts do lnot foul the carb inlet shroud.

Thread lengths on either end are the same as original studs (shown in photo, heavily corroded)

Spring washers fractionally less outer diameter than plain type washers - correct fit for reduced head washers

0898_2 Douglas Induction Pipe - short stud set

Kit includes:

2 x 17/64" x 25tpi studs x 0.875" long

2 x 0.445" AF (i.e. 3/16" Whit AF spanner size) 'Normal Head' slim linished nuts

2 x split washers (SS)


As above - the originals these are based on came from an original and untouched Douglas 2 3/4hp induction pipe, with primer tap fitted (although no carb fitted when I obtained it). Although I do not have an early carburettor to fit these inlet manifold stud - I know from looking at other period and original models that these studs would have been used with some early bikes (pre WW1?), as well as some later models which used down facing carburettors - see some photos I have attached. 

This original manifold was also fitted with original 'normal' width 0.445"AF nuts - hopefully if you already own a 2 3/4hp model you will know yourself if your carb is of the type that requires this shorter stud.

(Incidentally, I also own a later Douglas 2 3/4hp engine of approx 1924 year - this is fitted with a later small bore Amal carburettor and would also use studs of this size as well)

0898_3 17/64 x 25tpi x 2.125" SS Studs

Stud (Each) - 2.125" overall length.

Stainless Steel

As used to mount Douglas Amac carb on 2 3/4hp models, of the type where the stud goes through full body of carburettor.

Threaded 17/64" x 25 tpi both ends, one thread slightly longer than the other, as per original stud.

CNC Machined along full length (as a non standard diameter), in stainless steel, so also look very pretty

0898_4 17/64 x 25tpi x 0.875" SS Studs

Stud (Each) - 0.875" overall length.

Stainless Steel

 I believe this stud was used to retain other types of carburettor to Douglas 2 3/4hp manifolds (this one was copied from an original stud still attached to an early manifold fitted with petrol primer tap and green army paint that looked original - indicating it may have been of WW1 WD era).  

0898_5 SS Steel 17/64 nuts - Reduced Head Type

Stainless Steel milled hex - 0.375 AF.

Depth = 0.181"/4.6mm

These nuts are CNC manufactured (including milled hex flats, from round bar) with a 'reduced hex' of 0.375" AF.

This nut was copied from a very corroded original nut which was retaining the Amac carb to the inlet manifold on the carb/manifold shown in the photo.  This came from a large collection of Douglas 2 3/4hp parts and when I got it looked like it had not been stripped - possibly since new.  it had WWI paint on it and the studs/nuts were heavily corroded but looked original.  I believe the reduced nuts are fitted to this type of carb, as there is very little clearance and full size nuts would foul the side.  

More expensive than other nuts, as additional machining to mill the hex for this unusual size - but very pretty in stainless steel

0898_6 SS Steel 17/64 slim nuts - Std Head Type

Stainless Steel milled hex - 0.445 AF.

Depth = 0.181"/4.6mm

These nuts are CNC manufactured in stainless steel with 0.445" AF (across flats) hex - which is a standard Whitworth imperial spanner size - 3/16"W.

this particular nut is a 'shallow' or 2/3rd depth nut and was copied from an original (25tpi) one that came fitted to an original Douglas 2 3/4hp manifold - as shown in the photograph.  Again, this looks like it may be an WD WW1 manifold, but I have also seen later 2 3/4hp models with down facing carburettors fitted with short studs, for which I would expect these nuts are ideal.

The flats are linshed before machining, unlike other commercially available stainless nuts


SS Steel 17/64 Std width nuts - Std Head Type


Stainless Steel milled hex - 0.445 AF.

Depth = 0.248"/6.3mm

These nuts are CNC manufactured in stainless steel with 0.445" AF (across flats) hex - which is a standard Whitworth imperial spanner size - 3/16"W.

Similar to the nuts above, these nuts however are 'Full Depth' normal type nuts, so can be used on a variety of other fastenings that adopt the 17/64" x 25tpi thread form.

The flats are linshed before machining, unlike other commercially available stainless nuts

0898_8 SS Steel 17/64" Spring Washers (Pack of 10) 

Spring Washers to fit 17/64" bolts/studs

Outer Diameter of washer: 0.480"/12.2mm

These stainless steel spring washers have a smaller outer diameter than the plain washers, therefore are a closer fit washer ideally suited to the 'reduced wdth' 0.375"AF nuts.

However, can also be fitted with the larger nuts and always useful if worried about vibration .. . maybe less of an issue on a Douglas twin engine - but I like them on big racing Norton singles!

0898_9 SS Steel 17/64" Plain  Washers (Pack of 10) 

Plain Washers to fit 17/64" bolts/studs

Outer Diameter of washer: 0.539"/13.7mm

These stainless steel plain washers have a standard width and are ideal for most general purpose fittings.

They are fractionally thinner than the spring washers at 1.4mm.


By the way - I am indebted to an article written by Christopher Wright on the Douglas Forum website in 2004, which list all the main Douglas 2 3/4hp fastenings, including 17/64" fastenings:

The fittings listed here may also fit some of those other uses of this thread size.  If you have an original fitting you would like to see included, you can email us on and we may consider producing it


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