One of the first challenges of an early Douglas restoration is dealing with the non-standard threads. Not just because you don’t have the right taps and dies, but because the last person to work on the machine didn’t either! An M7 x 1.0 metric thread is very close to Douglas’ 1/4 x 25 tpi – just not close enough.

So why did Douglas use 5/16 x 25, 3/16 x 27 etc.? The received wisdom is that the Douglas factory wanted to prevent owners buying what today we call ‘pattern parts’. Strange threads and odd spanner sizes ensured that owners had to go back to the dealers for their spare parts. It must have seemed like a great wheeze, 90 years ago …

4 thoughts on “Those Strange Douglas Threads …

    1. Hi – I faced the same difficulty myself. The 1/4 x 25 tpi taps and die are available from Tracy Tools in Torquay, UK. If you are a user of the Douglas Forum, Doug Klephart, the site moderator, has had a few taps made in 5/16 x 25 and 3/16 x 27 sizes; I do not know if these are still available. Finally, the Club is in the process of having a batch of taps and dies made but I do not know when they will be available. The Spares Secretary may be able to advise.
      Regards, Andrew

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  1. Hi Sloper 29, Very interesting your piece ”Those Strange Threads”. Many moons ago when working at BAC in Bristol, I sometimes asked the Tool Room Foreman to do the occasional little job for my vintage motorcycle projects. One day he asked me if I had a Douglas, I replied that I did n’t but had friends who did. I asked him why? he told me that he had worked in the toolroom at Douglas in Kingswood & had liberated a lot of ‘crazy’ TPI taps & dies when the factory closed & if I could put them to use I could have them. He gave them to me & I promised to give them a good home & passed them to a vintage Douglas owner in the Bristol Section of the VMCC. We supposed that the reason for using non standard threads was to protect their spares business. I’m sure this must have caused Douglas machines having threads ruined by cycle threads forced in or on by mistake. I have been attempting to make a variable pulley for my single speed Triumph. To move a pulley flange axially by approx. 5/16in. I am using a 30’s Douglas flywheel clutch 3 lobed cam arm, it works well. What goes around —–. Regards, Reg.

    On Sun, 2 Feb 2020 at 13:10, The London Douglas Motor Cycle Club wrote:

    > sloper29 posted: ” One of the first challenges of an early Douglas > restoration is dealing with the non-standard threads. Not just because you > don’t have the right taps and dies, but because the last person to work on > the machine didn’t either! An M7 x 1.0 metric thread is ” >

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