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 …


