Hi Kevin
The most common bolt size on the 1930s models relating to cylinder head nuts and studs etc. is the 5/16" x 25tpi. There are also examples of the other Douglas special thread size of 17/64" x 25tpi. Taps and dies for these sizes are held in pre-1939 Spares by the LDMCC. You may also find examples of 1/8" and 1/4" BSP and another odd size used on oil and petrol unions being 7/16" x 19tpi. The carburettor base connection on many carburettors is 9/16" x 20tpi. Some timing cover screws are 3/16" x 27tpi. Again taps are kept by LDMCC for this size. Even where standard Whitworth threads are used Douglas often selected the non approved sizes such as 7/32" x 24tpi screws used on many pre-1939 flat tank models for holding the petrol tank to the top tube. I am sure it was all part of Mr Douglas' plan to ensure that if you wanted spares for your Douglas, even down to a nut, you had to buy them from him.
Chris.
