Hi All
Asked about the screws used to suspend the petrol tanks on early machines especially the 2.3/4hp I have done some research and provide the following information. These screws together with the screws used to secure the timing cover of the early engines have traditionally been described as Instrument Head having a countersunk head with a slight dome on top. The correct designation for these screws should be 90 degree Raised Countersunk Head Machine Screws and they generally conform to a much later British Standard B.S.450:1958 in which this type of screw was listed. The Standard provides for Unified thread versions having an 80 degree countersunk head and both Whitworth (BSW) and Fine (BSF) versions having the 90 degree countersunk head. However, as usual Douglas, in their pursuit of originality and to ensure that owners purchased spares only from the makers chose non standard or non preferred sizes. The screws supporting the petrol tank are 7/32" Whitworth (24 tpi) and the Standard lists only BSF (28 tpi) as a non preferred size and does not include a Whitworth version in this size. Nonetheless the head size and form meet the Standard and the details are as follows. Diameter of head nominally 3/8" (0.360"-0.383"). Depth of head 0.083" (excluding raised upper surface which is unspecified), width of screw driver slot 0.046" - 0.055" depth of slot 0.048". distance from under head to start of thread is two pitches. For the petrol tank application, a length of screw under head is about 0.5". I have made small changes to reflect the change of name in my original posting of 2006 linked in my previous message above. Chris.
