Hi Håkan,
If your Mark is a mark 1 with the original heads, the studs and nuts are 5/16" BSF, the heads are easily identified as the rocker covers are more rounded and have "DOUGLAS" cast onto the aluminium cover. The later Mk3 to Mk5 which are much more numerous. The heads have the rocker cover with an indent to allow access to the spark plug. These heads have 3/8" BSF studs.
If you are building up the bike and do not have a parts manual these are available from the London Douglas Club. The exploded diagrams of all parts, engine and frame carry most of the information you would need.
The Club does also hold a fair stock of many of the engine parts.
I for my sins hold the stock of manuals and parts lists, should you need one or both please email me directly via the personal email.
Dave
