From what I understand, their is a "quill" fitted in to the end of the crankshaft and the oil is fed to the big ends through it. BSA Bill's version has some piping and a "top hat" section to get the oil to the quill. However, SRM does the same thing but has the oil traveling through a modified timing cover. Some time since I looked at BSA Bill's configuration so stand to be corrected. BSA Bill's has the advantage the engine can be run with the timing cover off.