The drawings for my 3″ Foden show the position for drive pegs for the eccentrics at 131 degrees, giving full gear cut-off at 65%, but then in the notes on the drawing gives a figure of 122 degrees with different lap & lead.

Can anyone suggest any reasons for or against using 122 degrees instead of the 131 degrees as drawn?

I have considered putting the valve dimensions in to one of the valve gear simulators, but not 100% certain if I will be able to ascertain which set of results are actually “best”, would it be worth doing?

Another question, which may be better in it’s own thread, is how to put a keyway in the eccentric in the correct location, I can’t think of a way of lining up the broach so it is in line with the 2 centres, any ideas on how to do that? The only way I can think of at the moment is to take the blank eccentric, put it on the mill, find the centre, offset in the X (or Y) direction by half the throw, bore it out, then use a mill the size of the broach to start the keyway hole having moved in the X dimension half the bore diameter. I have seen axles broached using a custom guide with a flat on them, with a parallel to the crank pin, with suitable shims between the parallel & crankpin to get the correct alignment, but can’t do that here.

Thanks in advance.

Baldric.