hi brian,

if i can add a little bit more to jason's reply,

if you have launch links the eccentrics must be advanced by an amount equal to the lap on the valve plus any lead. this is relatively easy to work out with launch links. i should imagine that for a traction engine there will be no lead and no pre admission in full gear, so all you have to do is advance the eccentrics by the amount of lap.

with launch links, and depending on the amount of lap, the angle of advance in degrees may be more than that stated by jason. however i am applying miniature loco practice, and it may be that jason's figure is quite correct if there isnt much lap on your valves on your traction engine.

however guessing the angle is an imprecise method and it can be worked out very accurately.

if you use the 'sight' method for centralising the valve, a better method is to do this with air checking both leads and cut offs.

cheers,

julian