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Jim, The term used for the point in the stroke that the valve gear closes the inlet valve in a steam engine is known as cut off.This is expressed as a percent of piston stroke, so 100% cut off means that the valve stays open the full length of the stroke and is the max power for a given cylinder.Looking at the diagram in post 32 by eye the cut off point is less that 50% of the stroke it is about 30% of the stroke which gives good economy.Steam locomotives have a cut off of around 85% for full gear. The dotted line in figure 4 shows the effect of hooking up or notching back the valve gear. The cut off is chosen by the designer for the type of work the steam engine will be doing.Further reading:https://en.wikipedia.org/wiki/Cutoff_(steam_engine)Dan