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Reversing a Piston Valve Engine

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"....I don't think lap will make much difference but to get equal running in both directions you will effectively end up with no lap. Lead would give problems as when you change direction that would become lag but as you have your eccentric shown at 90deg there is no lead to worry about....."

Without steam lap, you will have cut off at the end of stroke.   Not good for steam economy....most models are set between 0.75 and 0.88 of stroke.   Lead, the amount the valve is open at dead center,  is usually set to "just opening"   Linking up will shorten the cut off, depending on the valve gear geometry but up to 30 percent or perhaps more! also adds some compression.   Exhaust lap is usually not added on model engines as the condensation losses are so great, you end up with "water lock"...which isnt good

My first steam engine was a small single cylinder 3/4 bore x 1" stroke engine for a model tugboat.   It was a single cylinder, and to get reverse, it had a tumbler crash gear box....the engine just kept running

Good luck with your project


Thank you, Dave.

I hope to make a start on this fairly soon.

Kind regards,



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