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Stuart No. 9 slide valve

Am unclear about steam valve dimensions and setting up

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mike robinson 229/11/2009 13:54:55
18 forum posts
7 photos
The Stuart drawing item 31 shows a milled pocket in the gun metal valve that is 1/2 wide and 5/8 tall * 1/8 deep. The dimension fixing a start position for the 1/2 inch wide pocket is not shown so it could be milled out anywhere - clearly not correct.
 
I have read Tubal Cains book on building a No.9 and he refers to the valve having a line and line exhaust lap and a 3/32 steam lap. According to TC the start of the steam transfer pocket is 7/32 from the "toe" of the slide valve The eccentric has a 5/16 throw.
 
Having modelled in CAD the cylinder ports at the as cast dimensions and the  valve and I am still uncertain of dimensions for the pocket such that the ports are "fully" open at each end or have the correct "lap". If one end is set up to be fully open then a the other end of the 5/16 stroke the port is only half open,
 
I would like to understand steam lap can anybody describe what it is and or how to calculate this so I can be certain of the dimensions to cut the pocket.
 
Appreciate any help - I have illustrations from CAD but cant upload - to this site.
 
Thanks , Mike

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