The position on the left is straight through ram air and on
the right is the filtered air source selected.

It will probably have to be bored on a large lathe after
the three 3 inch dia. tubes are welded to the barrel.
The Barrel is 4.3 inches ID so you need a thick wall chunk of aluminum
tube 4.5 inches in dia. or about a 3.5 inch long 4.5 inch dia. bar stock.

Nothing precision about it as you are dealing with huge volumes
of air so a little internal leakage here and there won't hurt much.

A lot fewer small parts than the dual butterfly valve.
 
Paul Lamar
 
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