I have done an old Welch pump too...but it was a single stage... not two stage.
There are 2 clearances to set.. the rotor side clearance ( and that of the vanes, too) and the rotor circumferential seal... the "duo " part of the seal.
I think the total side clearance is no more than 25 microns (0.001"). The only way to adjust it is by lapping the chamber shell casing and the vanes and the rotor. I used 600 grit SiC paper with Kerosene on a surface plate to do the lapping. I surfaced the OD of the rotor in the lathe, but resisted the temptation to dress the faces,as it would have necesitated a huge amount of lapping on the shell.
The Duo seal part is set my just bringing the shell down into actual light contact with the rotor. You then let it run with a small amount of oil, and flush several times, till it runs clean. Meaning the rotor has ground itself to the match the shell.
The most important thing for a good repair is to use NO gaskets or RTV.... just metal to metal seals. It really is amazing how rugged these workhorses are.
Dave Cooper
Created on Friday, December 15, 2000 3:58 AM EDT by David Cooper