I spoke once with the California rebuilding facility for Welch pumps. What I got from the technical support person, was botht official and an unofficial but definitely workable techinique which I described earlier, here. The bolt holes on the plates and pump shells have clearance by design to allow the adjustming the rotor shell clearance. The 5 tenths gap can only be "set" with the special jigs Welch uses in the rebuild. This is the official rebuild position.
But, O.0005" (12.5 microns) clearance is not a lot. And by setting up the rotors and shells with a zero or nearly zero clearance, the rotor, vane ends and the shell will come to a nice match in just a few hours of run-in. What I did for my single stage pump rebuild, was to set the shells for a light drag on the TOP of the rotors. This ensures that the seal area between the intake and exhaust ports has metal to metal contact. Then using a light oil (break-in oil, ATF or even kerosene) I ran the pump until it turned freely. Now there is a bit of art and finesse in this method... since it takes some judgement and mechanical feel to tell that it is a light not too heavy drag. But otherwise the process is somewhat like seating new piston rings.
During the seating time, there will be a bit of noise, some heat, and metal wear particles in the oil. Change oil after about 5 - 10 minutes the first time and run the same time and drain a second time. The third time double the run time as the oil gets cleaner. When you see no more darkening, switch to regular pump oil flush and run it long enough to get hot and check pressure. If things are working right... base pressures should march downward until you approach the limits.
Richard's right on the base pressure for these little ones...although mine has gotten to about 4 microns when the oil is clean and dry...normal will be somewhere in the 10 to 20 micron range.
The above method may seem a bit crude, but short of a full set of dowel pinned hardened jigs, I don't know if you can do it another way. Further, it deinitely works. Your suggestion about using the plastic wrap is interesting, and might work.
Dave Cooper
Created on Wednesday, April 25, 2001 7:32 PM EDT by David Cooper