A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Date: 2015 May 28, 16:59 -0700
I don't know of an overhaul manual.
I think it has been mentioned before but check Bill Morris' site sextantbook.com. Near the top of the index check Aircraft Bubble Sextants. About 2/3 of the way down he works on a 5851 averager and he references several other 5851 projects.