Some Notes on Repair of the Parker 51 
The Parker 51 can be a joy to work on. Nonetheless, I frequently run across 51s that have been permanently damaged by inappropriate techniques of assembly and disassembly.
One of the most vulnerable areas is the hood, which is normally cemented in place with a heat-sensitive sealant. If that sealant is not first softened, forcibly unscrewing the hood with section pliers can lead to disaster.
Although original Parker manuals advocate softening the sealant over an alcohol lamp, this is not advisable. Simply put, the margin for error with open flame is very, very thin. All too often one finds 51s (and 61s and 45s) with hoods distorted by excessive heat, the middle portion shrunken where it is thinnest and least supported. It is much safer to heat the hood-barrel joint with a heat gun, blow drier, or, safest of all, a hot water bath. The 51's acrylic can withstand water temperatures up to boiling, but a half a minute at 160°F should be sufficient to open all but the most stubborn joints (note that 61 and 45 hoods are not made of the same material, and must be treated with greater care).
It is also important to avoid excessive force when replacing the hood. Screwing the hood down tight may seem logical, but this can lead to minute stress cracks around the threads cracking that may only become visible hours or even days later. Later hoods which have recesses for an o-ring are even more vulnerable. Better to screw the hood down only until it seats, using only gentle finger pressure, relying on a thin band of shellac to hold it in place and seal it.
The same applies to replacing the filler units of Vac-filling 51s. Screwing down the retaining ring wedges the skirt of the diaphragm into its tapered seat. Screwing it down tight will give a good seal, but will also put tremendous stress on the seat virtually guaranteeing the development of cracks. The solution is to run a thin ribbon of shellac around the skirt of the diaphragm; this will insure a good seal even though the retaining ring is not tightened further after resistance is felt. If the ring doesn't turn freely, you won't be able to gauge if it is properly seated, so make sure the threads are clean on barrel and ring alike. Gently warming the ring and the barrel with a heat gun or blow drier before reassembly will also help.
If the retaining ring stops before it seats deeply enough, STOP! Remove the filler unit and look at the tapered seat: there is a layer of petrified rubber there which must be removed. Celluloid is more elastic than acrylic, so Vacs are much less likely to develop cracks around the diaphragm's seat. Nonetheless, the same warnings about torquing down the retaining ring apply. Many a Vac has had its barrel bulged in this way. Once again, the damage may not be apparent immediately the material may take some time to distort, as it creeps under stress.
Copyright © 2000, 2003 David Nishimura. All rights reserved