INSTALLATION, OPERATION, &
MAINTENANCE MANUAL
TITLE: F70U Series Rev. E
Unloader Valve Page 9 of 12
3.2 REASSEMBLY OF THE F70U SERIES MAIN VALVE
(See illustrations on Pages 7 and 8.)
1) Prepare new softgoods for installation. Take care in handling of all seats, seals, gaskets, and
metal sealing surfaces. All parts should be clean and free of any debris or contaminants.
2) Reinstall the liner (except 1-1/2 x 2) and liner seal(s) into the body. Do not nick or excessively
stretch the liner seal O-rings. The bottom O-ring (for 4 x 6 and larger) may be carefully set in its
sealing area in the body before the liner is inserted, or it may be fitted onto the liner and, if
necessary, held in place with a suitable lubricant (e.g., Dow Corning 33 or Molykote). For 2 x 3
and larger valves, the top liner seal should be lightly pressed into place in its groove, such that it
is evenly spread around the liner circumference and will not be pinched when reinstalling the
valve cap. For the 1-1/2 x 2 size, position the gasket on top of the body.
3) Insert the seat into the bottom of the piston, using care to position the seat completely within the
groove. Ensure that there are no scratches, gouges, or particles on the seat, and that it is not
twisted. Carefully position the retainer plate over the seat.
4) Apply Vibra-tite or Loctite to the retainer screw threads. Insert the screw(s) into the piston and
tighten firmly, as follows: Retainer
Valve size screw size Torque
1-1/2x2, 2x3, 3x4 1/4-20UNC 80 – 90 in-lbs
4x6, 6x8, 8x10 1/2-20UNF 30 – 35 ft-lbs
12x16 3/8-24UNF 12 – 15 ft-lbs
IMPORTANT: Retainer screw must meet firm resistance against locking Heli-Coil in
piston, and require a wrench to completely install. Replace Heli-Coil if worn or damaged.
5) For all sizes except 12 x 16, install the Teflon piston seal by pressing it evenly over the beveled
retaining lip at the top of the piston. For the 12 x 16, lightly lubricate the O-ring piston seal with
Dow Corning 33 or equivalent and fit into its groove.
6) Fit the wear rings in their grooves around the piston assembly. While holding the wear rings
(especially the free ends) in place, carefully lower the piston assembly into the liner. The piston
should move freely up and down, with some resistance from the seal and wear rings. It is
critically important that this installation result in even seating of the soft seat onto the machined
nozzle surface in the valve body.
7) After checking placement of the top liner seal or gasket [See Step (2)], as applicable, the cap may
then be positioned on top of the body.
8) The cap bolts, with washers, can then be reinserted. Apply anti-seize compound to bolt threads.
The following torque values are recommended:
Valve Size Cap Bolt Size Torque (ft-lbs)
1-1/2x2 1/4-20UNC 8 - 10
2x3, 3x4 1/2-13UNC 55 - 60
4x6 1/2-20UNF 55 - 60
6x8 5/8-18UNF 80 - 90
8x10, 12x16 3/4-16UNF 150 - 160
9) The main valve is now assembled. It should be tested per Section 4.0 before reinstalling control
valve(s) and dome tube, at which time tests described in Section 5.0 should be performed prior to
return to service.
