1
2
3
4
5
4
5
4
5
4
5
6
7
8
9
Piston
Rod
9
Piston bottom dead center
Piston
Piston
(Y)
(Y)
Piston top dead center
Piston
Piston
(X)
(X)
Piston Assembly Bill of Materials
Appendix E—D791, T791, D891 and T891 Piston Assembly Details
Ref.
No.
Part No.
Description
Qty
D/T791 1st Stage
3879-X1 (6"
)
D/T791 2nd
Stage
3884-X1 (3-1/4"
)
D/T891
3925-X1 (4-1/2")
1.
7002-025-TP100A 7002-010-TP100A 7002-025-TP100A
Screw, orlo gr. 8
(torque to 8 ft•lbs)
4
2.
3562
a
3561
a
3927
a
Piston cap
1
3.
3625
3731
2902
Shim washer (thick)
As
req.
3625-1
3731-1
2902-1
Shim washer (thin)
4.
1752
1756
1739
Piston rings
4
1752-2
1756-4
1739-2
Alloy 50 rings (opt.)
1752-3
1756-3
1739-3
Peek rings (opt.)
5.
1753
1757
1740
Expander ring
4
6.
3604
Lock nut (torque to
150 ft•lbs)
1
7.
3730
Thrust washer
2
8.
3879
a
(6" diameter)
3884
a
(3-1/4" diameter)
3925
a
(4-1/2" diameter)
Piston
1
9.
3603
Shim washer (thick)
As
req.
3603-1
Shim washer (thin)
10.
3812 (not shown)
Loctite tube (620)
1
Model
(Y) Bottom Min.
c
(Y) Bottom Max.
c
(X) Top Min.
b
(X) Top Max.
b
D791/D891
0.010" (0.25 mm)
0.020" (0.50 mm)
0.084" (2.13 mm)
0.104" (2.64 mm)
T791/T891
0.005" (0.13 mm)
0.015" (0.38 mm)
0.089" (2.26 mm)
0.109" (2.77 mm)
Piston Clearance (Cold)
b
The distance from the bottom of the head to the top of the piston.
c
The distance from the bottom of the piston to the top of the packing barrel.
CAUTION: Always relieve pressure in the unit before attempting any repairs.
a
MC1002 coated piston and cap available. Add “C” to end of a standard part number (e.g. 3562C)
38
