Sundstrand Series 20, 21, 22 and 23 Pump and Motor Rebuild Guide: Teardown, Seal Kits, Torque and Start-Up

To rebuild a Sundstrand Series 20, 21, 22 or 23 heavy duty pump or motor, pull the shaft seal first, then the charge pump and check valves (or the motor manifold), the control, the end cap with its valve plate, and finally the bearing plate and cylinder block kit. Replace every gasket and O-ring with the overhaul seal kit for your exact series and unit type, set shaft end play to 0.08-0.18 mm (0.003-0.007 in), torque to the Danfoss chart, and do not call it done until charge pressure is 190-210 psi in neutral and at least 160-180 psi in forward or reverse.

These four series are the small end of the Sauer-Sundstrand heavy duty family, which runs from Series 20 to Series 27. Danfoss's newer manuals call the same frames by displacement: 033, 052, 070 and 089. Unlike the Series 90 manual, the Danfoss Series 20 service manuals actually walk you through the major repair: valve plate, bearing plate, cylinder block, swashplate, servo and shaft. That is what this page follows. If you have a Series 24 through 27, use our Series 24-27 rebuild guide instead; the steps are the same but the torques, end play and kits are not.

Which Sundstrand Series 20 frame do you have?

Read the nameplate. On a Sauer-Sundstrand heavy duty unit, the first two digits of the model number are the series (20 through 27) and the third digit is the unit type: 2 is a variable displacement pump, 3 a fixed displacement motor and 4 a variable displacement motor. The rest of the number identifies the control, shaft, pressure settings and rotation in the factory records, and the model number is what parts are ordered against.

Series	Danfoss frame code	Displacement	Max speed at max displacement	Max recommended working pressure
20	SPV2/033, SMF2/033, SMV2/033	2.03 in³ (33.2 cc)	3,800 rpm	5,000 psi (345 bar)
21	SPV2/052, SMF2/052, SMV2/052	3.15 in³ (51.6 cc)	3,500 rpm	5,000 psi (345 bar)
22	SPV2/070, SMF2/070, SMV2/070	4.26 in³ (69.8 cc)	3,200 rpm	5,000 psi (345 bar)
23	SPV2/089, SMF2/089, SMV2/089	5.43 in³ (89 cc)	2,900 rpm	5,000 psi (345 bar)

Sources: BLN-9630 page 3 (displacement, speed, pressure); 520L0910, 520L0911 and 520L0912 (frame codes: SPV2 is the variable pump, SMF2 the fixed motor, SMV2 the variable motor). Sundstrand notes that running these units at 6,000 psi (414 bar) requires welded (non-filled) pistons.

The serial number matters as much as the model number. Its first two digits are the dash number, which changes whenever a design change affects parts interchangeability, and the build year and month (earlier units) or year and week (later units) follow. The parts manuals split parts by that date: the 23 Series trunnion bearing kit is 9510260 on pumps built before 88-01, and the servo piston link on 21, 22 and 23 Series pumps built 87-13 through 97-23 is a different part (9210620, 9221367 and 9230787) from the current one. Our Sundstrand and Sauer-Danfoss identification guide explains the date code rule.

How do you tell if a Series 20-23 needs a rebuild or just a valve?

Run the Sundstrand fault logic first. On these transmissions a lot of the trouble lives in parts you can reach without opening the end cap.

Symptom	Check first (per BLN-9630 and L1003506)
Works in one direction only	Control linkage, then swap the two system relief cartridges and the two charge check valves. If the fault changes direction, the part you swapped is bad. Then the shuttle valve and the control.
Charge pressure low in neutral only	The charge relief valve in the charge pump: debris holding the poppet open, galling or wear on the poppet and seat.
Charge pressure low in forward and reverse only (below 160 psi)	The second charge relief in the motor manifold. Its setting must stay below the pump charge relief, or the cooling circuit through the shuttle valve does not work.
Neutral hard to find	Disconnect the external linkage. If the pump centers on its own, the problem is the linkage, not the transmission. If not, inspect the control for a missing, plugged or wrong orifice.
Running hot	Oil level, heat exchanger, inlet filter, motor bypass valve if used, and system pressure against the machine load.

The full gauge-by-gauge version is in our hydrostatic transmission troubleshooting guide. If the valves check out and charge pressure still falls off in stroke, the case drain flow test tells you whether the rotating groups are worn before you tear anything down.

What order do you take a Series 20-23 pump apart?

Clean the unit before it comes apart, tag and cap every line, and work on a fixture. Danfoss recommends a mounting block (dimensioned in L1003506) to hold the unit while you work. Protect every sealing surface and open cavity as you go.

1. Shaft seal. These units use a face seal, not a lip seal. Remove the large retaining ring at the shaft end, side opposite the tangs first. Pry out the aluminum seal retainer, which is held only by its O-ring, and make sure all the seal springs come out with it. Then work the bronze rotating ring off the shaft by hand without touching the lapped face. Danfoss is specific that the end cap screws must not be loosened until the shaft seal is out.
2. Charge pump. Loosen only the four screws that form a rectangle. The screws at the top and bottom hold the charge pump sections together. Lift the pump off, and catch the spacer in the idler shaft bore before it drops into the main pump. Discard the gasket.
3. Charge relief and charge check valves. The charge relief comes out with a 1 in wrench: plug, spring and poppet. Earlier units set pressure with shims in the plug, later units with an external adjusting screw. Do not change the shims unless you replace parts. The two check valves are cartridges under the charge pump and need a draglink socket; keep them in their original positions.
4. Control. Remove the nine cap screws with a 7/16 in wrench, swing the control away and slip its pin out of the swashplate link. Watch the three O-rings and the orifice plate so they do not fall into the housing.
5. End cap and valve plate. Remove all but two end cap screws. The end cap is spring loaded from inside, so loosen the last two alternately until the load is off. Hold the valve plate as the end cap comes away so it cannot drop. Pump valve plates have two tapered slots and are rotation specific (CW and CCW).
6. Bearing plate and cylinder block. Lift off the bearing plate and pilot ring. Pull the tapered bearing off the shaft with a puller, using a spacer so the puller screw does not damage the slot in the shaft end, then slip out the spacer, lay the pump horizontal and slide the cylinder block assembly off while holding the outer end of the shaft.
7. Swashplate, servo and shaft (only if needed). Remove the front cover and lift out the shaft and bearing. Mark how deep each servo cylinder is screwed in and mark the servo sleeve to the housing before unscrewing it. Mark each trunnion and keep its plastic shims with it. The swashplate then comes out with the servo piston.

Motor differences

Motors have no charge pump or displacement control (variable motors do have a control). Instead there is a valve manifold on the end cap carrying the two system relief cartridges, the shuttle valve and the second charge relief. Remove the four corner capscrews first, hold the manifold, then remove the last two. There is no gasket; it seals with O-rings and back-up rings (or square-cut rings on some units), and the port with the counterbore takes the orifice plug and the smaller O-ring. The relief cartridges come out with a 1-3/8 in wrench and swap side to side if the settings match. Motor valve plates have four tapered slots, so check you have a motor plate before it goes back in.

What do you inspect inside a Series 20 pump or motor?

• Shaft seal: replace the complete seal. Never pair an old bronze ring with a new stationary seal, or the reverse. The lapped faces must be perfectly clean at assembly (BLN-9593).
• Valve plate and bearing plate: any damaged bearing or valve plate is replaced, and gaskets and O-rings are always replaced (L1003506 page 25).
• Thrust plate: pull it off the swashplate and check the surface. Any damage and it is replaced. A new one goes in chamfered side toward the swashplate.
• Cylinder block: replace the assembly complete or just the damaged parts, depending on how far the damage goes.
• Bearings: if the shaft bearings are damaged, the bearing cup in the front cover (pump) or housing (motor) is replaced along with the cone.
• Charge pump: check for debris holding the relief valve open, galling or wear on the valve cone and seat, and broken springs. The shaft and gears must turn freely. Danfoss does not permit further disassembly of the charge pump or the servo control valve.
• Charge check valves: push the ball off its seat; you should feel slight spring resistance and the ball should snap back. Replace them in pairs.
• Shuttle valve (motor): the spool must slide freely. Wear, dirt or cracks mean the whole manifold, because the spool and manifold are a matched select fit and are not sold separately.
• Pistons: depending on model, these units were built with plastic filled, hollow, solid or welded pistons. Welded pistons are the service part, and when they replace another type they must go in symmetrically in multiples of three (three, six or all nine). A unit built with welded pistons must be repaired with welded pistons. Solid pistons cut speed capability by roughly 20% and must not go into a unit that did not have them (BLN-9736).

Scored valve plate faces, slipper damage and worn swashplates are judged by eye and experience, because Danfoss publishes no figures for them. That is the honest reason to have a rotating group looked at before you buy parts for it.

Which overhaul seal kit fits a Series 20, 21, 22 or 23?

Every series has three different overhaul kits: one for the variable pump, one for the fixed motor and one for the variable motor. They are not interchangeable, so the third digit of the model number matters.

Part	Series 20 (033)	Series 21 (052)	Series 22 (070)	Series 23 (089)
Overhaul seal kit, variable pump	9510224	9510227	9510230	9510233
Overhaul seal kit, fixed motor	9510225	9510228	9510231	9510234
Overhaul seal kit, variable motor	9510226	9510229	9510232	9510235
Shaft seal kit	050088	050088	050088	050302
End cap gasket	9200088	9210204	9220425	9231081
Charge pump gasket, gear pump	9801235	9801235	9801235	9801235
Charge pump gasket, gerotor pump (not in overhaul kit)	n/a	9803241 with IPOR, 9803252 without IPOR
Control gasket	9803340 (all four)
Motor manifold O-ring kit	9510049 (all four)

Source: Danfoss 520L0910 page 5 (pumps), 520L0911 page 5 (fixed motors), 520L0912 page 5 (variable motors). Linked part numbers are kits and gaskets we carry; the rest we source against your nameplate. The 9801235 gasket is for the gear-type charge pump only. Series 24 through 27 use a different gear pump gasket, 9801604.

If you are replacing hard parts too, the parts manuals list valve plates by rotation. Pump valve plates (CW / CCW): Series 20 002881 / 002899, Series 21 003368 / 003376, Series 22 003889 / 003897, Series 23 004333 / 004341. Motor valve plates: 002907, 003384, 003905 and 004358. Cylinder block kits: 510149, 593251, 596890 and 9230973. Kits and parts we carry by series are in the Series 20, Series 21, Series 22 and Series 23 parts collections, and every Sundstrand kit is in the Sundstrand seal kit collection.

How do you reassemble it and what are the torque values?

Use new gaskets and O-rings throughout, lightly coated with clean petroleum jelly. Lubricate the thrust plate, slippers, pistons and bores with clean oil before the block goes on.

• Cylinder block: the undersized tooth in the block spline lines up with the missing tooth in the ball guide spline, and both line up with the missing tooth on the shaft. Slide the block onto the shaft against the thrust plate, stand the pump vertical, fit the spring guide, and support the shaft from below while pressing the tapered bearing on so the spring guide is not compressed.
• Valve plate and end cap: the valve plate slot goes over the dowel pin in the end cap. Turn the end cap so the two housing dowels meet their holes, draw it down evenly with two opposing screws, then fit and torque the rest.
• Swashplate and servo: refit the trunnions with new O-rings and their original plastic shims, line up your marks, screw the servo cylinders back to their marked depth, and check neutral with a depth micrometer at the outer edge of the thrust plate, 90° to the trunnions, at two points 180° apart. Danfoss allows the two readings to differ by no more than 0.03 (the manual prints no unit). Peen the retainers into the locking slot when done.
• Shaft end play: whenever the shaft, bearings, housing, front cover or end cap are replaced, assemble them with gaskets and check end play: 0.08-0.18 mm (0.003-0.007 in) on 033 to 089 frames. Adjust with a shim under the bearing race in the end cap. The shaft seal goes on last, after the end cap is torqued.
• Shaft seal: lubricate the O-rings, wrap the spline or key, fit the bronze ring lapped face out, line up the notch in the stationary seal with the pin in the retainer, and push the retainer in until the retaining ring groove shows. Ring goes in beveled side out. The spring count is six on 20 through 22 Series and eight on 23 Series (BLN-9593).

Fastener	Series 20, 21, 22 (033-070)	Series 23 (089)	Sauer-Sundstrand BLN-9630 figure
End cap screws	36.6-50.2 N·m (324-444 lbf·in)	61.0-73.2 N·m (540-648 lbf·in)	Not given
Front cover screws	36.6-50.2 N·m (324-444 lbf·in)	61.0-73.2 N·m (540-648 lbf·in)	Not given
Charge pump screws	13.6-14.9 N·m (120-132 lbf·in)		10-11 ft·lb (BLN-9592: 8-11 ft·lb, 1/4 in UNC)
Charge pump suction port	18.6-27.5 N·m (165-243 lbf·in)		Inlet hose no more than 14-21 ft·lb
Charge check valves	40.7-54.2 N·m (360-480 lbf·in)		30-40 ft·lb
Servo control valve screws	13.6-14.9 N·m (120-132 lbf·in)		10-11 ft·lb (BLN-10195 new MDC: 8-11 ft·lb)
Spring assembly	13.6-14.9 N·m (120-132 lbf·in)		Not given
Servo piston locking screw	18.6 N·m (165 lbf·in)		Not given
Motor valve manifold screws	21.7-28.5 N·m (192-252 lbf·in)		16-21 ft·lb (BLN-9591: 19-21 ft·lb)
Pump charge relief plug	22-24 N·m (195-212 lbf·in)		30-60 ft·lb
System relief cartridges (motor manifold)	Not given		30-70 ft·lb
Shuttle valve hex plugs and motor charge relief plug	Not given		60-100 ft·lb
Gauge port plugs	Not given		10-20 ft·lb
Control handle nut	Not given		100 lbf·in maximum (BLN-10195)

Sources: Danfoss L1003506 page 35 and page 22 (charge relief plug), L1003443 page 22 (motor manifold), BLN-9630 pages 28-31 and 35, BLN-9591, BLN-9592 and BLN-10195. The pump charge relief plug is the one real disagreement: the 2014 Danfoss manual says 22-24 N·m, the older Sundstrand manual says 30-60 ft·lb. Check the current manual for your unit before you lean on it.

What should charge pressure be on a Series 20 heavy duty pump?

The Sauer-Sundstrand heavy duty manual (BLN-9630) gives 190-210 psi (13-15 bar) in neutral and 160-180 psi (11-12.5 bar) in forward or reverse, both measured above case pressure. The lower in-stroke figure comes from the second charge relief in the motor manifold, which takes over through the shuttle valve whenever the pump is on stroke and dumps hot loop oil to the case. Sundstrand notes some machines have special settings, so the machine spec wins if it says otherwise.

The 2014 Danfoss manual L1003506 states it differently: about 15 bar (217.6 psi) in neutral at 1500 rpm and about 2 bar lower with the pump stroked, with 15-18 bar (218-261 psi) in neutral in its troubleshooting section, and it says to stop and find the fault if charge pressure drops below 8 bar (116 psi).

System requirement (BLN-9630)	Value
Charge pressure, neutral	190-210 psi (13-15 bar) above case
Charge pressure, forward or reverse	160-180 psi (11-12.5 bar) above case
Case pressure	40 psig maximum (3 bar)
Inlet vacuum at charge pump inlet	10 in Hg maximum (0.7 bar absolute); may be exceeded on a cold start
Suction filtration	10 micron nominal, no bypass, Beta 10 = 1.5 to 2.0

Gauges per BLN-9630: 500 psi on charge pressure, 10,000 psi on system pressure, a vacuum gauge teed into the inlet line and 500 psi on case pressure at the bottom drain port, all with snubbers and calibrated.

How do you start up a rebuilt Series 20-23 transmission?

This is Sundstrand's start-up from BLN-9630, condensed:

1. Inspect both units for shipping damage and make sure the reservoir, hoses, valves, fittings and heat exchanger are clean.
2. Fill the reservoir through a 10 micron (nominal, no bypass) filter. Never reuse fluid. Fill the inlet line and check it for loose fittings, restrictions and air leaks.
3. Fill the pump and motor housings with clean filtered oil through the uppermost case drain port.
4. Install a 500 psi gauge in the charge pressure port and leave the external control linkage disconnected so the pump stays in neutral. Do not start the prime mover unless the pump is in neutral, and block the machine against movement.
5. Start and run at the lowest possible rpm until charge pressure comes up. Air can be bled from the high pressure lines at the high pressure gauge ports on the motor manifold.
6. Bring the engine to normal operating speed. Charge pressure should be at least 190-210 psi. If not, shut down and find out why.
7. Shut down, connect the control linkage, restart and confirm the pump stays in neutral. Then cycle slowly forward and reverse for five minutes. Charge pressure should hold at least 160-180 psi in stroke.
8. Shut down, remove the gauges, plug the ports and top up the reservoir.

For fluid changes BLN-9630 calls for 2,000 hours on a sealed reservoir and 500 hours on an air-breathing reservoir unless the machine maker says otherwise. The 2014 Danfoss manual is stricter on a new system: first change at 80-100 hours, the second by 500 hours, then every 1,000 hours or yearly. Change the inlet filter every time the fluid is changed. Our pump start-up guide and fluid cleanliness and flushing guide cover the general side.

Should you rebuild a Series 20-23 yourself?

Shaft seals, charge pumps, check valves, controls and the motor manifold are honest bench jobs with the torque chart above. The rotating group is where it goes wrong: a valve plate for the wrong rotation, a pump plate in a motor, welded pistons mixed in the wrong pattern, or end play never checked after a bearing change. If there was metal in the filter or the case drain flow is high, have the unit torn down and quoted first. For the rest of the Sundstrand line, see the Series 90 pump rebuild guide and our Sundstrand seal kit guide.

Frequently asked questions

Related reading

• Sundstrand Series 24-27 rebuild guide
• Sundstrand Series 90 pump rebuild guide
• Sundstrand and Sauer-Danfoss hub
• Sundstrand piston pumps
• Sundstrand parts catalogs, Series 15 through 90
• Sundstrand document reference index
• Sundstrand and Sauer-Danfoss pump identification
• Sundstrand Series 20 and 90 seal kit guide
• Hydrostatic transmission troubleshooting
• All guides and resources