Bottom Cover
Rear Cover O-ring

Unloader Kit Part Number K203786
Item

Qty

Item

Qty

Item

Qty

1

1

Description
Small Balance Piston O-ring

4

1

Description
Medium Unloader Piston O-ring

7

1

Description
Unloader Cover Gasket

2

1

Large Unloader Piston O-ring

5

1

Return Spring

8

2

Unloader Cover Cap Screws

3

1

Balance Piston

6

1

Unloader Piston

9

1

Lubricant (not shown)

Safety Valve Kit Part Number K039542
Item

Qty

10

1

Description
ST-4™ Safety Valve 250 psi

Figure 10 – Bendix BA-3800 Closed Room Compressor Exploded View

11

CLEANING OF PARTS

ASSEMBLY

All parts should be cleaned in a good commercial grade of
solvent and dried prior to inspection.

All torques specified in this manual are assembly torques
and typically can be expected to fall off after assembly is
accomplished. DO NOT re-torque after initial assembly
torques fall unless instructed otherwise. Refer to the
Technical Specifications section on page 14.

Crankcase
To clean the crankcase, complete the following steps:
1. Carefully remove all sealant gasket material adhering
to the machined front face of the crankcase. See
Figure 3.
2. Make certain not to scratch or mark the mounting
surface. NOTE: Keep the compressor opening
covered to prevent any sealant material from
entering.
3. Repeat this process on the engine mounting face.
In lieu of the instructions and procedures presented in this
document, follow the instructions contained in the vehicle
maintenance documentation.
INSPECTION OF PARTS
Unloader Mechanism
1. If the unloader assembly has been removed from
the cylinder head, the unloader assembly must
be serviced using an unloader kit. Refer to the
Maintenance Kits section on page 10.
2. If large amounts of carbon build-up are present
on the unloader piston (6) seat or orifice or if the
return spring exhibits compression set, the unloader
components must be replaced with an unloader kit.
3. Carefully inspect the two (2) bores in the unloader
cavity of the cylinder head for gouges or material
transfer. If this is detected, the compressor should
be replaced.

UNLOADER INSTALLATION
To install the unloader, complete the following steps:
1. Apply a coating of the lubricant provided in the
unloader kit into the unloader bores (both diameters).
2. Apply additional lubricant to the two o-ring grooves
on the unloader piston (6) and the o-ring groove on
the balance piston.
3. Install the three o-rings (2, 4, 1) into the appropriate
o-ring grooves on the unloader piston (6) and
balance piston (3).
4. Apply another coating of the lubricant onto the
exposed o-ring surfaces and onto the large diameter
of the balance piston (3).
5. Apply additional lubricant to the larger internal bore
in the unloader piston (6).
6. Install the unloader piston (6) with the pre-installed
o-rings into the cylinder head unloader bore, making
certain not to damage them in the process.
7. Install the return spring (5) in the unloader piston.
8. Apply a coating of lubricant to the largest diameter
of the balance piston (3).
9. Install the small diameter end of the balance piston
(3) through the center of the return spring (5).
10. Install the balance piston (3) and return spring (5) into
the internal bore of the unloader piston (6).
11. Install the unloader cover gasket (7) on the cylinder
head making certain both screw holes align.
12. Position the unloader cover on top of the balance
piston (3).
13. Press and hold the unloader cover in place on the
cylinder head.
14. Install both unloader cover cap screws (8).
15. Torque the cover cap screws (8) to 62 to 71 in-lbs
(7-8 N•m).

12

INSTALLING THE COMPRESSOR
To install a compressor, complete the following steps:
1. Install the appropriate gasket or o-ring on the front
flange of the compressor.
2. Make certain oil supply or return holes in the gasket
are properly aligned with the compressor and engine.
NOTE: In some applications, an additional o-ring is required
to seal the oil feed passageway between the compressor
and engine gear case cover. Gasket sealants are not
recommended.
3. Secure the compressor on the engine and tighten
the mounting bolts per the engine manufacturer's
recommended torque requirements.
4. Install any supporting brackets on the compressor
in the same position(s) noted and marked during
disassembly. If a rear support bracket was on the
original installation, hand tighten the bolts on both
ends before torquing the bolts.
NOTE: It is important that the rear support bracket is flush
to both surfaces before the bolts are torqued.

5. Inspect all air, oil, and coolant lines, seals, and fittings
before reconnecting them to the compressor.
6. Make certain o-ring seals are in good or new
condition, the threads are clean, and the fittings are
free of corrosion. Replace as necessary.
7. Install the discharge, inlet, coolant, and governor
adapter fittings, if applicable, along with mating seals,
in the same position on the compressor noted and
marked during disassembly.
NOTE: Refer to the Technical Specifications section on page
14 for various fitting sizes and types of thread.
8. Tighten all hose clamps.
9. Before returning the vehicle to service, perform the
tests specified in the Operation and Leakage Tests
sections. Pay particular attention to all lines and
hoses disconnected during the maintenance and
check for air, oil, and coolant leaks at compressor
connections.
10. Check for noisy operation.

Sequence
1

5, 11

4, 10

Non-Lubricated Head Bolt Torque

2
2, 8

3
4

14.8 ft-lbs
(20 N•m)

5
6

6, 12

7
8
9

1, 7

3, 9

10
11

18.4 ft-lbs
(25 N•m)
60°±5°

12
• For the sequence 1-6, tighten all bolts to 14.8 ft-lbs (20 N•m).
• For the sequence 7-12, tighten all bolts to 18.4 ft-lbs (25 N•m)
then tighten all bolts to 60°± 5°.
NOTE: To confirm both bolts were tightened properly, the
final torque angle recorded by the equipment must measure
between 26.5 N•m and 46.0 N•m.

Figure 11 – Bendix BA-3800 Closed Room Compressor Head Bolt Torque Sequence

13

TECHNICAL SPECIFICATIONS
Specification

Value

Typical weight

33 lbs

Number of cylinders

1

Torque Specifications

Value

Bore diameter

3.622 in. (92 mm)

M8x1.25-6g cylinder head

221 in-lbs (25 N•m),
then turn 60°± 5°

Stroke

2.244 in. (57 mm)

M5x0.75-6g unloader cap
screws

62-71 in-lbs (7-8 N•m)

Calculated displacement at 1500 RPM

20.0 CFM

Discharge port fittings
Two M8x1.5-6H screws

221 in-lbs (25 N•m)
maximum

Water port fittings
Two M8x1.5-6H screws

221 in-lbs (25 N•m)
maximum

6.4 HP (4.8 kW)

Unloader port fittings
M10x1-6H

203 in-lbs (23 N•m)
maximum

1.0 HP (0.8 kW)

Safety valve port
1/4"-18 NPT

Flow capacity @ 1800 RPM and
145 psi (10 bar)

18.3 CFM
(520 L/min)

Flow capacity @ 3000 RPM and
145 psi (10 bar)

25.0 CFM
(710 L/min)

Loaded power @ 1800 RPM and
145 psi (10 bar)
Unloaded power @ 1800 RPM
Minimum coolant flow at
maximum RPM

1.6 gal/min

Maximum inlet air temperature

140°F (60°C)

Maximum discharge air temperature

320°F (160°C)

Minimum oil pressure required

Inlet Air Port Fittings
Two M8x1.5-6H screws

200-249 in-lbs (22.5-28 N•m)
221 in-lbs (25 N•m)
maximum

14.5 psi (1.0 bar)

Minimum unloader line size

1/4 in. I.D.

Recommended unloader line size

1/4 in. I.D.

Minimum governor cut-out pressure

109 psi

RELATED DOCUMENTATION

BENDIX TECHNICAL SUPPORT

Go to B2Bendix.com to download the latest version of related
technical documentation.

Contact the Bendix Tech Team at techteam@bendix.com
or by phone at 1-800-AIR-BRAKE (1-800-247-2725), option
2, Monday through Thursday, 8:00 a.m. to 6:00 p.m. and
Friday, 8:00 a.m. to 5:00 p.m. ET. Follow the instructions in
the recorded message.

Document
Number

Document Title

BW1971

Advanced Troubleshooting Guide for Air Brake
Compressors

BW5057

Air Brake Handbook

BW1396

Bendix® Air Brake System Troubleshooting
(ABS without ESP® Stability System)

SD-01-503

Bendix® D-2® Governor Service Data Sheet

Unless otherwise indicated, the trademarks used in this document, including Bendix, are United States trademarks
owned by, or licensed to, Bendix Commercial Vehicle Systems LLC.

14

SD-11-550 Rev 000 © 12/2025 Bendix Commercial Vehicle Systems LLC, a member of Knorr-Bremse • All Rights Reserved