4.236 & T4.236 DIESEL ENGINES Section R: Compressor Caterpillar

Section R: Compressor
1.1. S.C.6. Compressor
2.2. Introduction
3.2. Preventive Maintenance
4.2. Service Check Inspection
5.2. Operating Tests
6.2. Air Leakage Test
7.2. To Remove Compressor
8.2. To Replace Compressor
9.2. Important
10.2. Check after Installation
11.2. Dismantling
12.2. Cleaning
13.2. Inspection of Parts
14.2. Reassembly
15.2. Fault Finding


S.C.6. Compressor


The S.C.6 compressor is a single cylinder unit which is bracket supported from the cylinder block, the compressor being flange mounted to the rear of the compressor drive housing. This compressor drive housing is mounted on the rear of the timing case and incorporates a geared drive from the engine idler gear to compressor.

The compressor draws air through the engine induction system. The air is then compressed and fed to the air reservoir. The compressor cylinder head incorporates an unloader device which is operated by air pressure and controlled by a governor valve connected to the reservoir and the compressor by a small bore pipe.

Preventive Maintenance

Every 4 Months or Every 5,000 Miles (7,500 km)

Make a visual check of all joints, unions, etc., for leakage or looseness and rectify where necessary. If the cylinder head has recently been removed, check that the cylinder head nuts are fully tightened down to a torque of 10 lbf ft (1,38 kgf m) -13 Nm.

Check compressor mounting and couplings for alignment.

Clean air filter.

Every 10,000 Miles (15,000 km) or 500 Hours

Clean the oil supply line to the compressor.

Remove delivery valve cap and delivery valve seat retaining spring and check for presence of excessive carbon. Withdraw and check condition of delivery valve. If excessive carbon is found, remove and clean the cylinder head; also check compressor discharge line for carbon and clean or replace the line if necessary.

Every 60,000 Miles (90,000 km) or 2,500 Hours

Dismantle compressor, thoroughly clean all parts and inspect for wear and damage. Repair or replace all worn or damaged parts or replace the compressor with a Factory Reconditioned Unit.

Service Check Inspection

Ensure that the air cleaner or filter is clean and correctly installed.

With compressor running, check for noisy operation and oil leaks.

Reduce the pressure in the reservoir by operating the brakes and check that governor and unloader mechanisms are functioning at correct pressure. (If possible the vehicle gauge should be replaced during the test by a master gauge).

Check to be sure compressor mounting bolts are secure.

Operating Tests

If leakage in the remainder of the system is not excessive, failure of the compressor to maintain the normal air pressure in the system usually denotes loss of efficiency due to wear. Another sign of wear is excessive oil passing through to the reservoir. If either condition develops, and inspection shows the remainder of the air brake equipment to be in good condition, the compressor must be overhauled or replaced with a Factory Reconditioned Unit.

Air Leakage Test

Excessive leakage past the delivery valve can be detected by charging the air system to just below the governor cut-out setting, and then with the engine stopped, carefully listening at the compressor for the sound of escaping air. If this test is satisfactory, fully charge the system and again stop the engine. Check once more for audible leaks, which if present indicate leakage at the unloader plunger.

Leakage at the delivery valve can be remedied by cleaning, lapping or replacing the valve and/or valve seat. Unloader plunger leakage can be remedied by replacing the plunger seal or valve plunger.

To Remove Compressor

Release all air pressure from the system.

Disconnect oil pipe and air pipes at the compressor.

Remove the setscrews securing the compressor drive and cover to the compressor drive housing.

Remove the compressor mounting bolts and remove compressor from engine.

To Replace Compressor

Fit compressor to compressor drive housing and secure with setscrews.

Replace bracket which supports compressor to cylinder block.

Reconnect oil pipe and air pipes to compressor.


Clean the oil supply line to the compressor and run the engine for a few seconds to be sure the oil supply is flowing freely.

Connect oil and/or air pipes, ensuring that the air cleaner or filter is clean and properly installed.

Check after Installation

With the compressor running, check for noisy operation and oil and air leaks.


Marking before dismantling

The compressor should have the following items marked to show the correct relationship prior to dismantling.

1. Position of cylinder head in relation to cylinder and crankcase.
2. Position of end-cover(s) in relation to crankcase.
3. Position of crankshaft in relation to crankcase.

Removing and Dismantling Cylinder Head and Cylinder

Remove the unloader cap and copper washer and withdraw the unloader plunger assembly and spring.

Remove the delivery valve cap and copper washer, and remove delivery valve spring and seat retaining spring.

Unscrew the four nuts and washers from cylinder head studs and lift off cylinder head. Remove the joint.

Remove the delivery valve and screw out the valve seat.

Withdraw inlet valve spring guide. (A simple extractor can be made from two 1/4 in U.N.F. bolts and a strip of metal formed to bridge the guide). Remove the inlet valve spring, inlet valve and valve seat.

Withdraw cylinder and remove the joint.

Removing and Dismantling Piston and Connecting Rod Assemblies

Remove the compressor mounting bracket and joint.

Turn the crankshaft to B.D.C. position and release the tabs of the locking strap. Unscrew the two bolts and remove the connecting rod cap. Withdraw piston assembly and replace connecting rod cap.

Remove the piston rings from the piston. If the piston is to be detached from the connecting rod, release one gudgeon pin retaining circlip and press the gudgeon pin from the piston and connecting rod.

Removing Crankshaft

Remove compressor drive gear.

Remove drive key from crankshaft.

Unscrew the four setscrews or nuts together with washers securing the rear end-cover to crankcase. Withdraw the end-cover, plain bearing, thrust washer and joint.

Unscrew the four setscrews or nuts securing the drive end-cover, and withdraw the end-cover complete with crankshaft and joint. Tap crankshaft with bearing from drive end-cover.


Ensure that all carbon is removed from the cylinder head. Check that the air passages in the head and the oilways in the crankcase, where applicable, rear end-cover and crankshaft are clear and clean.

Clean inlet and discharge valves, not damaged or worn excessively, by lapping them on a sheet of crocus cloth held on a flat surface.

Inspection of Parts


Check cylinder bore for excessive wear, out-of-round or scoring. If scored or out-of-round more than 0.002 in (0,05 mm) or tapered more than 0.003 in (0,08 mm) cylinder should be rebored. The original cylinder bore is to the limits 2.6255/2.6265 in (66,69/66,71 mm) and the clearance for the piston is 0.002/0.003 in (0,05/0,08 mm). Check for wear in cylinder bore and rectify in accordance with following table:-

Piston and Connecting Rod

Inspect piston for scores, cracks or damage of any kind. Check fit of rings in ring grooves, clearance should be 0.0005/0.0025 in (0,01/0,06 mm). Install rings in cylinder and check that gaps are 0.002/0.007 in (0,08/0,18 mm). Check fit of gudgeon pin in piston and connecting rod. Gudgeon pin should be a light press fit in piston and clearance in the connecting rod bush should not exceed 0.0015 in (0,04 mm).

Inspect connecting rod bearing for correct fit on crankshaft journal. Clearance between rod journal and bearing must not be less than 0.001 in (0,02 mm) and not more than 0.003 in (0,08 mm). Check connecting rod for cracks or damage.

Crankshaft and Bearings

Examine ball bearings for discoloration, pitting, wear and cracked races. Rotate slowly to check for roughness. Defective bearings should be removed, using a well-fitting extracting tool. Press new bearing on to crankshaft, using a suitable length of tube, until it contacts shoulder.

Inspect crankshaft for wear and check threads, shaft ends, keyways and drive keys for damage. The crank pin diameter should be within the limits 0.874/0.0735 in (22,20/22,19 mm).

Crankshaft and End-covers

Inspect oil seal carefully, ensuring that sealing edge is intact and sharp. If an oil leak has been observed at the crankshaft end, a new seal must be fitted. Lip of seal should face inwards.

Examine crankcase, end-cover and mounting bracket for damage and cracks. Check bearing bores for wear. The ball race should be a light press fit in end-cover and the crankshaft should be a neat sliding fit in the plain bearing. Inspect crankshaft thrust washer for wear.

Cylinder Head

Inspect cylinder head for cracks and unloader plunger guide bush for wear. Check that unloader plunger is a neat sliding fit in the guide. Examine unloader plunger seal ring for wear. Inspect inlet and delivery valves and seats. If valves are grooved deeper than 0.003 in (0.08 mm) where they contact the seat, they should be replaced. If not badly grooved they can be refaced by lapping on crocus cloth. Valve seats, if showing slight scratches, may be reclaimed by lapping with fine grinding paste. If badly pitted or scratched, use a seating reamer before lapping.

Renew delivery valve spring and check remaining springs for corrosion, fatigue or pemanent set.


Lubricate all internal parts with clean engine oil to prevent possible damage until the oil supply is functioning.

Install the crankshaft, complete with bearing, into the drive end-cover. Insert the crankshaft into the crankcase and secure the drive end-cover, ensuring that the joint is correctly positioned over the oil drain ports.

Position the thrust washer in the rear end-cover with the steel face towards the plain bearing and the tab located in the slot. Assemble the rear end-cover with joint and secure. Check the crankshaft to ensure free rotation and then tighten end-cover nuts or bolts. Fit the drive key to the crankshaft and fit compressor drive gear.

Refit the piston rings, ensuring that sides marked 'Top' are uppermost, and assemble the piston to the connecting rod. Assemble the connecting rod on the crankshaft, tighten the bolts to a torque of 3.75/4 lbf ft (0,51/0,55 kgf m) -51/54 Nm, and turn up the tabs of the locking strap. Space the piston ring gaps and assemble the cylinder, with joint, over the piston.

Assemble the cylinder head. Lightly smear the outside diameters of the inlet valve seat and spring guide with 'Loctite', or equivalent, sealing compound. Insert the inlet valve seat, inlet valve and valve spring and press the spring guide into position. Screw in the delivery valve seat, using a wrench inserted in the hexagonal hole through the centre of the fitting, and tighten securely. Place the delivery valve on the seat and position the springs. Screw in the valve cap together with the copper washer. Lightly smear the unloader plunger with MS 200 (Midland Silicone) grease, and insert the spring and plunger complete with the spring circlip. Screw in the unloader cap together with copper washer.

Place the joint on the cylinder and correctly position the cylinder head on the studs. Tighten nuts progressively to a torque of 9/10 lbf ft (1,24/1,38 kgf m) -12/13 Nm. Invert the compressor and apply clean engine oil over the crankshaft and on the cylinder wall. Assemble the mounting bracket and joint.

Fault Finding

Compressor Fails to Maintain Adequate Pressure in the System

Dirty air cleaner or filter.Excessive carbon in cylinder head or discharge line.Delivery valve leaking.Excessive wear in compressor.Inlet valve or unloader plunger stuck open.Excessive leakage at inlet valve.

Compressor Passes Excessive Oil

Excessive wear.Dirty air cleaner or filter.Excessive oil pressure.Oil return to engine crankcase obstructed.Back pressure from engine crankcase.Piston rings incorrectly installed.

Noisy Operation

Excessive wear.Excessive carbon in cylinder head.

Compressor does Not Unload

Defective unloader plunger seal.Unloader plunger sticking or binding.Passage in cylinder head obstructed.