Jeep CJ/Scrambler 1971-1986 Repair Guide

Pistons and Connecting Rods

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REMOVAL



See Figures 1 through 13

In most cases, this procedure is easier with the engine out of the vehicle.

  1. Remove the head(s).
  2.  
  3. Remove the oil pan.
  4.  
  5. Rotate the engine to bring each piston, in turn, to the bottom of its stroke. With the piston bottomed, use a ridge reamer to remove the ridge at the top of the cylinder. DO NOT CUT TOO DEEPLY!
  6.  
  7. Matchmark the rods and caps. If the pistons are to be removed from the connecting rod, mark the cylinder number on the piston with a silver pencil or quick drying paint for proper cylinder identification and cap-to-rod location. Remove the connecting rod capnuts and lift off the rod caps, keeping them in order. Install a guide hose over the threads of the rod bolts. This is to prevent damage to the bearing journal and rod bolt threads.
  8.  
  9. Using a hammer handle, push the piston and rod assemblies up out of the block.
  10.  

PISTON PIN REMOVAL & INSTALLATION



Use care at all times when handling and servicing connecting rods and pistons. To prevent possible damage to these units, do not clamp the rod or piston in a vise since they may become distorted. Do not allow the pistons to strike against one another, against hard objects or bench surfaces, since distortion of the piston contour or nicks in the soft aluminum material may result.

  1. Remove the piston rings using a suitable piston ring remover.
  2.  
  3. Remove the piston pin lockring, if used. Install the guide bushing of the piston pin removing and installing tool.
  4.  
  5. Install the piston and connecting rod assembly on a support, and place the assembly in an arbor press. Press the pin out of the connecting rod, using the appropriate piston pin tool.
  6.  
  7. Assembly is the reverse of disassembly. Use new lockrings where needed.
  8.  



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Fig. Fig. 1: Checking for a cylinder bore ridge



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Fig. Fig. 2: Number each rod and cap to keep them in order



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Fig. Fig. 3: Installing the piston pin locking rings



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Fig. Fig. 4: Piston pins must be pressed in with an arbor press



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Fig. Fig. 5: Bottom of the engine view, with the oil pan removed



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Fig. Fig. 6: Install short pieces of rubber hose on the connecting rod bolts, to protect the cylinder bore



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Fig. Fig. 7: Use a wooden dowel or hammer handle and gently tap the piston out of the block



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Fig. Fig. 8: Removing a main bearing cap, with the engine still in the vehicle



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Fig. Fig. 9: Removing a connecting rod bearing cap, with the engine still in the vehicle



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Fig. Fig. 10: Use a short blunt punch to tap the connecting free of the crankshaft journal



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Fig. Fig. 11: This connecting rod journal has a thin piece of plastic gauging material on it



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Fig. Fig. 12: When installing the connecting rod bearing, align the tang on the bearing with this groove



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Fig. Fig. 13: Use the paper gauge that is supplied with the gauging material, to check the connecting rod bearing clearance

INSPECTION



See Figures 14 through 24

Cylinder Block

Check the cylinder walls for evidence of rust, which would indicate a cracked block. Check the block face for distortion with a straightedge. Maximum distortion variance is 0.005 in. (0.13mm). The block cannot be planed, so it will have to be replaced if too distorted. Using a micrometer, check the cylinders for out-of-roundness.

Connecting Rods and Bearings

Wash connecting rods in cleaning solvent and dry with compressed air. Check for twisted or bent rods and inspect for nicks or cracks. Replace connecting rods that are damaged.

Inspect journals for roughness and wear. Slight roughness may be removed with a fine grit polishing cloth saturated with engine oil. Burrs may be removed with a fine oil stone by moving the stone on the journal circumference. Do not move the stone back and forth across the journal. If the journals are scored or ridged, the crankshaft must be replaced.

The connecting rod journals should be checked for out-of-round and correct size with a micrometer.

Crankshaft rod journals will normally be standard size. If any undersized bearings are used, the size will be stamped on a counterweight.

If plastic gauging material is to be used:
  1. Clean oil from the journal bearing cap, connecting rod and outer and inner surfaces of the bearing inserts. Position the insert so that the tang is properly aligned with the notch in the rod and cap.
  2.  
  3. Place a piece of plastic gauging material in the center of lower bearing shell.
  4.  
  5. Remove the bearing cap and determine the bearing clearances by comparing the width of the flattened plastic gauging material at its widest point with the graduation on the container. The number within the graduation on the envelope indicates the clearance in thousandths of an inch or millimeters. If this clearance is excessive, replace the bearing and recheck the clearance with the plastic gauging material. Lubricate the bearing with engine oil before installation. Repeat the procedure on the remaining connecting rod bearings. All rods must be connected to their journals when rotating the crankshaft, to prevent engine damage.
  6.  

Pistons

Clean varnish from piston skirts and pins with a cleaning solvent. DO NOT WIRE BRUSH ANY PART OF THE PISTON. Clean the ring grooves with a groove cleaner and make sure oil ring holes and slots are clean.

Inspect the piston for cracked ring lands, skirts or pin bosses, wavy or worn ring lands, scuffed or damaged skirts, eroded areas at the top of the piston. Replace pistons that are damaged or show signs of excessive wear. Inspect the grooves for nicks or burrs that might cause the rings to hang up.

Measure piston skirt (across center line of piston pin) and check piston clearance.

MEASURING THE OLD PISTONS

Check used piston-to-cylinder bore clearance as follows:

  1. Measure the cylinder bore diameter with a telescope gauge.
  2.  
  3. Measure the piston diameter. When measuring the pistons for size or taper, measurements must be made with the piston pin removed.
  4.  
  5. Subtract the piston diameter from the cylinder bore diameter to determine piston-to-bore clearance.
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  7. Compare the piston-to-bore clearances obtained with those clearances recommended. Determine if the piston-to-bore clearance is in the acceptable range.
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  9. When measuring taper, the largest reading must be at the bottom of the skirt.
  10.  

SELECTING NEW PISTONS
  1. If the used piston is not acceptable, check the service piston size and determine if a new piston can be selected. (Service pistons are available in standard, high limit and standard oversize.
  2.  
  3. If the cylinder bore must be reconditioned, measure the new piston diameter, then hone the cylinder bore to obtain the preferred clearance.
  4.  
  5. Select a new piston and mark the piston to identify the cylinder for which it was fitted. On some vehicles, oversize pistons may be found. These pistons will be 0.010 in. oversize.
  6.  



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Fig. Fig. 14: Checking the piston ring end gap



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Fig. Fig. 15: Checking the piston ring side clearance, be sure the ring groove is clean



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Fig. Fig. 16: Check the connecting rod length along its side



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Fig. Fig. 17: Check the connecting rod side clearance with a feeler gauge and the connecting rod installed



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Fig. Fig. 18: Be sure you have proper ring gap spacing when installing the rings



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Fig. Fig. 19: 6-225 left side piston assembly, note the markings for installation



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Fig. Fig. 20: 6-225 right side piston assembly, note the markings for installation



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Fig. Fig. 21: Connecting rod numbering on the 6-232 and 6-258 engines



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Fig. Fig. 22: Connecting rod numbering on the 4-151 engine



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Fig. Fig. 23: On the 4-151 engine, the arrow on the piston faces forward



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Fig. Fig. 24: Measuring the piston diameter

CYLINDER HONING



See Figures 25, 26 and 27

  1. When cylinders are being honed, follow the manufacturer's recommendations for the use of the hone.
  2.  
  3. Occasionally, during the honing operation, the cylinder bore should be thoroughly cleaned and the selected piston checked for correct fit.
  4.  
  5. When finish-honing a cylinder bore, the hone should be moved up and down at a sufficient speed to obtain a very fine uniform surface finish in a cross-hatch pattern of approximately 45-65° included angle. The finish marks should be clean but not sharp, free from imbedded particles and torn or folded metal.
  6.  
  7. Permanently mark the piston for the cylinder to which it has been fitted and proceed to hone the remaining cylinders.
  8.  

Handle the pistons with care. Do not attempt to force the pistons through the cylinders until the cylinders have been honed to the correct size. Pistons can be distorted through careless handling.

  1. Thoroughly clean the bores with hot water and detergent. Scrub well with a stiff bristle brush and rinse thoroughly with hot water. It is extremely essential that a good cleaning operation be performed. If any of the abrasive material is allowed to remain in the cylinder bores, it will rapidly wear the new rings and cylinder bores. The bores should be swabbed several times with light engine oil and a clean cloth and then wiped with a clean dry cloth. CYLINDERS SHOULD NOT BE CLEANED WITH KEROSENE OR GASOLINE. Clean the remainder of the cylinder block to remove the excess material spread during the honing operation.
  2.  



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Fig. Fig. 25: Use a ridge cutting tool to remove the ridge from a cylinder bore



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Fig. Fig. 26: Use a ball type hone and a power drill, to lightly hone a cylinder bore



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Fig. Fig. 27: A properly honed cylinder will have a light cross-hatched appearance

CHECKING CYLINDER BORE



See Figure 28

Cylinder bore size can be measured with inside micrometers or a cylinder gauge. The most wear will occur at the top of the ring travel.

Reconditioned cylinder bores should be held to not more than 0.001 in. taper.

If the cylinder bores are smooth, the cylinder walls should not be deglazed. If the cylinder walls are scored, the walls may have to be honed before installing new rings. It is important that reconditioned cylinder bores be thoroughly washed with a soap and water solution to remove all traces of abrasive material to eliminate premature wear.



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Fig. Fig. 28: Checking the cylinder bore with a cylinder gauge

RING TOLERANCES



See Figures 29 and 30

When installing new rings, ring gap and side clearance should be checked as follows:

Piston Ring and Rail Gap

Each ring and rail gap must be measured with the ring or rail positioned squarely and at the bottom of the ring travel area of the bore.

Side Clearance

Each ring must be checked for side clearance in its respective piston groove by inserting a feeler gauge between the ring and its upper land. The piston grooves must be cleaned before checking the ring for side clearance specifications. To check oil ring side clearance, the oil rings must be installed on the piston.



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Fig. Fig. 29: Cleaning the ring groove with a cleaning tool designed for this purpose



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Fig. Fig. 30: You can use a piece of an old ring to clean the groove. Be careful using this method, the ring edge is sharp

RING INSTALLATION



See Figures 31, 32 and 33

For service ring specifications and detailed installation productions, refer to the instructions furnished with the parts package.



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Fig. Fig. 31: You can install the rings using an installation tool



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Fig. Fig. 32: Most rings will have markings showing which side is up



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Fig. Fig. 33: Use a ring compressor and a hammer handle to install the piston assembly into the block

PISTON ASSEMBLY AND INSTALLATION



  1. Using a ring expander, install new rings in the grooves, with their gaps staggered to be 270° apart.
  2.  
  3. Using a straightedge, check the rods for straightness. Check, also, for cracks. Before assembling the block, it's a good idea to have the block checked for cracks with Magnaflux® or its equivalent.
  4.  
  5. Install the pins and retainers.
  6.  
  7. Coat the pistons with clean engine oil and apply a ring compressor. Position the assembly over the cylinder bore and slide the piston into the cylinder slowly, taking care to avoid nicking the walls. The pistons will have a mark on the crown, such as a groove or notch or stamped symbol. This mark indicates the side of the piston which should face front. Lower the piston slowly, until it bottoms on the crankshaft. A good idea is to cover the rod studs with length of rubber hose to avoid nicking the crank journals. Assemble the rod caps at this time. Check the rod bearing clearances using Plastigage®, going by the instructions on the package.
  8.  
  9. Install the bearing caps with the stamped numbers matched. Torque the caps to the figure shown in the Torque Specifications Chart. See the accompanying illustrations for proper piston and rod installation.
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