Mazda Trucks 1972-1986 Repair Guide

Pistons and Connecting Rods

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REMOVAL



See Figures 1, 2, 3, 4, 5, 6, 7 and 8

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Fig. Fig. 1: Example of the cylinder bore ridge



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Fig. Fig. 2: Using a ridge reamer to remove the cylinder bore ridge will ease removal and installation of the piston and rod assemblies



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Fig. Fig. 3: Place rubber hose over the connecting rod studs to protect the crank and bores from damage



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Fig. Fig. 4: Carefully tap the piston out of the bore using a wooden dowel



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Fig. Fig. 5: Matchmarking the connecting rods to their caps using a scribe



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Fig. Fig. 6: Matchmarking the connecting rods to their caps using a number stampyou can also use a center punch and mark the appropriate number of dots on each rod and rod cap



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Fig. Fig. 7: Always protect the crankshaft journals from scratches and dingscover the connecting rod bolts with lengths of rubber hose or commercially available rod bolt covers



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Fig. Fig. 8: Carefully push the piston out with a hammer handle or wooden dowelnever push on the soft bearing that sits in the center of the rod, as this will render it unusable

  1. Remove the engine assembly from the truck, see Engine Removal and Installation.
  2.  
  3. Remove the intake manifold and cylinder head.
  4.  
  5. Remove the oil pan.
  6.  
  7. Remove the oil pump assembly.
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  9. Stamp the cylinder number on the machine surfaces of the bolt bosses of the connecting rod and cap for identification when reinstalling. 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.
  10.  
  11. Examine the cylinder bore above the ring travel. If a ridge exists, remove the ridge with a ridge reamer before attempting to remove the piston and rod assembly. Never cut into the ring travel area in excess of 0.8mm (0.0315 in.) when removing the ridges.
  12.  
  13. Remove the rod bearing cap and bearing.
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  15. Install a guide hose over threads of rod bolts. This is to prevent damage to bearing journal and rod bolt threads.
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  17. Remove the rod and piston assembly through the top of the cylinder bore.
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  19. Remove the other rod and piston assemblies in the same manner.
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PISTON PIN REPLACEMENT



See Figures 9 and 10

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Fig. Fig. 9: Use a ring expander tool to remove the piston rings



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Fig. Fig. 10: Use needlenose or snapring pliers to remove the piston pin snaprings

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 Then, on all engines, press the piston pin out of the piston with tools designed for this purpose.
  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.
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  7. Assembly is the reverse of disassembly. Use new lockrings where needed.
  8.  

CLEANING AND INSPECTION Connecting Rods



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.

Pistons See Figures 11, 12 and 13



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Fig. Fig. 11: Clean the piston grooves using a ring groove cleaner



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Fig. Fig. 12: You can use a piece of an old ring to clean the piston grooves, BUT be careful, the ring is sharp



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Fig. Fig. 13: Measure the piston's outer diameter using a micrometer

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



See Figures 14 and 15

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Fig. Fig. 14: Cylinder bore measuring points



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Fig. Fig. 15: Measuring the cylinder bore with a dial gauge

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.
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  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, and oversizes of 0.25mm and 0.50mm.
  2.  
  3. If the cylinder bore must be reconditioned, measure the new piston diameter, then hone the cylinder bore to obtain the preferred clearance.
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  5. Select a new piston and mark the piston to identify the cylinder for which it was fitted.
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CYLINDER HONING



See Figures 16, 17, 18 and 19

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Fig. Fig. 16: Proper cylinder for cross-hatching after honing



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Fig. Fig. 17: Using a ball type cylinder hone is an easy way to hone the cylinder bore



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Fig. Fig. 18: A solid hone can also be used to cross-hatch the cylinder bore



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Fig. Fig. 19: As with a ball hone, work the hone carefully up and down the bore to achieve the desired results

  1. When cylinders are being honed, follow the manufacturer's recommendations for the use of the hone.
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  3. Occasionally during the honing operation, the cylinder bore should be thoroughly cleaned and the selected piston checked for correct fit.
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  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 45-65º included angle. The finish marks should be clean but not sharp, free from imbedded particles and torn or folded metal.
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  7. Permanently mark the piston for the cylinder to which it has been fitted and proceed to hone the remaining cylinders.
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Handle pistons with care. Do not attempt to force pistons through cylinders until the cylinders have been honed to 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.
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CHECKING CYLINDER BORE



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 0.025mm (0.00098 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.

PISTON RING REPLACEMENT



See Figures 20, 21, 22, 23 and 24

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Fig. Fig. 20: A telescoping gauge may be used to measure the cylinder bore diameter



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Fig. Fig. 21: Most rings are marked to show which side should face upward



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Fig. Fig. 22: Check piston ring end-gap with a feeler blade



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Fig. Fig. 23: Compression ring installation



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Fig. Fig. 24: Oil ring installation

The pistons have three rings (two compression rings and one oil ring). The oil ring consists of two rails and an expander.

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

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.

ROD BEARING REPLACEMENT



See Figures 25, 26, 27, 28, 29, 30 and 31

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Fig. Fig. 25: Checking the ring-to-ring groove clearance



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Fig. Fig. 26: The notch on the the side of the bearing cap matches the groove on the bearing insert



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Fig. Fig. 27: Inspect the rod bearings for scuffing or other wearalso check the crankshaft journal



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Fig. Fig. 28: Measure the connecting rod length at these points



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Fig. Fig. 29: Measuring the connecting rod bearing oil clearance with a strip of Plastigage material



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Fig. Fig. 30: Apply a strip of gauging material to the bearing journal, then install and torque the cap



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Fig. Fig. 31: After the cap is removed again, use the scale supplied with the gauge material to check clearances

The connecting rod bearings are designed to have a slight projection above the rod and cap faces to insure a positive contact. The bearings can be replaced without removing the rod and piston assemblies from the engine.

If you have already removed the connecting rod and piston assemblies from the engine, follow only Steps 3-7 of the following procedure.

  1. Remove the oil pan. See the Oil Pan procedures, earlier in this section.
  2.  
  3. With the connecting rod journal at the bottom, stamp the cylinder number on the machined surfaces of the connecting rod and cap for identification when installing, then remove the caps.
  4.  
  5. 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.
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  7. The connecting rod journals should be checked for out-of-round and correct size with a micrometer.
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Crankshaft rod journals will normally be standard size. If any undersized bearings are used, all will be 0.25mm undersize and 0.25mm will be stamped on the number 4 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.
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  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. Undersized bearings are available in sizes of 0.25mm, 0.50mm and 0.75mm. Lubricate the bearing with engine oil before installation. Repeat Steps 2-7 on the remaining connecting rod bearings. All rods must be connected to their journals when rotating the crankshaft, to prevent engine damage.
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INSTALLATION



See Figures 32, 33, 34 and 35

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Fig. Fig. 32: Typical piston and connecting rod assembly



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Fig. Fig. 33: Using a wooden hammer handle, tap the piston down through the ring compressor and into the cylinder bore



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Fig. Fig. 34: Most pistons are marked to indicate positioning in the engine (usually a mark means the side facing front)



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Fig. Fig. 35: Installing the piston into the block using a ring compressor and the handle of a hammer

  1. Install some lengths of rubber tubing over the connecting rod bolts to prevent damage to the journals.
  2.  
  3. Apply engine oil to the rings and piston, then install a piston ring compressing tool on the piston.
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  5. Install the assembly in its respective cylinder bore.
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  7. Lubricate the crankshaft journal with engine oil and install the connecting rod bearing and cap, with the bearing index tang in rod and cap on same side.
  8.  

When more than one rod and piston assembly is being installed, the connecting rod cap attaching nuts should be tightened only enough to keep each rod in position until all have been installed. This will aid installation of the remaining piston assemblies.

  1. Torque the rod bolt nuts to specification. Using a feeler gauge and small prybar, check connecting rod side clearance.
  2.  
  3. Install all other parts in reverse order of removal.
  4.  
  5. Install the engine in the truck. See Engine Removal and Installation.
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