Isuzu Cars and Trucks 1981-1991

Piston and Connecting Rod

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REMOVAL AND INSTALLATION

It is not advisable to remove the piston from the connecting rod unless part replacement is necessary. Whenever a piston is removed, the piston pin should be replaced. When examining a piston, look for scuffs, cracking or wear. The rings should be removed with a ring expander and should be kept separately to avoid interchanging parts. All clearances should be checked with a micrometer or comparable precision gauge. Assemble the piston rings to the piston so that the NPR or TOP marks are turned up. Every piston has a mark to designate proper installation, this FRONT MARK is located on the top edge, in line with the piston pin bore. In addition, the cylinder number that the piston came from is stamped on the connecting rod and the bearing cap.

Before removal of connecting rod(s) and cap(s), mark them with their respective cylinder number. This will insure a proper match during reinstallation.

Cylinder Bore Ridge

NOTE
This procedure is easily completed if the engine has been removed from the car. If the oil pan can be removed without removing the engine assembly from the vehicle, the piston/connecting rod can be removed with the engine in the vehicle.

  1. Remove the cylinder head(s), intake manifold, exhaust manifold, oil pan, and oil pump as outlined in this Section.
  2.  
  3. Mount the engine on a stand. In order to facilitate removal of the piston and connecting rod, the ridge at the top of the cylinder (unworn area; see illustration) must be removed. Place the piston at the bottom of the bore, and cover it with a rag. Cut the ridge away using a ridge reamer, exercising extreme care to avoid cutting too deeply. Remove the rag, and remove cuttings that remain on the piston.
  4.  


CAUTION
If the ridge is not removed, and new rings are installed, damage to rings will result.

  1. Remove the connecting rod bearing caps and bearings.
  2.  
  3. Install a section of rubber hose over the connecting rod bolts to prevent damage to the crankshaft.
  4.  
  5. Slide the piston/connecting rod assembly through the top of the cylinder block.
  6.  


CAUTION
Do not attempt to force the piston past the cylinder ridge (see above).



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Fig. Cylinder wear patterns measuring points. The ridge must be removed to remove the pistons

POSITIONING

NOTE
Most pistons are notched or marked to indicate which way they should be installed. If your pistons are not marked, mark them before removal. The pistons should be installed with the aligning mark facing front.



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Fig. Position mark towards the front of the engine



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Fig. Connecting rod and cap positioning both marks go on the same side

CLEANING AND INSPECTING

A piston ring expander is necessary for removing piston rings without damaging them; any other method (screwdriver blades, pliers, etc.) usually results in the rings being bent, scratched or distorted, or the piston itself being damaged. When the rings are removed, clean the ring grooves using an appropriate ring groove cleaning tool, using care not to cut too deeply. Thoroughly clean all carbon and varnish from the piston with solvent.


CAUTION
Do not use a wire brush or caustic solvent (acids, etc.) on piston.

Inspect the pistons for scuffing, scoring, cracks, pitting, or excessive ring groove wear. If these are evident, the piston must be replaced.

The piston should also be checked in relation to the cylinder diameter. Using a telescoping gauge and micrometer, or a dial gauge, measure the cylinder bore diameter perpendicular (90and#x00B0;) to the piston pin, 64mm (2 1 / 2 in.) below the cylinder block deck (surface where the block mates with the heads). Then, with the micrometer, measure the piston perpendicular to its wrist pin on the skirt. The difference between the two measurements is the piston clearance.

If the clearance is within specifications or slightly below (after the cylinders have been bored or honed), finish honing is all that is necessary. If the clearance is excessive, try to obtain a slightly larger piston to bring clearance to within specifications. If this is not possible obtain the first oversize piston and hone (or if necessary, bore) the cylinder to size. Generally, if the cylinder bore is tapered 0.127mm (0.005 in.) or more or is out-of-round 0.076mm (0.003 in.) or more, it is advisable to rebore for the smallest possible oversize piston and rings. After measuring mark pistons with a felt-tip pen for reference and for assembly.


NOTE
Cylinder block boring should be performed by a reputable machine shop with the proper equipment. in some cases, "clean-up" honing can be done with the cylinder block in the car, but most excessive honing and all cylinder boring must be done with the block stripped and removed from the car.



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Fig. Piston ring positioning all 4-cylinder gasoline engines



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Fig. Piston ring positioning V6 engines



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Fig. Piston ring positioning diesel engines

PISTON PIN REMOVAL AND 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. Install the book bushing of the piston pin removing and installing tool.
  4.  


NOTE
The front mark on the piston and the capital letter ISUZU mark on the connecting rod should face the same direction for the four cylinder engines.

  1. 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.
  2.  
  3. When installing the new piston, apply clean engine oil to the pin and press in with a piston pin installing tool. Make sure the connecting rod moves freely without binding after pin is installed. If not, reaming the pin hole may have to be performed.
  4.  



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Fig. Removing the piston pin



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Fig. Installing the piston pin

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 in the "Piston and Connecting Rod" chart in the beginning of this Section. 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.
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  11. If the measurement is not within specifications, the cylinders should be bored and new oversize pistons should be installed.
  12.  



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Fig. Piston skirt measurement



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Fig. Installing piston pin and snapring diesel engines

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 0.254mm (0.010 in.) 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.
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  5. Select a new piston and mark the piston to identify the cylinder for which it was fitted. (On some cars, oversize pistons may be found. These pistons will be 0.254mm (0.010 in.) oversize). After market piston manufactures supply oversized pistons 0.030 in., 0.040 in., and 0.060 in. in most cases.
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  7. After the cylinder has been reconditioned and new pistons purchased, remeasure bore and piston to ensure proper piston fit.
  8.  

CHECKING RING END GAP

Piston ring end gap should be checked while the rings are removed from the pistons. Incorrect end gap indicates that the wrong size rings are being used; ring breakage could occur.

Compress the piston rings to be used in a cylinder, one at a time, into that cylinder. Squirt clean oil into the cylinder, so that the rings and the top 50mm (2 in.) of cylinder wall are coated. Using an inverted piston, press the rings approximately 25mm (1 in.) below the deck of the block. Measure the ring end gap with a feeler gauge, and compare to the "Ring Gap" chart in this Section. Carefully pull the ring out of the cylinder and file the ends squarely with a fine file to obtain the proper clearance.



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Fig. Measuring piston ring gap typical

INSTALLATION AND SIDE CLEARANCE MEASUREMENT

Check the pistons to see that the ring grooves and oil return holes have been properly cleaned. Slide a piston ring into its groove and check the side clearance with a feeler gauge. Make sure the feeler gauge is inserted between the ring and its lower land (lower edge of the groove), because any wear that occurs forms a step at the inner portion of the lower land. If the piston grooves have worn to the extent that relatively high steps exist on the lower land, the piston should be replaced, because these will interfere with the operation of the new rings and ring clearances will be excessive. Piston rings are not furnished in oversize widths to compensate for ring groove wear.

Install the rings on the piston, lowest ring first (oil ring), using a piston ring expander for the compression rings. There is a high risk of breaking or distorting the rings, or scratching the piston, if the compression rings are installed by hand or other means.

Position the rings on the piston as illustrated; spacing of the various piston ring gaps is crucial to proper oil retention and even cylinder wear. When installing new rings, refer to the installation diagram furnished with the new parts.



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Fig. Measuring piston ring side clearance



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Fig. Installing a piston using a piston ring compressor

 
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