Pistons, Piston Pins and Piston Rings

Fitting Pistons

Pistons (6108) are available for service in standard size and oversize shown in the Section 03-01A (3.8L) or Section 03-01B (5.0L) under Specifications.

The standard-size pistons are color-coded red, blue or yellow on the dome.

Measure the cylinder bore and select the piston to ensure the proper clearance. When the bore diameter is in the lower one-third of the specified range, a red piston should be used. When the bore diameter is in the middle one-third, a blue piston should be used. When the bore diameter is in the upper one-third, a yellow piston should be used.

NOTE: Cylinder bore must be clean and dry, and cylinder block (6010) must remain at room temperature (21° C/70°F) for eight hours before taking cylinder measurements.

Measure the piston diameter to ensure the specified clearance is obtained. It may be necessary periodically to use another piston (within the same grade size) that is either slightly larger or smaller to achieve the specified clearance.

If none can be fitted, refinish the cylinder to provide the proper clearance for the piston .

When a piston has been fitted, mark it for assembly in the cylinder to which it was fitted.

If the taper, out-of-round and piston-to-cylinder bore clearance conditions of the cylinder bore are within specified limits, new piston rings will give satisfactory service.

If new piston rings are to be installed in a used cylinder that has not been refinished, remove the cylinder wall glaze using only spring-loaded Engine Cylinder Hone Set T73L-6011-A, and only if there is no visible sign of cross-hatch markings on the cylinder walls. (Refer to Cylinder Block,Cylinder Walls, Refinishing.) Always clean the cylinder bore thoroughly with detergent and water solution.

NOTE: After any refinishing operation, allow cylinder bore to cool, and ensure piston and bore are clean and dry before piston fit is checked.

1. Calculate size of piston to be used by taking a cylinder bore check. Follow procedures outlined previously.

2. Select proper size piston to provide desired clearance. Measure piston diameter in-line with centerline of piston pin (6135) and at 90 degrees to piston pin axis.

3. Ensure piston and cylinder block are at room temperature, 21°C (70°F).

   Measure the piston diameter to ensure the specified clearance is obtained. It may be necessary periodically to use another piston (within the same grade size) that is either slightly larger or smaller to achieve the specified clearance.

Fitting Piston Rings

NOTE: Always use a piston ring expanding tool to install piston rings on a piston .

1. Select proper piston ring set for size of cylinder bore.

2. Position piston ring in cylinder bore in which it is going to be used.

3. Push piston ring down into bore area where normal piston ring wear is not encountered.

    CAUTION: Use care to avoid damage to piston ring or cylinder bore.

4. Position piston ring in bore so piston ring is square with cylinder wall.

5. Measure gap between ends of piston ring with a feeler gauge. If piston ring gap is less than or greater than specified limits, try another piston ring set. For Specifications, refer to Section 03-01A (3.8L) or Section 03-01B (5.0L).
   
   
   
   

   NOTE: If lower lands have high steps, piston should be replaced.

6. Check piston ring side clearance of compression piston rings with a feeler gauge inserted between piston ring and its lower land. Gauge should slide freely around entire ring circumference without binding. Any wear that occurs will form a step at inner portion of lower land.

7. Piston rings should be staggered on the piston to ensure the piston ring end gaps are not aligned.
   
   
   
   

Cleaning

 CAUTION: Do not use a caustic cleaning solution or a wire brush to clean pistons.

Remove deposits from the piston surfaces. Clean gum or varnish from the piston skirt, piston pins , and piston rings with solvent.

Clean the piston ring grooves with Piston Ring Groove Cleaner D81L-6002-D or equivalent. Ensure oil ring slots (or holes) are clean.

Inspection

Carefully inspect the pistons for fractures at the piston ring lands, skirts, oil ring slot corners, and pin bosses, and for scuffed, rough or scored skirts. If the lower inner portion of the ring grooves have a high step, replace the piston . The step will interfere with ring operation and cause excessive ring side clearance.

Spongy, eroded areas near the edge of the top of the piston are usually caused by detonation or pre-ignition. A shiny surface on the thrust surface of the piston , offset from the centerline between the piston pin holes, can be caused by a bent connecting rod (6200) . Replace pistons that show signs of excessive wear, wavy ring lands or fractures or damage from detonation or pre-ignition.

Check the piston-to-cylinder bore clearance by measuring the piston and cylinder bore diameters. Refer toSpecifications in the appropriate engine section for the proper clearance. Refer to Cylinder Block Inspection for the bore measurements procedure. Measure the OD of the piston and check the piston ring side clearance following the procedure under Fitting Pistons, Piston Pins and Piston Rings.

Replace piston pins showing signs of fracture, etching, scoring or wear. Check the piston pin fit in the piston and connecting rod .

Check the OD of the piston pin and the ID of the piston pin bore in the piston . Replace any piston pin or piston that is not within specifications. For Specifications, refer to Section 03-01A (3.8L) or Section 03-01B (5.0L).

 CAUTION: Piston rings should not be transferred from one piston to another, regardless of mileage.

Replace all piston rings that are scored, broken, chipped or cracked. Check the end gap and side clearance.