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Fitting Pistons

  1. Clean carbon deposits off piston crown and from ring grooves. Pistons for 1986 model year and later have 4 equal sized valve reliefs to allow interchangeability. DO NOT  damage metal surfaces during cleaning. Clean gasket material and foreign matter from crankcase lower section mating surfaces and cylinder head gasket surfaces of block.
    NOTE: An uneven Blue mark or one-sided traces of deposit on piston body (at right angle to piston axis) may be caused by bent connecting rod. DO NOT  reuse piston (or rod) with this condition.
  2. Pistons and pins are paired according to weight. Mark pistons and pins to ensure parts are not interchanged. If parts are mixed, rearrange by checking total weight of piston and pin assembly.
  3. Piston total weight is measured with pin, rings and pin retainers. Piston weight is 25.1-25.4 oz. (718-726 g). An oversize piston has same weight as standard piston.
  4. When checking pistons for excessive wear, measure diameter about 2.4" (61 mm) down from top of piston crown, at 90 degree offset from piston pin bore. Piston repair sizes are standard, first oversize and second oversize, with 3 tolerance groups for each size.
  5. Tolerance group stamped on piston crown is either "0", "1" or "2". Tolerance group stampings indicate if piston is standard, 0.0004" (.010 mm) or .0008" (.020 mm) oversize of its particular repair size. See PISTON OVERSIZE & TOLERANCE GROUP CODE  table.
    NOTE: Cylinder location and tolerance group are stamped on 2 raised bosses on cylinder block, at head gasket surface, in line with each bore centerline. Tolerance group mark is closest to bore. Only match pistons and cylinders having same tolerance group. Different tolerance groups may be used in one engine.
    PISTON OVERSIZE & TOLERANCE GROUP CODE

    Piston Repair Size/Group Code Piston Diameter In. (mm) Cylinder Bore Diameter In. (mm)
    Mahle Standard
    "0" 3.7386 (94.960) 3.7399-3.7404 (94.995-95.005)
    "1" 3.7389 (94.970) 3.7404-3.7407 (95.005-95.015)
    "2" 3.7394 (94.980) 3.7407-3.7411 (95.015-95.025)
    1st Oversize
    "0KD1" 3.7583 (95.460) 3.7596-3.7600 (95.495-95.505)
    "1KD1" 3.7587 (95.470) 3.7600-3.7604 (95.505-95.515)
    "2KD1" 3.7591 (95.480) 3.7604-3.7608 (95.515-95.525)
    2nd Oversize
    "0KD2" 3.7779 (95.960) 3.7793-3.7797 (95.995-95.005)
    "1KD2" 3.7783 (95.970) 3.7797-3.7801 (96.005-96.015)
    "2KD2" 3.7787 (95.980) 3.7801-3.7805 (96.015-96.025)
    KS Standard
    "0" 3.7385-3.7390 (94.957-94.971) 3.7399-3.7404 (94.995-95.005)
    "1" 3.7389-3.7394 (94.967-94.981) 3.7404-3.7407 (95.005-95.015)
    "2" 3.7392-3.7398 (94.977-94.991) 3.7407-3.7411 (95.015-95.025)
    1st Oversize
    "0KD1" 3.7581-3.7587 (95.457-95.471) 3.7596-3.7600 (95.495-95.505)
    "1KD1" 3.7585-3.7591 (95.467-95.481) 3.7600-3.7604 (95.505-95.515)
    "2KD1" 3.7589-3.7595 (95.477-95.491) 3.7604-3.7608 (95.515-95.525)
    2nd Oversize
    "0KD2" 3.7778-3.7784 (95.957-95.971) 3.7793-3.7797 (95.995-96.005)
    "1KD2" 3.7782-3.7787 (95.967-95.981) 3.7797-3.7801 (96.005-96.015)
    "2KD2" 3.7786-3.7792 (95.977-95.991) 3.7801-3.7805 (96.015-96.025)
  6. When measuring cylinder bore and out-of-round tolerances, ensure crankcase lower section is installed and tighten to specification. Using dial bore gauge, measure bore about 2.4" (61 mm) from upper edge of cylinder block, 90 degrees to piston pin bore.
  7. Standard out-of-round tolerance is .0004" (.010 mm) and maximum tolerance is 0.0008" (.020 mm). Standard piston-to-cylinder bore clearance is 0.0009-.0019" (.024-.048 mm) and maximum clearance is 0.0032" (.080 mm).
  8. If piston-to-cylinder clearance or out-of-round tolerance is beyond specification, cylinders may be bored to next oversize. Cylinders may be bored separately since oversized pistons are the same weight as standard pistons.
    NOTE: Due to cylinder block composition (aluminum alloy) and bore finish (silicone), vehicle manufacturer recommends using only Sunnen brand Automatic Cylinder Honing Machine (CK-10) with Honing Head (CK-3000) and special Honing Stone Set (EHU-123, C30-J84 or JHU-820 and C30-Co3-81) which includes Silicone Paste (AN-30).
    CAUTION: Before honing procedure, ensure upper and lower cylinder block sections are bolted together to specified torque or cylinder bore distortion will result.
  9. When honing cylinders to size, ensure initial honing procedure brings cylinder bore size to within .004" (.10 mm) of finished size. Second (final) honing procedure should bring cylinder to within .0008" (.020 mm).
  10. After final honing procedure, polish bore to final size, and treat with Sunnen silicone mixture according to manufacturers instructions. After honing and polishing procedures are complete, ensure upper cylinder edge is cut to a 30 degree chamfer that is .02" (.5 mm) wide.
  11. Before assembly, all upper and lower block assemblies must be thoroughly cleaned of all emery particles and silicone paste.
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