Engine - Single Size Pistons
88pontiac07Bulletin Number: 88-6-31A
Reference Number: 836107R
Publish Date: 5/88
Subject: "SINGLE SIZE" PISTONS
Models
Affected: 1988 GRAND AM WITH 2.5L L4 ENGINE (VIN CODE U)
"This bulletin supersedes bulletin 88-6-31, dated 3/88. The part numbers have been changed and additional information regarding the part numbers added."
This bulletin addresses the recent piston size change made on the 1988 2.5L (VIN U) engine only. Improved process control has reduced the piston-to-bore clearance variation to the point were pistons of one size may be used. Serviceability has also been improved by reducing the number of replacement piston sizes.
Currently produced engines are being assembled with "single size" pistons which are stamped with the letter "C".
The VIN breakpoint corresponding to the change to "single size" pistons is JC718522.
The "C" size piston, cylinder bore diameter, and clearance
specifications are as follows:
PISTON SIZE: 3.9985 +/- .0005 Inch (101.5619 +/- .0127 mm)
BORE SIZE: 4.0010 +/- .0005 Inch (101.6254 +/- .0127 mm)
PISTON-TO-BORE
CLEARANCE : .0015 - .0035 Inch (.0381 - .0889 mm)
Replacement piston and pin assemblies are available as follows:
STANDARD: P/N 10101486
.005 Inch
OVERSIZE : P/N 10044248 *
*P/N 10101488 will supersede P/N 10044248 at some future date.
Previously, the VIN Code U engines were assembled with a range of six possible different piston sizes stamped with letters "L", "M", "P", "S", "T" and "U". Each piston size was different by .0004 inch (.0010 mm).
The "L" size had the smallest diameter - 3.9982 inch (101.555 mm).
The "U" size had the largest diameter - 4.0006 inch (101.615 mm).
Prior to the piston size change, the piston-to-bore clearance specification was .0014 - .0022 inch (.0355 - .0558 mm).
Please be aware of the preceding information when servicing 1988 Grand Am vehicles with 2.5L engines.