SINGLE SIZE PISTONS USED IN PRODUCTION

MODELS AFFECTED: 1988 SKYLARKS WITH 2.5L L4 ENGINES (VIN CODE U)

NOTICE: This bulletin supersedes 88-6-13 dated April, 1988. The replacement piston size and part numbers have been changed. Please discard 88-6-13 and replace with 88-6-13A.

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 breakpoints corresponding to the change to "single size" pistons is as follows:

Buick Skylark - JMO29410

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: Part Number 10101486 .005 Inch OVERSIZE: Part Number 10044248**

** Part Number 10101488 will supersede Part Number 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 Skylarks with 2.5L engines.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.