SERVICE MANUAL UPDATE NEW PISTON DESIGN

Model and Year: 1990 B, F, Y CARS

The new piston is a low-friction, lightweight design for improved fuel economy. It is not interchangeable with the 1989 (or previous) piston. This piston is a flat top design that gives a compression ratio of 10.25: 1 in the 5.7L PFI (VIN 8) for the Y-car ONLY and 9.75: 1 in the 5.7L TBI (VIN 7) and PFI (VIN 8) for the F-car ONLY. Piston weight is 540 grams. The ring pack is also a new design and not interchangeable with the 1989 (or previous) piston.

The top ring is 2 mm thick but has a narrower radial width and shallower ring groove depth than 1989 (or previous).

The second ring is 1.5 mm thick and also has a narrower radial width and shallower ring groove depth than 1989 (or previous).

The oil ring assembly width is 4 mm, also new for 1990.

This piston is fit to the same clearance as the 1989 piston however, the new shape of the skirt requires the piston to be size measured differently. It is measured at a point 11.5 mm (.45") above the bottom of the skirt. The skirt has a "barrel-shaped" profile so the piston skirt taper **cannot** be measured.

Piston orientation markings and size measurement points are illustrated below:

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.