The federal government said yesterday it has made an "initial determination" that nearly one million jacks that come as original equipment on certain 1975 and 1976 GMC and Chevrolet light-duty trucks are defective, and can fall apart with no prior warning.

Because of that finding, the National Highway Traffic Safety Administration has scheduled a public hearing here on Oct. 4 to allow General Motors Corp. to present data and arguments as to why the jacks shouldn't be recalled.

In a letter to GM, the NHTSA said the jacks "when used under roadside conditions . . . are subject to failures, which can result in injuries and property damage."

The NHTSA said it has received 5 reports of failures from consumers, including 3 cases where the jack dropped the vehicles they were supporting. GM has received 57 other reports of similar failures, "resulting in 4 injuries," the agency noted.

Involved in the investigation are Chevrolet C-10, P-10 and G-20, and GMC C-15, P-15 and G-25 light-duty trucks. "Approximately 954,000 of these vehicles were equipped with the jacks in question, which General Motors designates as model 344788," the NHTSA said.

The jack is a screw-type design, "but has the appearance of a hydraulic jack," the NHTSA said.It is positioned under the axle of the truck and used to raise the chassis in order to change tires.

The agency said it tested 27 of the jacks under roadside conditions. Two of the jacks dropped the vehicles they were supporting, and 12 others were found to be severly deformed or cracked.

In addition, the NHTSA said that the number of warranty claims filed for failure of the jacks was "excessive," considering the jack is not a regularly used part.

The NHTSA also warned owners of the jacks not to use them because there are "no known pre-failure symptoms."

If a motorist must use the jack to change a tire, NHTSA recommends the following precautions:

Select a flat, even surface.

Lighten the vehicle load.

Stop raising the vehicle as soon as the jack begins to bend or deform.

Place sturdy wooden blocks or other support under the axle after the jack is extended.

Never get under the vehicle while it is being supported by the jack.

Individuals seeking to participate in the hearing should contact Joanne Murianka at 202-426-2850 before Sept. 28.