WASHINGTON (AP) _ The Navy has temporarily suspended its acceptance of new F-14 Tomcat jet fighters following the discovery in March of a small crack in a fuselage bulkhead inside a plane that was already in service.

Both the Navy and the Grumman Aerospace Corp., which manufactures the plane, stressed Friday that the discovery of the crack did not pose any safety problems and that the Navy's fleet of 425 F-14s had not been grounded.

''We want to stress that there is no safety problem here, because we know what to look for and how to remedy the problem,'' said Joseph Vranich, a Grumman spokesman.

The order ''discontinuing acceptance'' of new planes is fairly limited in its effect. Grumman's current production schedule calls for delivering two planes per month.

The Navy said the crack was discovered during routine maintenance on an F- 14 at the Miramar Naval Air Station in California, on March 15. The service said it immediately ordered its entire fleet of F-14s inspected.

Those inspections found three other planes with similar cracks in the bulkhead, the Navy said, and those four planes have been grounded. In addition, the inspections determined that 238 of the planes - manufactured before certain design changes were made in the mid-1970s - did not run any risk of bulkhead cracking.

The bulkhead is a structural member that runs laterally down the length of the fuselage's interior to give it strength. The Navy said Grumman has already designed and started manufacturing a new type of structural fitting that will be added temporarily to the exterior of existing planes to strengthen the affected bulkhead.

That process should be completed by early June, the Navy continued. Starting in mid-May, the Navy said teams composed of Grumman and Navy workers will gradually begin replacing the temporary external fittings with newly designed interior fittings to permanently fix the problem.

That process will likely take four to eight months, the Navy said.

Vranich said the company has already completed most of the design work on the permanent fitting, which will also be incorporated into the construction of new planes.

The spokesman said he could not estimate how much it would cost to strengthen the bulkheads - or whether the company or the Navy will pay for the fix - until the precise cause of the cracks was determined.

The Navy's decision to delay further acceptance of the planes has so far affected only one aircraft, Vranich added.

The F-14 is a twin-engine, two-seat supersonic fighter that is the mainstay of the Navy's fleet air defense force. As such, it must endure the enormous stress that comes from flying on and off aircraft carriers.

A similar problem involving cracks in an F-14 bulkhead occurred in 1975. Grumman fixed the problem then by strengthening the bulkheads of 238 planes that had been delivered and redesigning the piece for later planes.

Until the discovery of the cracks in March, the redesigned bulkhead had been thought to have solved the problem.