CAPE CANAVERAL, Fla. (AP) _ Technicians working in cramped conditions drilled holes in space shuttle Discovery's cargo bay wall and an engine compartment Wednesday in an attempt to stem a tiny leak in a fuel line.

Plugging the leak is one of two technical hurdles NASA must overcome before it can schedule the first post-Challenger shuttle launch. The agency is aiming for a liftoff in late September or early October.

The second hurdle is the final pre-launch test firing of a redesigned solid fuel booster rocket, scheduled Thursday at the Morton Thiokol plant in Brigham City, Utah.

The technicians used a routing tool to cut two holes in the rear bulkhead of Discovery's 60-foot-long cargo bay and two more in the adjoining hull of a steering engine compartment.

Because Discovery is standing vertically on the launch pad, the rear bulkhead is parallel to the ground, forcing the technicians to work on their hands and knees and on their stomachs. They worked in a small area surrounded by a tent-like canvas to prevent loose debris from entering other areas of the shuttle.

The dual openings, separated by structural supports, allow a worker to reach about 12 inches into the engine compartment. The goal is to clamp a 4- inch-long clamshell-shaped device over the leak and fill it under pressure with a sealing compound.

The small leak is in a metal fitting on a nitrogen tetroxide gas line that serves an engine system that steers the shuttle in orbit. The leak of gas was detected more than a month ago and has been stopped by putting pressure on the line. However, the hole must be plugged before launch to prevent additional leakage.

The repair and subsequent testing are expected to take about a week, said NASA spokeswoman Lisa Malone.

NASA had delayed fixing the leak until after it had completed last week's successful test firing of Discovery's three main engines on the launch pad.

The main engine test turned up one fault - a small amount of hydrogen gas that was detected after the firing in an area where electrical and fuel lines join the shuttle with the external fuel tank. For the firing, the tank was loaded with liquid hydrogen and liquid oxygen propellant.

Malone said the source of the hydrogen remained a mystery and that pressure tests had failed to disclose a leak. She said it was not considered a serious problem and that engineers ''feel they might be able to live with it'' without taking any action.

For Thursday's test in Utah, engineers have deliberately introduced flaws by cutting holes in insulation, seals and O-rings in several of the joints between rocket segments. They want to see how effectively backup systems contain the blazing hot gases during the two-minute ignition.

A faulty booster rocket joint was blamed for the explosion of Challenger and the loss of seven crew members on Jan. 28, 1986.