Up to 40 aging Metrobuses will be leased to Pittsburgh's transit authority to ease a shortage of road-worthy buses there, Metro operations chief Theodore Weigle said yesterday.

Each bus will earn Metro $25 for every day it is leased, injecting as much as $150,000 into the financially strapped system by next fall, the earliest the buses will be returned. The money will be set aside for a capital program to rebuild more than 600 older Metrobuses by the mid-1980s.

The deal will not affect service here, according to Weigle, because the buses will come from Metro's normally inactive reserve fleet. One-hundred eighty buses will remain in reserve, enough for emergencies that may arise. Moreover, the lease allows the buses to be recalled on 30 days' notice.

Last year New York City leased 160 reserve Metrobuses after its brand new Grumman Flxible 870 buses were recalled en masse to have their bodies reinforced. In general, the Metrobuses "were returned in better condition than they were sent," according to Weigle. "We got some (12) new engines out of it."

Two of the vehicles did not come back, however. One was stolen and set afire. The other rolled off a pier into deep water and sank. New York will send Metro two replacement buses.

Michael Scanlon, marketing manager for the Port Authority of Allegheny County, which runs Pittsburgh's buses, said that age, hilly driving, potholes and this year's unusually severe winter have taken a toll on the city fleet. The authority often is 15 buses short of 785 it needs for peak-hour service.

The first of 410 new buses it ordered from a Colorado subsidiary of the West German company Neoplan were expected more than a year ago, Scanlon said.

Until then, 1966 model buses from Washington, produced by Grumman Flxible and among the less reliable that Metro owns, will fill the gap. "An old bus is a welcome sight to someone on a cold corner, as opposed to no bus," Scanlon said.

Metro has about 2,000 buses, of which less than 1,800 are on the streets at rush hour, held as spares or used internally. In addition, 226 buses in its reserve fleet are parked long-term at Metro garages.