One out of every three Metro subway cars is out of service following a sudden epidemic of failures of a six-inch part in the wheel assembly of the cars, the Metro board was told yesterday.

Metro owns 300 rail cars. Its peak-hour schedule calls for having 244 in operation - a goal met only one day in the past three weeks. On Wednesday, as record crowds of commuters continued to swamp the trains, it was able to dispatch only 200.

The sharp drop in the number of cars in service resulted mainly from the failure of an aluminum part, called a caliper link, that attaches part of the brake mechanism to each car's wheel assembly. Erich Vogel, chief of Metro car maintenance, said routine inspection showed the parts, under warranty, to be dangerously loose on 40 cars, apparently due to vibration.

Metro had experienced sporadic problems with caliper links in the past, Vogel said, but only recently - as thousands of extra riders began riding the trains because of the gasoline shortage - did the problem reach epidemic proportions.

Cars will be returned to service as fast as spare parts can be shipped from the manufacturer in New Jersey, Vogel said. Enough parts to equip 12 cars had been received by yesterday.

To get quick delivery, Metro had to gas up the Manufacturer's truck for the return trip to the factory, Metro spokesman Cody Pfanstiehl said.

Richard S. Page, Metro general manager, said all scheduled trains are being run, but only by shortening many from eight to six cars. The resulting space shortage has crowded most passengers and left some off the train altogether.

Hundreds of passenger were left waiting on station platforms yesterday when they could not crowd aboard trains that filled up near the outer ends of both lines, officials said. They waited and caught later trains.

Page appealed urgently for the government and private industry to stagger working hours so the trains and buses can haul more people. Of the 300,000 people who work in downtown Washington and the Pentagon-Rosslyn area of Arlington County, 125,000 are due at work at 8:30 a.m., causing a load that Metro simply cannot handle.

"There is plenty of room on the trains before 7 o'clock in the morning and after 6 o'clock at night," Page said.

Metro carried 306,059 passengers on Tuesday, an apparent record for the mnufacturer, four are being used for Three-year-old system, and 286,908 on Wednesday, a spokeswoman said.

Having one-third of a transit system's fleet out of service at one time is unusual. The American Public Transit Association said a general rule of thumb is that no more than 10 percent should be out at one time.

Ralph Wood, head of metro rail operations, said about 30 cars are in the shops for repairs or maintenance not related to the caliper links, and another eight are there for routine inspections.

Of the other 22, Wood said, four have not yet been accepted from the late-night revenue collections from stations, six are sidetracked because of major damage caused by accidents, four are being used or equipment tests and four began shuttling back and forth this week in test trips between Rosslyn and the new Ballston station in Arlington.

Service on the four-mile Ballston extension is scheduled for next November. Since no new cars are expected from the manufacturer when that extension opens, Metro's fleet will be stretched even thinner, Page said.

The surge of new Metro riders, however, has increased fare collections without adding anything to the cost of operations. As a result, officials said fares paid for 63 percent of the cost of running the trains in April and May instead of the 55.7 percent contemplated in the system's budget.

In money terms, the increase amounts is about $1 million. But Eckhard Bennewitz, assistant Metro comptroller, said this will be more than eaten up by the sharply rising cost of diesel fuel for the bus system.

Page also said the bus system is short about 100 of the authorized 2,700 drivers, and must recruit more to assure continued full service. "We pay good wages - I'm a little surprised we don't have people knocking on our doors to become drivers at $9 an hours," Page said. "That's an open invitation . . ."

The board averted a financial crisis yesterday by voting to borrow from Metro's own construction and trust funds to pay $38.7 million interest due July 2 on bonds that were sold to build the system. The U.S. Department of Transportation approved the action in advance, since Congress has not yet voted the $23.2 million federal share of the interest payment, and Metro would have defaulted on the payment. CAPTION: Picture 1, Lathe operator Frank Neff works on a caliper link that has caused problems on Metro's subway trains; By James M. Thresher - The Washington Post; Picture 2, Mechanic Cliff Overton inspects subway train Before it is returned to service. James M. Thresher - The Washington Post; Picture 3, Pile of caliper links with the bearings removed await repairs in Metro shop.