An arbitration panel ruled yesterday that Metro's 4,800 unionized bus and subway employes are entitled to a cost-of-living pay increase retroactive to July 1, just as those employes had insisted when they went out on a wildcat strike July 19 and tied up the city for six days.

The increase will be less than the 20 cents or 21 cents an hour strikers had said they should receive because of a new revision in the formula by which the raise is figured, William A. Boleyn, Metro's assistant general manager for finance said late yesterday.

Regardless of the amount, the arbitration panel's ruling suggested in the strongest language that Metro simply was wrong when it did not pay a cost-of-living increase agreed to in an expired contract just because a new contract was under arbitration.

Metro's bus drivers, subway operators, merchants, station attendants and some office clerks will receive a pay increase retroactive to July 1. The money will appear in weekly paychecks as soon as possible, probably Aug. 16, Boleyn said.

The pay raise was being figured by Metro accountants last night from the Bureau of Labor Statistics (BLS) consumer price index for metropolitan Washington. BLS has revised that index in recent days, Boleyn said, which is why Metro was recomputing last night. During the strike, drivers and merchanics said they are entitled to 20 or 21 cents, but "that number is overstated," Boleyn said. "We don't know by how much yet."

Metro officials were gloomy about the ruling and even the unionized employes were taking it with a grain of salt.

Driver Bob Kirk, interviewed at the Bladensburg garage, the system's largest said, "I don't really think [the strike] was effective . . . even though the arbitrator ruled in favor of us. There were many other factors involved" besides the pay raise, he said.

William T. Scoggin, the Arlington garage bus driver who emerged as a moderate leader of the strikers and whose division was the first to return to work, said, "We're very please . . . It's what we expected. The union leadership allowed Metro to get away" without paying the cost of living increase.

"I'm very disappointed in the decision," Metro board chairman Joseph S. Wholey said yesterday. "I don't think it's a fair award for the taxpayers of the region."

Experienced Metro bus drivers and subway operators earn $8.16 per hour and, with overtime, average about $18,800 annually. Other salary rates among unionized employes are computed as a percentage of the driver salary.

Metro's contract with local 689 of Amalgamated Transit Union expired April 30. A colause said "all of the conditions in this contract shall remain undisturbed during the arbitration proceedings." Binding arbitration is required by U.S. law as the means of settling contract disputes.

Metro interperted "undisturbed" to mean that it did not have to pay the cost-of-living increase because cost-of-living increases are under arbitration in the new contract. The striking drivers said that undisturbed meant undisturbed. The arbitrator, Richard J. Bloch, agreed.

Bloch was the third-party "neutral" arbitrator in the three-man panel. Metro and Local 689 each chose a representative and they agreed on Bloch. Naturally, Metro's representative dissented from the final opinion.

Failure to pay the cost-of-living escalator on July 1, when it was due, the arbitrator ruled, "resulted in a substantial disturbance of existing conditions and, therefore, is a contract violation."

The arbitration on the issue of what "undisturbed" means was ordered by a federal judge here as part of the legal actions that followed the beginning of the strike. Both Metro and Local 689 initially had agreed to settle the cost-of-living question as part of the new contract rather than settle the dispute over the meaning of "undisturbed" in the old contract. Arbitration on the new contract is not scheduled to begin until Aug. 21.

That schedule was unacceptable to many strikers, particularly since there was no guarantee of retroactivity in the new arbitration.

U.S. District Judge Louis F. Oberdorfer ordered arbitration to begin speedily, specifically on the question of the meaning of "undisturbed" in the old contract.

As a result of the ruling yesterday, it is considered conceivable that the arbitration panel deciding the, new contract could change the cost-of-living formula. That possibility was one of the key reasons Local 689 chose not to file a grievance on the withholding of the cost-of-living increase, local president George R. Davis said yesterday.

"I think the decision is very good," Davis said, "but it doesn't finally decide the issue. While we've won the battle we haven't won the war."

As a result of the illegal strike, 10 Metro employes have been fired and 85 others have been suspended for between one and nine days, spokesman Marilyn Dicus said yesterday.

"The only advantage this strike had," driver Scoggin said, "was that it got us a quick hearing in court. You know how long it takes to get a civil suit heard in a federal court without a strike? If we're going to sacrifice the right to strike, we should be able to get a quick hearing."