About 90 machinists and mechanics for Metro's bus and subway operations walked off the job or called in sick yesterday in a wildcat job action that was all but unnoticeable to the riding public.

However, about 140 mechanics and bus drivers met at an RFK Stadium parking lot last night and shouted their approval of a proposal to put up picket lines at the Metro bus garages and rail yard. There were no picket lines yesterday, and buses and trains ran full schedules without difficulty.

Ron Kuba, a mechanic from the Royal Street Garage in Alexandria, told the crowd at the stadium, "A lot of people didn't want to go out today. Metro ignored all the sickouts. We'll just have to wildcat."

The walkout yesterday fell far short of the shutdown of the transit system that a group of dissident Metro employes had promised after they captured control of a regularly scheduled union meeting Tuesday night.

Neither Metro nor union officials were willing to assess the probable effectiveness of a bus or subway job action today for the simple reason that neither could speak for the dissidents, and it is nuclear how strong that faction is and who is leading it.

Leaders of Local 689 of the Amalgamated Transit Union, the official bargaining agent for about 4,500 Metro bus drivers, subway motormen, machinists, mechanics, station attendants and clerks, denounced the job action from the beginning as illegal.

Senior Metro officials indicated that they are prepared to move against the walkout with suspensions, firings and legal action, but they declined to detail their plans. "We will take the appropriate action on an individual basis," assistant general manager Nicholas Roll said.

There was universal agreement that the issue had nothing to do with security on the buses and everything to do with a 20-cent-an-hour pay raise union employes feel they should have received in yesterday's weekly paycheck, but did not. Inadequate security for bus drivers was the key issue in a one-day wildcat strike May 18 that seriously disrupted bus service.

Metro's unionized employes have been without a contract since April 30, when a two-year pact with Local 689 expired. However, that old contract continued a clause requiring both parties to submit to binding arbitration.

A key part of the old contract was a provision calling for an automatic, full cost-of-living pay increase every quarter. The union dissidents complained yesterday that under the binding arbitration clause, the automatic cost of living increase should have been paid automatically by Metro effective July 1. That would amount to 20 cents an hour over the present $8.16 hourly rate for fully experienced bus drivers and mechanics.

Metro general manager Theodore C. Lutz said yesterday that "in reviewing out negotiating position we felt it was not appropriate to automatically pay the cost-of-living increase." Lutz said that "freezing the contract" until the arbitration process was completed had been past practice here.

If the arbitration panel awards the cost-of-living increase retroactively, it will be paid, Lutz said.

George R. Davis, president of Local 689, said, "I asked Metro to pay this even though I know they don't have to and they said no." Davis and Lutz have agreed that the cost-of-living payment will be arbitrated before other issues and that a final resolution could be reached by the end of August.