A three-week-old dispute between construction equipment operators and major contractors has slowed work on building projects throughout the Washington area and allegedly ignited violence at several sites.

A Fairfax County judge, citing a contractor's complaints, has issued an order barring violence and limiting the number of pickets at two major projects in the county. The judge acted last week after the contractor said workers hired to replace union engineers at the huge Mobil Oil headquarters and a Metro subway station were being threatened, assaulted and verbally abused.

"It upsets the men a great deal when someone comes in and takes their job, yes sir," said Jack Clapsaddle, business agent for Operating Engineers Local No. 77. But Clapsaddle said that members of his union had not engaged in violence.

Neither side could agree yesterday on whether the dispute was a strike or a lockout or one how widespread was its impact. The contractors argued it was a strike and the engineers claimed it was a lockout.

It was apparent, however, that the dispute's impact would grow as the regular construction season arrives with the warmer spring weather. Meanwhile, picketing engineers are faced not only with replacement at some sites but also with craft unions -- whose contracts contain a no-strike clause -- crossing their lines.

A key issue in the dispute is a guarantee of a 40-hour work week that the engineers say has been a part of their contracts for 40 years. The contractors have sought to remove the guarantee, saying they should not have to pay the engineers for a full 40-hour week when weather or other conditions make it impossible for the engineers to put in that much time on the job.

"It's a tough situation," said federal labor mediator Charles Scott yesterday, as the two sides convened for a fifth weekly session under government auspices. "I'm not overly optimistic at this point."

The local Operating Engineers' 1,200 members have been working under the terms of a contract that expired April 30. Other ouilding trades, meanwhile, have signed an unusual one-year contract ceding many traditional work practices and containing a wage freeze in order to compete with lower-priced nonunion workers.

Local 77 and the Construction Contractors Council have since met 35 times without reaching a settlement, according to Frank E. Williams Jr., head of the employers bargaining committee.

With the other crafts contracts due to expire soon, Williams said, "We felt it was necessary to bring this [operating engineers] situation to a conclusion."

In late January, the contractors council declared an impasse in negotiations and soon afterwards implemented its last proposal, dropping the 40-hour guarantee.

"We were willing to negotiate and work under our old contract until settlement," Clapsaddle said. "We did not strike; they locked us out."

Pickets later appeared on job sites, incuding the Blue Plains sewage treatment plant, the new Library of Congress addition, and the National Institutes of Health in addition to the Fairfax sites. Neither side was able yesterday to guage precisely the impact of the dispute. The number of affected companies was said to be 14 by the union and 39 by the contractors' bargainer.

Clapsaddle said that some jobs had been shut down entirely by the absence of union members who run the bulldozers, cranes, shovels and other equipment essential to large-scale construction work.

The Bethesda-based George Hyman Co. said in obtaining its Fairfax court injunction that "threats and intimidation" by picketers had resulted in several nonunion replacements "quitting their jobs and refusing to come back to work." Pickets, Hyman charged, had engaged in "force, threats of violence, intimidation, and the use of insulting or threatening language in an attempt to induce employes to quit."

According to court testimony, picketers allegedly spread nails across the road at the entrance to the Huntington Metro site near Alexandria and blocked an entrance at the new Mobil office on Gallows Road with a backhoe. Union witnesses denied the allegations and said they had been instructed to act within the law.

Williams, the employers' bargainer, said there have been scattered other acts of violence and some vandalism at different job sites.

Clapsaddle said his workers' plight has been exacerbated by a lack of strike benefits and by employers' decisions to stop pension and health insurance payments.

The employers, Clapsaddle said, "want to see this thing put away as well as we do. It doesn't help anybody in the industry."