Defense contractors made more than twice as much profit on government work as on commercial business when their gains were measured against what they had invested in plants and other assets, according to a study the Navy released yesterday that conflicts with an earlier one by the defense secretary's office.

Profits on sales, as distinguished from those on assets, also were higher when the contractors built ships and other weaponry for the Defense Department, averaging 9.2 percent in 1984 compared with 8 percent profit on nondefense work.

Some firms are making so much money on defense work, the study concluded, that those profits more than make up for losses in commercial endeavors.

The Boeing Co., General Dynamics Corp., Lockheed Corp., McDonnell Douglas Corp. and Northrop Corp. "clearly rely on U.S. government business for 90 percent or more of their operating profits," the report said in analyzing 1984 data. The Grumman Corp. "is very close at about 87 percent" and Todd Shipyards Corp. "is probably 90 percent or more," it added.

Defense contractors generally have argued that profits on government work fluctuate and over a period of years are no higher than commercial profits. They also contend that defense work has certain unattractive features, including onerous government paper work, public hostility and reliance on one customer who can arbitrarily cancel contracts.

Navy Secretary John F. Lehman Jr., in response to the profits report, last week issued orders restricting Navy contractors trying to charge the government for tooling costs for construction of ships and other weaponry.

The Defense Department in August released a report entitled "Defense Financial and Investment Review" that concluded that the defense industry was not making "unreasonable" profits but instead receiving "an equitable return for its involvement in defense business."

That study was ordered by the deputy secretary of defense and chaired by Mary Ann Gileece, the former deputy undersecretary of defense who resigned under pressure after seeking consulting work from defense contractors while still in her Pentagon post.

The new Navy study showed that the return on assets in 1984 for government work, mostly for the Pentagon, was 53.6 percent for FMC Corp., 35.5 percent for General Dynamics, 35 percent for Martin-Marietta Corp., 30 percent for Tenneco Inc., which includes Newport News Shipbuilding, 29.4 percent for Rockwell International Corp. and 29.3 percent for the Boeing Co.

Overall, the return on assets averaged 26.7 percent from military work compared with 10.7 percent for commercial work in 1984 for the 22 firms analyzed for the Navy.

Profits on sales, the study said, were "about 15 to 20 percent higher" on government work for the major defense contractors than that yielded on commercial business.

The high return on assets for defense work has been true for at least the last eight years, according to the study. The profit ratios on sales for military and commercial work have fluctuated in that same period.

The study shows commercial work was more profitable than defense contracts until late 1980 when the Pentagon budget began to grow toward its present plateau of about $300 billion a year.

Everett Pyatt, assistant secretary of the Navy for shipbuilding and logistics, commissioned the profits study. In a letter to Lehman, Pyatt said the findings document the need to prod contractors to invest more heavily in capital equipment. He added that profits should be "significantly reduced."

The Navy and defense panel used different data to reach their conflicting conclusions about profits on military contracts.

RRG Associates, in conducting the Navy study, relied on data filed by the selected companies with the Securities and Exchange Commission. The Defense Financial and Investment Review had an accounting firm collect voluntary data from the corporations; it was then compiled in a collective presentation that did not identify the profit patterns of particular firms.