Charles E. Nichols general treasurer of the 800,000-member United Brotherhood of Carpenters and Joiners of America, pleaded guilty to yesterday contempt of the U.S. District Court here in connection with a grand jury probe of an alleged scheme to sell group automobile insurance to labor unions.

Nichols, 57, who was reelected to his union post last week, admitted that he destroyed a document that had been subpoenaed by the grand jury, and then made "two or more" statements before the grand jury "which were inconsistent to the degree that one of them was necessarily false."

U.S. District Judge Aubrey E. Robinson Jr. told Nichols that "such a simple act [as he committed] undermines our whole system," but said that "nothing would be gained" by his imprisonment. Robinson placed Nichols on two years of unsupervised probation.

Nichols was originally charged with lying to the grand jury about his knowledge of the involvement of Charles T. (Red) Muntain in the insurance scheme.

Muntain, a former assistant to the secretary of the U.S. Department of Housing and Urban Development for labor relations, was convicted in August of four counts of receiving illegal gratituties, two counts of making false statements to the government and one count of conspiracy to receive the illegal gratituties and to defraud the government. He also was placed on probation in the case.

In exchange for recommending the insurance policies to labor union, Muntain received $2,000 in cash, airline tickets and other benefits from various persons promoting the sale of the insurance, according to Assistant U.S. Attorney Robert R. Chapman.