Sen. Charles E. Grassley, a right-wing Republican farmer from New Hartford, Iowa, is hardly your usual Pentagon scourge. He's a Reagan loyalist who votes for the MX and the B1 bomber, and doesn't fret about warships in Central America. The only weapons system he has disapproved of lately is the nerve gas program, and not for the high-minded reason given by other critics. He just says it's not economical.

Grassley is, nonetheless, making life miserable for the money-spenders across the river, and chipping away at Defense Secretary Caspar W. Weinberger's reputation as "Cap the Knife." Weinberger is as astonished as the Democrats, who so bitterly mourned Grassley's 1980 victory over liberal incumbent John C. Culver.

Grassley, a gawky, inarticulate former state legislator, is proving as much a problem as Culver ever was, and nobody can call him "soft on defense," the charge he leveled against Culver in their campaign.

Weinberger has only himself to blame for this situation. It all began last December, when Grassley, who is usually preoccupied with agricultural matters, heretically proposed a freeze on all spending, including the Pentagon's. Grassley said he wanted to subject Defense Department ledgers to the same scrutiny his kind usually reserves for the food-stamp program.

He got that off-the-wall idea from having read the cost analyses prepared by a program analysis and evaluation officer named Franklin Charles (Chuck) Spinney. Spinney pointed out, to the fury of his superiors, that the $1.5 trillion defense expenditures projected over the next five years, will cost at least $500 billion more.

Grassley wanted to talk to Spinney.

Repeated applications to the Pentagon resulted only in the proffer of surrogate witnesses, presumably people with more tolerant views of the Pentagon's procurement practices.

Finally, in his frustration, Grassley, one January day, climbed into his car and drove over to the Pentagon. He negotiated his way through the maze to the anteroom of Spinney's office. There the door was barred. Spinney's boss, Dr. David Chu, came flying out to flag him down.

"You can't talk to Spinney, he's a civil servant," Chu told Grassley.

The next day, Weinberger was on the phone, assuring Grassley he really didn't want to talk to "them--the people who are out to get us."

With the help of three fellow members of the Senate Budget Committee, Grassley forced a Spinney appearance on the Hill. The cost-cutter, who was being subjected to the reprisals historically accorded Pentagon whistle blowers, told the senators that the Defense Department was soaking the taxpayers, and would continue to do so under its present procedures. Spinney got on the cover of Time magazine.

Sometime later, President Reagan invited Grassley, with other members of the Budget Committee, to the White House so he could talk "reason" to them about Pentagon spending. Grassley, eyewitnesses report, pounded the table and said, "You haven't made your case, Mr. President."

In the recent debate on the defense bill, a Grassley amendment passed that could clip the wings of the B1, the controversial bomber that Culver had helped to shoot down during the Carter years. Grassley's amendment exacts the reestimate of the whole $30 billion cost, and the release to Congress of all cost data. Armed Services Committee Chairman John G. Tower (R-Tex.) came up on the floor and told Grassley he didn't know what he was talking about, but Grassley just looked at him with his ice-colored eyes.

His new activity plays well in the cornfields. Grassley says that people who tell him they never voted for him and never will, let him know they like what he is doing. Ordinary citizens can't figure out the intricacies of Pentagon contracts, but they can follow the spare parts scandal without any trouble. Grassley brought to light such items as the 4-cent electric diode for which the Sperry Corp. charged $109. His Iowa approval rating has jumped from 40 to 70 percent.

When Weinberger issued a whole new set of "Ten Commandments" that he said would curb the greedy contractors, Grassley promptly held a news conference and denounced them as "toothless" and "vague."

Further, he is introducing a "creeping capitalism" bill, which would require that 70 percent of all Pentagon contracts eventually be subject to competitive bidding. At present, only 6 percent go through marketplace negotiation.

After his news conference, Grassley hurried to join a delegation bound for the White House. Its mission: to persuade Reagan to be faithful to his conservative principles, especially as applied to school prayer and abortion.