Sal Yvars, a reserve catcher for the New York Giants who took part in, and ultimately revealed, an elaborate sign-stealing scheme that might have helped propel the Giants to their storied 1951 National League pennant victory, died Wednesday in the Bronx. Yvars, who lived in Valhalla, N.Y., was 84.

The cause was amyloidosis, an accumulation of abnormal proteins in the organs, his son David said.

In his eight seasons in the major leagues, Yvars (pronounced E-vars) was mostly a backup for the outstanding defensive catcher Wes Westrum. His closest brush with baseball fame came against the Yankees in Game 6 of the 1951 World Series when his pinch-hit line drive to right field seemed certain to tie the score with two outs in the ninth inning. But Hank Bauer’s diving catch ended the game and the Series.

Long after Yvars’s playing days, a different kind of contribution he made to the Giants’ fortunes became known. It involved coded signals that he conveyed within full view of the Polo Grounds crowds but that went unnoticed by opposing players and the fans.

During the last 10 weeks of the 1951 season, the Giants’ batters were tipped off to the kinds of pitches they could expect during their home games. The details of the baseball espionage were ultimately revealed by Yvars and several teammates.

As the story was told, Giants Manager Leo Durocher stationed Hank Schenz, a substitute infielder, and later Herman Franks, a coach, behind an opening in the Giants’ clubhouse above center field, almost 500 feet from home plate, and they peered through a telescope to spy on the opposing catchers’ signs, made with their fingers. An electrician working for the Giants sat alongside them.

When the electrician was told by Schenz or Franks what type of pitch was coming, he activated a buzzer in the Giants’ bullpen, in right-center field. One buzz meant a fastball was coming, two meant a curveball. Yvars, seated in the bullpen, listened for the buzzes, and before each pitch the Giants’ batters could glance at him for a signal.

In his book “Pennant Races” (Doubleday, 1994), Dave Anderson recounted the sign-stealing operation and quoted Yvars as having told Giants batters: “Watch me in the bullpen. I’ll have a baseball in my hand. If I hold on to the ball, it’s a fastball. If I toss the ball in the air, it’s a breaking ball.”

Detailed accounts of the scheme were related by Joshua Prager in The Wall Street Journal in 2001 and in his book “The Echoing Green” (Pantheon, 2006), based on his interviews with Yvars and many of his former teammates.

The Giants, far behind the Brooklyn Dodgers in mid-August ’51, came back to tie them for first place and won the pennant in Game 3 of a playoff on Bobby Thomson’s Shot Heard ’Round the World off Ralph Branca. That three-run homer at the Polo Grounds with one out in the ninth inning was perhaps the most dramatic moment in baseball history.

When Branca delivered his fateful pitch, a high inside fastball, did Thomson know what was coming? “I gave him the sign,” Yvars told The New York Times in 2001. But Thomson told Prager that he had been concentrating so heavily that he had not looked toward Yvars.

A Manhattan native and a baseball star at White Plains High School, Yvars made his Giants debut in 1947. He concluded his playing days with the St. Louis Cardinals in 1954 and had a career batting average of .244. He was later an investment adviser.

In addition to his son, of Croton-on-Hudson, N.Y., Yvars is survived by his wife, Antoinette; his daughters Diane Yvars and Donna Saldi, both of Valhalla, and Deborah DiLorenzo, of Hawthorne, N.Y.; a brother, Jack, of Valhalla; five grandchildren; and three great-grandchildren.

Sign-stealing by mechanical means was outlawed by baseball in 1961. As for that episode a decade earlier, “I didn’t feel guilty about anything,” Yvars told Prager. “I was just doing my job.”

Salvador Yvars (1924)