When Neil Walker races out of the visitors’ dugout to take his position at second base at Pittsburgh’s PNC Park on Monday night, he plans to follow the ritual he established during his seven years with the Pirates.

The first-year Met first will take a few seconds to look at the rightfield wall, where No. 21 is displayed in Pirates black-and-yellow in honor of the legendary Roberto Clemente.

And then Walker will tip his cap.

It’s his way of saying “thank you” to the late Clemente for making a decision 44 years ago that — unbeknownst to anyone at the time — ultimately would save his dad’s life long before Neil was born.

On New Year’s Eve 1972, Tom Walker, a 24-year-old pitcher with the Montreal Expos, expected to join Clemente on a flight to Nicaragua to help him bring relief items there after an earthquake devastated the area.

But, Walker said, there was no room left on the plane, and Clemente turned to him, smiled and said, “Young Walker, go home and party. Have fun.” So that’s what Tom Walker did.

The overloaded DC-7 cargo plane crashed shortly after takeoff from San Juan, Puerto Rico. Clemente’s body was never found and all aboard are believed to have perished.

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Many memories fade over time, especially four decades’ worth.

Not this one.

“It’s just one of those things you get in your mind,” Tom Walker said, “and you can’t ever get rid of.”

THANKFUL HE DIDN’T

GET ON THE PLANE

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Tom Walker, now 67, said in an interview on May 23 that he became friendly with Clemente — a childhood idol of his — from two years of playing winter ball in San Juan. Single and with lots of free time, Walker said he enjoyed being around Clemente as much as he could.

Then 37 and larger than life in his native Puerto Rico, Clemente was distressed about the damage done by the Dec. 23 earthquake in Managua, a city he had visited just weeks earlier, according to his sons Roberto Jr., 50, and Luis, 49.

So Clemente, well known for his humanitarian efforts, asked for donations to send there. And people responded.

“Things were being donated and brought to Hiram Bithorn Stadium” in San Juan, Walker said. “You couldn’t imagine the amount of stuff that showed up. It was unbelievable.

“I told him I’d like to help, that I didn’t have anything else to do. I didn’t have to go home for the holidays.”

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Walker remembers mostly spending his time in the stadium’s parking lot, sorting goods such as clothing, food and medical supplies and loading trucks.

The supplies were sent on a series of three flights over the next week, but Walker remembers Clemente being displeased to learn that goods allegedly were being taken by corrupt government officials instead of going to people in need.

Clemente figured the best way to combat this was to go there himself. But he didn’t want to wait until the plane he was using to deliver goods came back from its last flight.

So on the spot he accepted an offer to use a local man’s cargo jet to go to Nicaragua the next day with another batch of relief items, Luis Clemente said on May 27.

According to author David Maraniss’ 2006 book “Clemente: The Passion and Grace of Baseball’s Last Hero,” the plane had issues. Three weeks earlier it rode off the runway into a drainage ditch. The owner of the plane had a litany of Federal Aviation Administration violations. He found a pilot and flight engineer hours after securing Clemente’s flight.

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Walker still remembers the sight of people working on the plane as they loaded it with goods.

“Me looking at an airplane, I don’t know anything about airplanes,” Walker said. “I remember they had people trying to do things to the plane, but that’s not unusual.”

Walker said he thought he was going on the flight until everything was loaded and he realized there were five seats and six prospective passengers. And the plane was loaded to the brim with goods everywhere else, leaving no room.

“I remember getting close to my car,” Walker said, “and looking back and waving to him.”

He believes he left the airport sometime in the middle of the afternoon and then went to a restaurant to get something to eat. He was back at his apartment that night when a neighbor knocked on the door in tears and told him what had happened. By then the news was on the radio, and considering it was Clemente, word traveled as fast as it could in 1972.

The sick feeling in the pit of his stomach returns every time he recites the story.

“It’s obviously something I can’t ever forget,” Walker said. “I’m thankful I didn’t get on that plane. I’m sorry that Roberto did.”

CLEMENTE’S LEGACY

CARRIED BY NEIL

Given the number of people who helped sort the donated goods, the Clemente family has heard many people through the years say that they too were supposed to be on that plane.

But Luis Clemente says Walker’s story has stood out because of his baseball ties to his father.

The Clemente family also believes that their father’s legacy on the field is, in some small way, being carried on by Tom Walker’s son, Neil.

Tom Walker met his wife, Carolyn, in spring training 1974, a little more than a year after the plane crash. That she was born and raised in Pittsburgh as a dedicated Pirates fan was a coincidence. Pittsburgh is where they have called home ever since they were married that November.

Neil Walker was born in 1985, the youngest of four children. That he was drafted by the Pirates in the first round in 2004 also is a coincidence.

Neil says his father told him and his siblings the story when they were kids, but he doesn’t think he completely grasped the gravity and potential ramifications of Clemente’s decision until he was in high school.

“If it doesn’t happen that way, if my dad went on that plane,” Walker said, “then me, my brothers and my sister don’t exist.”

It was on a trip to Cooperstown for a baseball tournament when Neil was in high school that father and son looked at Clemente’s plaque and did a double-take. His name is listed as Roberto Clemente Walker.

“Walker was my grandmother’s maiden name,” Luis Clemente explained. “We utilize our mother’s maiden name as a second last name.”

Another eerie coincidence: On his first Opening Day with the major-league club in 2011, Neil Walker hit a grand slam. The only other Pirate to do that? Roberto Clemente.

And in 2014 Walker hit his 21st home run over the 21-foot Roberto Clemente wall in rightfield on “Roberto Clemente Day” in Pittsburgh. Sitting next to each other in the stands were Roberto Clemente Jr. and Tom Walker. Tom said they looked at each other in disbelief.

“All these things happen and you kind of think to yourself, this is really strange,” Neil said. “You feel like somebody is there for you.”

Traded by the Pirates to the Mets last December, Neil will make his return to Pittsburgh on Monday for the first time as a visitor.

But in many ways, it’s still very much home.

“Understanding the impact of Dad not letting Tom get on that flight,” Roberto Jr. said, “will forever be a part of that family’s history.”