The Mets' Pete Alonso crosses homeplate after hitting a solo...

The Mets' Pete Alonso crosses homeplate after hitting a solo home run against the Diamondbacks in the second inning on Tuesday in Phoenix. Credit: Rick Scuteri

PHOENIX — In his march toward free agency, now just a month-plus away, Pete Alonso added another to his long list of historical franchise feats Tuesday night.

His home run in the second inning of the Mets’ 8-3 stomping of the Diamondbacks was the 220th of his career. That tied him with Hall of Famer Mike Piazza for third on the organization’s all-time list.

The only names ahead of them are Darryl Strawberry (252) and David Wright (242), both of whom played hundreds more games with the club than Alonso. But third is where Alonso is all but guaranteed to remain as he heads to the open market this offseason.

“Mike was a childhood hero of mine, so that’s really special. He was one of those guys I looked up to,” Alonso said. “I‘m just happy that I was able to help contribute tonight in a positive way, have good at-bats and get us on the board early. It’s really cool. Really, really cool.”

This long ball and attached factoid doubled as a reminder of what the Mets would be paying for, if they and Alonso agree to a long-term contract: the rest of the career (or most of the rest of the career) of one of their best ever. Reliable production at first base, plus a chart-climbing, record-breaking homegrown slugger. A face of the franchise, plus a legacy for player and team.

“It’s something that I don’t really like to think of during the season,” Alonso said of his place in Mets history. “Those certain moments, they really set in during the offseason. That’s when I have time to think about and reflect on some things. This moment is really special, but I don’t think it’s going to set in until the season’s done.”

It would help Alonso’s case, of course, if he and the Mets figure out how to get to the playoffs, which they’ve done just once in his five seasons in the majors.

With a win against Arizona (75-57), which entered with a six-game win streak and the best record of anybody since the All-Star break, the Mets stayed three games behind Atlanta for the last National League wild-card spot. They have 30 games across 33 days remaining.

The Mets (69-63) blew the game open with a six-run top of the fifth. They began that sequence with five consecutive hits, including an RBI single from Francisco Alvarez, his first knock since his walk-off home run against the Orioles on Aug. 19. He had been in an 0-for-17 funk.

They reached righthander Brandon Pfaadt (8-7) for eight runs (six earned) in 4 2/3 innings. The last of those were inherited by lefthander Jordan Montgomery, the former Yankee who has been so bad with the D-backs — who signed him in late March — that they recently relegated him to long relief.

Montgomery forced in runs with a bases-loaded hit batsman (Tyrone Taylor) and a bases-loaded walk (Jeff McNeil). Shortstop Geraldo Perdomo’s fielding error on Alvarez’s grounder — Perdomo’s third misplay of the inning — made it an eight-run advantage for the Mets.

“When you put the ball in play, good things happen,” manager Carlos Mendoza said. “Defense is going to have to make some plays. We caught some breaks there, but we continued to put together some really good at-bats.”

Lefthander Sean Manaea (10-5) continued his strong run with 6 2/3 innings (three runs). He struck out 11, walked none and carried a shutout into the seventh. Lourdes Gurriel Jr. and Corbin Carroll homered against Manaea before Mendoza pulled him.

Over the past month, Manaea owns a 2.87 ERA in six starts.

Manaea has four double-digit strikeout games this season. The rest of the Mets have one (Luis Severino against the Cubs in June).

“Amazing. Incredible,” Mendoza said. “It feels like as we progress with the season, he keeps getting better and better . . . It’s amazing what he’s been able to do. He’s been huge for us the whole year. He’s pretty much carried that rotation, him along with [Severino]. Huge. I’m proud of him.”

Alonso described Manaea as “absolutely carving” opposing lineups.

“I don’t know about that,” Manaea said. “I’m just feeling good.”

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