PITTSBURGH -- After two of his first three starts this season, Chris Archer remarked that the Pirates’ margin for error is small. Their pitching carried them to 12 wins in their first 18 games, but every run -- and every lead -- feels important as their short-handed lineup scraps to score.

That was the case again on a chilly, cloud-covered Sunday afternoon at PNC Park. Archer gave up a three-run homer to Giants catcher Buster Posey in the fifth inning, and that proved to be all San Francisco needed as Pittsburgh lost, 3-2, halting the Bucs’ winning streak at five games.

The Pirates are 8-2 this season when scoring at least four runs, a testament to the strength of their pitching staff, and on Sunday they fell to 4-5 when scoring three runs or fewer.

Archer threw 83 pitches on the day, and one mistake down the middle was the difference between another win and his first loss of the season. That’s what he meant about the thin margin for error, isn’t it?

“Pretty much, yeah,” Archer said.

First baseman Josh Bell continued to carry a lineup currently playing without its top four outfielders, starting second baseman and a pair of shortstops. With two outs in the fourth inning, Bell crushed a hanging changeup from Giants righty Dereck Rodriguez over the right-field seats. After his two-run blast to the riverwalk, Bell is batting .303 with a .976 OPS through 19 games.

“I feel good. Just trying to get my ‘A’ swing off, trying to focus on timing on starters,” Bell said. “It’s nice working in the cage pregame every day, working with [hitting coaches Rick Eckstein and Jacob Cruz] about just timing and nothing else. Staying inside the ball and nothing else. Kind of simplify things and let the rest take care of itself.”

Bell’s fourth home run of the season gave Pittsburgh a 2-0 lead, but it would not last long.

Rodriguez hit a leadoff single in the fifth, then Steven Duggar dropped a bunt single in front of the plate. Posey came to the plate with one out and a home run drought dating back to June 19 of last year. The former National League MVP ripped a 1-1 fastball to dead center, putting the Giants on top.

“I left a pitch down the middle to a good hitter,” Archer said. “I was trying to go in, get the ground ball for a double play. It just stayed in the middle, and he hit that one hard.”

The Pirates’ need for runs also led to Archer’s early exit. He could have pitched another inning, but he was due up in the fifth. He would have hit for himself with a runner on first, manager Clint Hurdle said, but Bryan Reynolds advanced into scoring position on a wild pitch.

“I’m not a big fan of taking him out there, either,” Hurdle said, “but I’m a bigger fan of winning games.”

Hurdle sent up pinch-hitter JB Shuck in Archer’s place with two outs, hoping to extend the inning, but Shuck grounded out to shortstop Brandon Crawford.

The Pirates’ bullpen kept them within striking distance for the rest of the afternoon. Richard Rodriguez worked a scoreless sixth inning, and Keone Kela struck out Posey to end a perfect seventh. Francisco Liriano struck out two in the eighth, and Nick Kingham stranded two runners in the ninth.

But the Pirates, averaging 3.58 runs per game this season, were kept in check by Rodriguez and a San Francisco bullpen that’s proven to be stingy all season. They gave themselves a chance in the ninth, when Reynolds came to the plate against lefty Will Smith with two outs and Jung Ho Kang on first base.

Reynolds laced a double to right field, and the ball took an odd bounce off the bottom of the Roberto Clemente Wall. Third-base coach Joey Cora, encouraged to be aggressive with his decision-making, waved Kang around third. Duggar played the carom well, however, and started a 9-4-2 relay that gave Posey plenty of time to tag Kang for the final out.

“The ball kicked funny off the wall, and Joey pushed it,” Hurdle said. “He’s aggressive over there, and they made a perfect relay.”