The old guard recaptured its citadel-New York-tonight. The New York Rangers, born in 1926 but ailing since 1974, conquered the upstart New York Islanders, best team of the National Hockey League's regular season, and advanced to the Stanley Cup Final.

Don Murdoch and Ron Greschner scored the second-period goals that carried the Rangers to a 2-1 victory. But it was a 17-man effort that limited the Islanders to only three shots on goal in the third period, preventing them from mounting an effective comeback on freshly made ice that resembled marshmallow topping in consistency.

By winning this shocking semifinal in six games, while Montreal was losing in Boston, the Rangers received a bonus. They look ahead to three days of rest before traveling to either Montreal or Boston for the start of the final series.

The Rangers last appeared in the finals in 1972, losing to Boston, and have not won the Cup since 1940. Yet few of the 17,372 fans who generated enough noise to scare the circus lions in the Madison Square Garden basement, could doubt, after these amazing victories over Philadelphia and the Islanders, their heroes' ability to win it all.

"It's a great feeling," Murdoch said. "I've never been this far in my life. I think we deserve to be this far and I don't know who's going to be ahead of us, but they'd better be ready."

The Islanders could have blamed the dreadful ice and some sorry officiating, but instead they chose to credit their conquerors' superior play.

"No matter how much blame anybody wants to put on us, and it will be easy to say we choked, I think we lost because the Rangers stood up to us and played better than us," said Mike Bossy, who gave the Islanders an early 1-0 lead with his only points of the series.

"I still don't think they have a better team, but they won the series and with a couple of breaks I think they can stand up to Montreal. We didn't expect to walk through the series, but we did expect to win."

"We underestimated what they were doing," said Islander goalie Chico Resch, who wound up 0-3 in the series by losing tonight.

"It was not so much that we were overconfident, but just the feeling that our talent could carry us. They seemed to catch the magic. It will be interesting to see if they can do it one more time."

The Islanders compiled 116 points during the regular season to the Rangers' 91 and few, including the Rangers themselves, had visions of such a startling upset.

"It really hasn't sunk in yet," said Ranger roookie Don Maloney. "If anybody had predicted before the series that we'd win like this, somebody would have thought they were smoking something funny or doing something funny."

The same could be said for anyone who thought Bossy, a 69-goal regular-season scorer, would enter the sixth game of this series without a point-or that the Islanders' power play, best in the NHL, would come in 0 and 19. Bossy ended both droughts in the first period, but that was the extent of Islander scoring.

Murdoch tied it on a brilliant rebound, after Resch had muffed Mario Marois' long shot. Murdoch, back to the net, held the puck while Resch came out, then spun and fired it in.

"That was my one mistake," Resch said. "The shot from the point by Marois was a routine shot and it went off the palm of my glove and bounced to Murdoch. I thought he'd shoot right away and I dove, but he spun and beat me."

"It was on my backhand and Resch was rushing me, so I waited, then turned and shot in the corner," Murdoch said.

Greschner's winner came on a 50-footer that eluded Resch while venerable Phil Esposito screened the little goalie.

Esposito had controlled the puck in the right-wing corner, then passed to Don Maloney and headed for the slot. Instead of passing back to Esposito, Maloney took his time and finally fed Greschner at the point.

"(Denis) Potvin has been coming for me, but this time Phil took him with him, and it left me with a lot of time," Maloney said. "Gresch was open, so I went to him and, fortunately, it worked out."

For the next 31 minutes, it was a matter of waiting-and praying.

"It seemed so long, so long," said Murdoch, who had missed the season's first 40 games because of a drug conviction. "Even the last 17 seconds, Steve Vickers and I were on our hands and knees on the bench, praying, 'God, I'll never do another bad thing again.'" CAPTION: Picture 1, Ron Greschner; Picture 2, Ranger Bobby Sheeshan flies skates-first toward boards as action grows rugged. Mike Bossy is Islander No. 22. AP