On the eve of yet another big game against the New York Islanders, the Washington Capitals made a move today that seemed designed to light a fire under a couple of underachieving defenders.

Paul Cavallini, a 6-foot-2, 202-pound defenseman, left the Canadian Olympic team and signed with the Capitals. Cavallini, 20, immediately joined the Binghamton Whalers and Coach Bryan Murray indicated that if injuries should strike, he would not be afraid to put him in a Washington sweater.

"Paul is big and strong, the type of guy who in this division will be a real asset," Murray said. "The scouts tell me he will make a few people on our team nervous when they see him play, as far as a spot on our defense is concerned."

Murray made it clear he is not happy with his defense, beyond the first two pairs of Rod Langway and Larry Murphy, Scott Stevens and Kevin Hatcher.

"No, I'm not satisfied," Murray said. "Darren Veitch and Peter Andersson and Dwight Schofield can play, but they limit themselves. Darren is just not trying to do things I know he's capable of doing.

"Peter plays well some nights and others not at all. He's too inconsistent. He plays better with Rod or Scott, but as the third man on the left side he hasn't been up to that standard.

"If we have injuries, they'll have to upgrade their play. Peter is reluctant sometimes to work in the corners and on the power play he gives up the puck too early. He's held back by his unwillingness to be involved and in this division you have to be involved and pay the price.

"Darren will go in the corners, but he's too tentative. He seems to be questioning himself and he's almost passive at times. He has to show more strength and intensity."

Murray indicated that for the rest of the season, Schofield would play only against Philadelphia, as a left wing to offset the highly physical Dave Brown or Rick Tocchet.

"Dwight is a great person and a great guy who knows his role," Murray said. "He has a chance to make a contribution by his presence against the Flyers, but otherwise he doesn't figure to play much."

Cavallini, a 10th-round draft choice in 1984, played a year at Providence College before joining the Canadian Olympic team in August. Although he was committed to that program until May, his departure was arranged amicably with Coach Dave King.

"We were attempting to keep Paul with the Olympic team, but he wanted to get on with his career," Murray said. "Dave King did not object, because he has nine or 10 defensemen, he felt Paul wouldn't be there in 1988 and anyway he didn't feel he had the quickness to play international hockey on the big ice surface. Paul is more suited to the hitting game in the NHL."

Another 1984 draftee, right wing Steve Leach, finishes his sophomore season at the University of New Hampshire this weekend and he then is expected to sign with the Capitals.

It is a big weekend on the ice, too, and the back-to-back games against the Islanders here Saturday and at Capital Centre Sunday should provide clues not only on whether the Capitals can wipe out the current three-point deficit and catch Philadelphia for first place, but whether they are ready at last to bounce the Islanders in playoff competition.

Although the teams have split four games this season, Washington has lost both decisions to Bill Smith, the goalie who has been instrumental in eliminating the Capitals from the playoffs three years in a row.

Smith is 21-6-3 against Washington during the regular season and 8-3-0 in postseason competition. He seems likely to start Saturday, although Coach Al Arbour can be unpredictable. Arbour went with Kelly Hrudey here Feb. 18, when the Capitals rallied for a 5-4 victory, the Islanders' only home-ice loss in the last two months.

Asked whether thoughts of the Islanders still make him uncomfortable, Murray said, "I think we're a better team and I thought so last year, too. I thought we'd challenge them in the playoffs two years ago.

"The playoffs are still that type of trial where if you lose one critical game, it can kill you. For us last year, it was the fourth game blowing a 4-2 lead ."

Although goaltender Al Jensen and right wing Craig Laughlin practiced today at Mount Vernon before the Capitals flew here, Murray said he was reluctant to use them Saturday unless they were 100 percent healthy. Laughlin has been bothered by a sprained ankle, Jensen by a bruised knee.

Center Bengt Gustafsson, who missed Tuesday's 4-2 victory over New Jersey with a bruised hand, will play Saturday. Oilers 5, Penguins 3

Craig MacTavish scored the tie-breaking goal midway through the final period and Wayne Gretzky broke his own NHL single-season assist record to carry Edmonton to a home victory over Pittsburgh last night.

With 40 seconds left in the game and Pittsburgh goalie Gilles Meloche pulled for an extra skater, Gretzky had a clear shot at the empty net. Instead, he passed to winger Jari Kurri and collected his 136th assist of the season. Gretzky broke the record he set in 80 games last year in his 67th contest.

Kurri's goal was his second of the game and league-leading 53rd of the season.

With the score tied, 3-3, MacTavish skated out alone from the corner and flipped a backhander over goalie Meloche's outstretched leg on his short side at 11:49.

Norm Schmidt had tied the score 3-3 on a rising slap shot that fooled Edmonton goalie Andy Moog at 5:13. Whalers 6, Sabres 2

Dave Babych had one goal and two assists for Hartford. The victory, Hartford's fourth in four trips to Buffalo this season, gives the Whalers 64 points, four fewer than Buffalo in the race for fourth place and the Adams Division's final playoff spot.

Dean Evason, Doug Jarvis, Paul MacDermid, Sylvain Turgeon and Kevin Dineen were the other scorers for the Whalers, who have won three straight games, all against Adams Division teams.

Gil Perreault scored the 499th goal of his career, and Doug Smith added the other goal for Buffalo.