The Reagan administration's capacity for borrowing nuclear trouble has been exercised once again in the threat to dispense later this year with the limits of the unratified SALT II treaty.

The timing is horrible -- as if some gremlins in Caspar Weinberger's shop could hardly bear the sight of the Russians stewing on the nuclear griddle after Chernobyl and had jumped to seize the spotlight for us.

Every NATO ally is alarmed and annoyed. Yet with truly sublime obtuseness, Secretary of State George Shultz dismisses this fraternal unrest as having more to do with ''imagery'' than with ''content.''

Depending on what one means by ''imagery,'' Shultz's distinction falls somewhere between absurd and untenable. Nuclear policy, especially in matters relating to deterrence, is 80 percent a question of impressions -- or ''imagery,'' if you prefer. Despite the occasional burst of madcap talk about the feasibility of successful nuclear war, nuclear weapons exist only to prevent their own use. In that connection, ''imagery'' is all-important.

For Europeans, the primary value of the SALT II treaty likewise has less to do with the esoteric technical limits it imposes than with what it symbolizes. In European eyes, arms control agreements betoken that the United States and the Soviet Union, with their globe-busting arsenals, can observe modest regimes of restraint.

The untimely U.S. threat to renounce the SALT II limits seems to be the byproduct of a struggle in the administration. After strenuous debate, President Reagan recently decided to observe the limits by scrapping two old nuclear submarines as the latest Trident begins sea trials. The anti-arms-control faction wanted the two Poseidons mothballed. It would appear that they extracted a presidential vow that this would be the last gesture of self-restraint -- that the United States will jump the fence next December when bombers armed with new cruise missiles will enter the fleet and push total U.S. ''launchers'' through the SALT ceilings.

The declared excuse for the breakout, however, is a remarkably hackneyed one -- Soviet ''violations'' of SALT II of which Perle, Weinberger & Co. have been complaining for years. One compliance issue is a Soviet missile that we call the SS-25. We claim that it is a second new heavy missile, violating SALT restraints. The Russians insist it's a remodeled SS-13.

But even if the violations are substantial, and resistant to the usual consultative procedures, they need balancing against the treaty provisions that are observed.

In the 1980 campaign, Reagan denounced SALT II as ''fatally flawed.'' But by May 1982, he had been persuaded that it included limits of genuine value to the United States -- even in the view of hard-headed military and intelligence professionals.

Those limits are still working. SALT II sets missile ceilings that the United States has been able to build up to, while the Soviets were forced to build down to them. The ''counting rules'' make the Soviet arsenal easier to verify. And the limitation on the numbers of missiles that can be ''MIRVed'' (armed with multiple warheads) is advantageous, as is the limitation of ''MIRVed'' missiles to 10 warheads. (U.S. experts believe the Soviet monster missiles are capable of carrying more than 10. That's part of what the throw-weight debate has been about.)

In the event of a breakout from the SALT II limits, the Russians could more easily and quickly augment their missile-striking power. Even if this merely stacked redundancy upon redundancy, in the usual Soviet fashion, it would not be reassuring to those who gauge nuclear dangers in quantitative terms.

But again, the greater danger is political. Theju embers of unilateralism in Europe are there to be fanned by abrupt and ill-considered changes of U.S. policy. European discontent could express itself in jeopardy to the strongly pro-NATO governments in Britain and West Germany, both of which face elections next year.

Thus the ''imagery'' that Shultz dismisses is terribly important. We can revoke this serious diplomatic blunder now, or pay the price for it later.