Social Security reserves will be razor thin in the years 1985 to 1987 and could fall a bit short even if Congress approves a proposed bipartisan rescue plan, experts warned the House Ways and Means Social Security subcommittee yesterday.

"The real danger is that the thing won't fly in 1985 and 1986," said Dwight Bartlett, former chief actuary of Social Security, after testifying for the American Academy of Actuaries, which supports the plan with some qualifications.

The plan proposed by a bipartisan presidential commission provides $168 billion over the next seven years to meet Social Security's short-run crisis. It also recommends steps to eliminate about two-thirds of its long-range deficit, which is estimated at 1.8 percent of taxable payroll over the next 75 years.

Bartlett said new estimates by Social Security actuaries, based on economic assumptions not originally available to the commission, could show that the plan as it stands will not produce quite enough money to keep the system solvent from 1985 to 1987 because the major new revenue source--advancing a portion of the scheduled 1990 Social Security tax increase--won't kick in until 1988.

Similar warnings were sounded by other actuaries and pension experts. Several also said they expect that the Social Security actuaries would estimate the long-term deficit as higher than 1.8 percent of taxable payroll.

Donald S. Grubbs, Washington director for George B. Buck Consulting Actuaries, said the possible short-term deficit makes it imperative to build an adequate "fail-safe" into the rescue legislation--a fact recognized by the commission.

He suggested allowing automatic borrowing from the Treasury, with an automatic Social Security tax increase to repay the loan if ordinary revenues fail to do so within a specified time. Others have suggested cutting Social Security recipients' cost-of-living increases to repay the loans.

In a related matter, Edwin C. Hustead, the former chief actuary of the Civil Service retirement system, told the subcommittee that while mandatory inclusion of new federal employes under Social Security would reduce new income somewhat for the existing Civil Service retirement fund it would also reduce future obligations enormously, since the federal government owes about $4 for each $1 a federal worker contributes.

He said short-term funding problems for the Civil Service fund would be "easily correctible" in a variety of ways at no cost to the federal government.

Hustead said the government cost for Civil Service pensions is about 29 percent of payroll, while a private company typically pays 17 to 22 percent of payroll to cover its share of Social Security payments for its employes plus a "better" type of private supplementary pension.