The Arlington County Board, in a vote along party lines Saturday, divvied up a $4.2 million surplus from the last fiscal year, appropriating most of it to one-time expenses and another chunk to shoring up the county's insurance policies.

In the past several years, the county has ended its fiscal year with a surplus, largely because of the conservative estimates of its financial staff, whose projections of revenue and costs for fiscal 1985, which ended June 30, were off only one-sixth of 1 percent.

Michael E. Brunner, the board's only Republican member and the dissenter at last week's meeting, tried unsuccessfully to get the four Democrats to agree to set aside a contingency fund of $1 million, then $500,000, for cuts in the personal property tax rate next year. The tax, currently $4.65 per $100 assessed value, is applied to motor vehicles, boats, planes and office equipment.

The Democrats said they would probably cut that tax rate and others next year anyway, but that they did not want to reduce reserve and contingency funds at this time. They did, however, set aside $350,000 toward cuts to be made next month in business and professional taxes.

Of the $4.2 million, $1.1 million came from unspent school funds, as revenues exceeded projections; $1.9 million from lower than expected health insurance costs for both the county and schools and $1.1 million from unanticipated income from taxes, said Mark Jinks, the county's budget director.

After recommendations from County Manager Larry J. Brown, the board agreed to allocate the surplus to:

*Increase the General Fund operating reserve account by about $2 million, leaving it just short of the goal of achieving a $5 million cushion for "rainy days" and emergencies, such as a dramatic reduction in federal aid or an economic slump, Jinks said.

*Add $500,000 to the General Fund contingency account to bring it to $1.5 million. This account sets aside money for expenses that were unforeseen when the budget was adopted. Up to $500,000 of this fund may be set aside for the transfer of over half of the police force to temporary quarters next year when construction begins on the Court House Plaza complex on a county employe parking lot.

*Add $500,000 to bring the county's self-insurance fund to $1.5 million. Jinks said the increase should help the county get extra insurance coverage if it assumes a larger deductible amount, thereby becoming a lower risk.

"Although the county's record is excellent, insurance companies tend to look at all markets the same way," Jinks said. "We think this will make them say they feel comfortable giving us an extra layer of coverage."

*Add $500,000 for increased liability and property insurance premiums at a time when there are increasingly higher claims and awards against governments. The county is trying to get a better umbrella policy for automobile coverage, which may exceed the $475,000 budgeted for it now.

Like other localities nationwide, Brown said, the county has experienced "extraordinary price increases or unavailability of certain levels of liability insurance" recently.

*Set aside almost $290,000 for tax refunds resulting from prorated personal property taxes, readjusted real estate assessments and business tax refunds.

*Transfer $185,000 back to the schools and another $96,000 for miscellaneous expenses.