The Teamsters union and major trucking companies renegotiated their week-old labor contract, adding a pay raise for part-time employees and converting a share of the hourly wage gain into a bigger benefits package.

The unexpected changes in the new labor agreement came less than a day before a scheduled ratification vote late Thursday by the union's 277 local presidents in Chicago. If the local leaders accept the contract, it next goes to the union's 200,000 drivers and freight handlers for final approval.The pact was reworked Wednesday after top union leaders apparently objected to the distribution of monetary increases negotiated in the original contract.

The hourly pay rate for casual, or part-time, workers had been frozen at $12 an hour under last week's agreement. Casuals now will receive wage increases of 35 cents an hour in each of the pact's three years.

The revised contract also will lower the first-year wage gain for full- time workers to 35 cents an hour from 50 cents an hour. That reduces the wage hike to about 2.4 percent from the previous 3.4 percent.

Cost-of-living increases in the second and third years, which could produce gains of up to 35 cents an hour each year, remain unchanged from the original pact.

To offset the lower wages, employer contributions for health and welfare and pensions will rise to 35 cents an hour in the first year from the previously negotiated 20 cents an hour. Second and third-year hikes of 20 cents an hour are unchanged.

The net effect of the revisions will vary for each company, depending on the use of casual workers and the amount of employee overtime.

The hourly wage increase for casuals, for example, is a big plus for the union and will raise the price of the new contract for trucking companies.

But shifting 15 cents from wages to benefits will help employers, who make health and pension contributions only on the first 40 hours worked but pay wages for regular and overtime hours.

The biggest factor in the new contract is the change in pay rates for employees hired during the last three years. These entry rates were unchanged during Wednesday's renegotiation.

Under the expired agreement, newly hired workers received 70 percent of the standard pay rate their first year, rising to 80 percent the following year, then to 90 percent before reaching top scale after three years.

The new pact sets an entry-rate scale of 85 percent, 90 percent and 95 percent, compressed within 18 months. The change applies to all current workers, which means thousands of employees hired after Oct. 1, 1986, will receive substantial raises, some as much as 30 percent.

For example, a worker hired eight months ago had been receiving 70 percent of the top rate of $14.71 an hour, or $10.29 an hour. Under the new scale, he now will earn 90 percent of $15.06, or $13.55 an hour.

Richard Holt, a transportation analyst with Prudential Bache Securities in New York, estimated the actual first-year wage and benefit increases for major trucking companies, based on the numbers of recently hired employees who will scale up immediately under the new entry rates.

ABF Freight System Inc. of Fort Smith, Ark., and Carolina Freight Carriers Corp. of Cherryville, N.C., will be hurt the most, facing overall labor cost hikes of 7 percent. Roadway Express Inc. of Akron, Ohio, and Yellow Freight System Inc. of Overland Park, Kan., each will absorb wage increases of more than 6 percent.

Mr. Holt's calculations do not include casual workers or the effects of subsequent scale increases later in the year.