International Harvester Co., the large farm equipment and truck manufacturer, yesterday reported a net operating loss for fiscal 1981 of $636 million compared with a loss of $375 million a year earlier.

The preliminary, unaudited figures also showed fiscal 1981 sales of $7.041 billion, up 18 percent from $5.968 billion in 1980, when the firm endured a prolonged strike.

However, the net loss for fiscal 1981 was $393 million, slightly less than the net loss of $397 million in fiscal 1980, largely because of the sale of the firm's Solar Turbine Division, as well as other considerations.

The net loss for the fourth quarter was $319 million compared with net income of $20 million (64 cents a share) for the same period in 1980. Salkes dropped to $1.777 billion from $2.211 billion a year earlier. The final quarter of fiscal 1981 was the low point of the year for demand in the company's major markets, company officials reported.

The company blamed depressed markets worldwide, high interest costs and industrywide price discounting for the continuing losses.

Harvester officials said the firm reported minimal income tax benefits in 1981 compared with a benefit of more than $300 million in 1980.

IH Chairman Archie R. McCardell said the figures are similar to projections given earlier this year to the firm's creditors as part of an effort to restructure Harvester's $4.1 billion debt.

"Terms of the restructuring arrangements are being placed in final form, and we intend to distribute signature copies to the banks next week," McCardell said.

IH has experienced several delays in putting together the new financial package. The company has asked the United Auto Workers for "considerations" that would cut its labor costs by up to $100 million. The union has not formally responded.