Financially troubled International Harvester Co. yesterday reported a $497.7 million loss for the first half of its fiscal year and predicted "substantial losses" for the last half.

The prediction was in marked contrast to the company's earlier forecast of a more prosperous future for the company.

Harvester's management said yesterday that the huge farm machinery manufacturing firm lost $198.3 million in the second quarter compared with a $79.2 million loss for the 1981 quarter. The six-month loss compared with a loss of $175.6 million in the same period a year before.

The company reported sales of $1.5 billion in the second quarter and $2.5 billion for the first half. That compared with second-quarter sales of $2.1 billion and first-half sales of $3.7 billion last year.

Harvester has "realistically revised its previous estimates of operating results for the full year," and "present expectations are for substantial losses from continuing operations in each of the last two quarters, though of much less magnitude than those of the first two quarters," said a statement by the company's two top officers.

Former Harvester chairman Archie R. McCardell had predicted "a return to profitability" by the end of the fiscal year when he addressed Harvester's annual meeting last February.