In a factory in Hekinan, a seaside town southwest of Tokyo, machines spit hot metal rods into baskets. On a recent Monday, afternoon light poured in through yellowed windows, and the air inside was hot and smelled of metal shavings.

The plant, operated by an 84-year-old Japanese automotive parts maker, Asahi Tekko, produces components that are fitted into Lexus and Land Cruiser models at nearby Toyota factories. Many of those vehicles are then shipped to the United States.

For decades, the route has been one of many supply lines that underpin the automotive industry. Now, however, it is a source of unease for Asahi Tekko, as the company grapples with recent fluctuations in U.S. trade policies.

When President Trump laid down a 25 percent tariff on Japanese automobiles and parts this year, Tetsuya Kimura, the chief executive of Asahi Tekko, thought, “Is he actually going to do something like that?” Mr. Kimura recalled. He added, “The thinking seemed so oversimplified.”

Asahi Tekko has not yet felt the full effect of the tariffs, which are to be set at 15 percent as part of Japan’s recent trade agreement with the Trump administration. Still, the uncertainty is unnerving. Mr. Kimura fears they could hurt American demand for cars and cut the need for his parts.

“When it comes to how much impact these tariff rates will have, we don’t have a calculus,” he said.