Fastenal Co.'s third-quarter results met investor expectations, but then executives of the nut-and-bolts seller hit them with a surprise.

They told investors to expect a gross profit margin in a range of 51 percent to 52 percent, a tightening from previous range of 51 percent to 53 percent. As a volatile week in the market ended, investors sent Fastenal's shares down 4.8 percent for the day.

Edward Jones analyst Logan Purk called the drop an overreaction. Fastenal's operating costs are also expected to drop significantly, Purk said. and could lift its operating margin in the next few quarters. "Still, the market is just really hung up on this gross margin issue for now," he said.

In explaining the change, Winona-based Fastenal said its gross profit margin has been in the 51 percent to 52 percent range for most of the quarters since 2010.

"We are optimists; therefore, we previously disclosed that we believe a range of 51 percent to 53 percent would be normal," the company said in a statement. "However, given our performance, we now believe a range of 51 percent to 52 percent, or simply 'around 51 percent' would probably be more appropriate."

Profit for the quarter that ended Sept. 30 rose 11.7 percent to $133.3 million, or 45 cents a share, in line with consensus estimates. Sales rose 14 percent to $981 million.

The seller of construction and factory products said it added 1,896 employees from a year ago due a strategy of bolstering customer service. It now has 18,425 workers nationwide.

Fastenal reduced the number of its retail stores by 1.5 percent to 2,647. It boosted the number of its industrial vending-machines placed inside customers' shops. Machines grew 16.4 percent from a year ago to 45,596 units. The machines cost less to operate than a store and allow customers to quickly resupply commonly used factory parts as needed, without having to schedule special deliveries or go to a Fastenal store.

"We believe industrial vending will be an important chapter in the Fastenal story," CEO Willard Oberton said in a statement. "We also believe it has the potential to be transformative to industrial distribution, and that we have a first mover advantage. Given this we have been investing aggressively to maximize the advantage."

Dee DePass • 612-673-7725