[2] From the village of Stone House at PA Route 8, PA 173 heads north-northeast in Butler County about 4 miles (6.4 km) to the borough of Slippery Rock, home to Slippery Rock University.

Routes 108 and 173 overlap, traveling northeasterly for 0.3 miles (0.48 km), where the concurrency ends and PA 173 turns northward.

After entering Mercer County, PA 173 continues northerly almost 3.5 miles (5.6 km) to the borough of Grove City, the site of Wendall August Forge and Grove City College.

This concurrency ends at the intersection of PA Route 208, where Routes 173 and 208 concurrency northward – for about 0.33 miles (0.53 km), the northbound and southbound lanes run on parallel one-way streets through the downtown area.

About 0.4 miles (0.64 km) later, the concurrency ends, PA 173 continues north, and leaves the borough.