A 300-pound intermediate pressure stop valve disk was on its edge, braced against a 6-by-6 timber, while an employee measured the internal seat angle with a protractor. He had reached into the valve disk bore with his right hand, from the bottom side of the disk. As he was removing his hand, the timber slid away, and his index finger was caught between the disk bore and the protractor as the disk slid down the timber. He suffered a partial amputation to the finger.