The commonest site of obstruction in Gallstone ileus is -

Although Harrison writes the most common site to be the ileocecal valve; Sabiston, Bailey & Maingot's write 'distal ileum' to be the most common site. "The site of obstruction is usually the terminal ileum, just proximal to the ileocecal valve. This corresponds to the segment of the intestine with the smallest diameter." Bailey & Love write, "classically there is impaction about 60 cm proximal to the ileocecal valve." Also, know the commonest site for gallstone impaction in CBD - Ampulla of Vater. Ref : Maingot's 10/e, p 1736