Mr. and Mrs. James S. Wey of Needham, Mass., have announced the engagement of their daughter, Susan M. Wey, to William Colyer Crum, son of Prof. and Mrs. Colyer Crum of Weston, Mass., and Sunapee, N.H.

A September wedding is planned in Needham. Miss Wey, an engineer at Honeywell Electro-Optics Operations in Lexington, Mass., was graduated cum laude from Wellesley College and next month expects to receive an M.S. degree in electrical engineering from Northeastern University.

Her father is with the trust department of Hale & Dorr in Boston.

Mr. Crum, who is a magna cum laude graduate of Harvard College, is a third-year student in the joint J.D.-M.B.A. degree program at the Harvard Law School and the Harvard Graduate School of Business Administration. His father is James R. Williston Professor of Investment Management at the Harvard Graduate School of Business Administration.