 After building a limited number of 1.5 L Twin Cams, the bore was increased to , raising the capacity to 1.55 L.

Assembly of the first 50 engines was contracted out to J.A.Prestwich.Prestwich also machined the raw cylinder head castings (cast by William Mills) on these early twin cam engines.The Twin Cam had its official debut at the Earls Court Motor Show in October 1962.