Not all THM350 or 400's for that matter had the same DRIVE gears. They were different. It also varied by year. In 1972, the BB, and BS 400 had a 16 tooth DRIVE gear. The BC, BT, and BU 400's(All big cars in 72) all had 18 tooth DRIVE gears. My 1967 BT switch pitch had a 17 tooth DRIVE gear.