There were continuous improvements in the design of the block (and heads, etc.), and each time a minor change was made, a new casting number was issued, as one or more of the cores or final machining operations were affected; standard GM practice. The same casting number block was frequently produced by two or more separate foundries during the same model year, as it took three foundries and five engine plants to produce the volume required for Chevrolet daily car and truck production and service requirements. The Saginaw Foundry fed Flint V-8 and the Flint Motor Plant (6-cylinders) and the Saginaw Foundry also fed the Saginaw Service Engine Plant, the Tonawanda Foundry fed the Tonawanda Engine Plant, and the McKinnon Industries (St. Catherines Engine Plant today) Foundry and Engine Plant in Canada produced many engines for both Canadian and U.S. assembly plants. Chevrolet built 20,000 cars and trucks every day in the 60's; Flint V-8 and Tonawanda each built 5500-6000 engines per day, and the other plants picked up the balance.