1. Casting ID number (D7TE-9425-SA). All cast parts originally designed for truck M-blocks use the D7TE prefix.

2. Casting date code (8G14) indicates this manifold was cast on July 14, 1978.

3. Foundry mark (MCC) and cylinder firing order.

4. Cylinder numbers corresponding to the firing order are marked on top of each intake runner, just before it meets the head.

1. Main manifold vacuum supply port at rear.

2. EGR port in carburetor mounting flange. This port is drilled into the exhaust crossover below the plenum.

3. Exhaust crossover access for automatic choke hot-air tube.

4. Auxiliary manifold vacuum port. Sometimes used to provide manifold vacuum for carburetor power valve.

5. Thermactor (AIR) air injection port.

This picture shows how the main manifold vacuum port on the back of the stock M-block intake manifold taps into both planes. From the main port, holes are drilled into the intake runners for the #4 and #8 cylinders.

If you had a problem with the intake valve on either of those cylinders, it could disrupt the vacuum supplied by the main manifold vacuum port. The major consumer of that vacuum is the power brake booster, but it is also used to provide a vacuum signal and vacuum supply for various emissions control devices.

Up to MY1974, the Thermactor AIR system on 335-series engines (351C and 400) used the ports at the rear of the cylinder heads to inject fresh air.

The M-block intake manifold was redesigned in 1975 to use the ports at the front of the cylinder heads for Thermactor AIR, but bosses for the earlier system remained at the rear of the manifold. They were adapted to serve as mounts for throttle brackets, automatic transmission kickdown rod guides, and body-to-engine electrical grounding straps.