This picture shows typical casting marks on a '75-up M-block cylinder head from the Michigan Casting Center (MCC).

1. Casting date code. "G11" indicates July 11th. Notice the year digit is missing.

2. Foundry mark. All later M-block cylinder heads were cast at either the Michigan Casting Center (MCC) or the Cleveland Foundry (CF).

3. No mark on the upper corner of the head. Some M-block heads have a raised "M" in that location.

1. Casting ID code. "D5AE A-2-A" identifies the specific casting design. All M-block cylinder heads manufactured after the 1974 model year (including all 351Ms and all truck engines) use the D5AE casting ID prefix.

2. No mark on the upper corner of the head. All 351C cylinder heads have either "2" or "4" in that location to identify 2V heads and 4V heads. Only (some) M-block heads were made with no mark in that location.

Starting in MY1978 through the end of M-block production in the 1982 model year, Ford used different valve springs and retainers on M-block intake and exhaust valves. Before that period, M-block intake and exhaust valves used the same springs and retainers.

This picture shows the difference between the intake valve spring retainer (right circled) and the exhaust valve spring retainer (left circled).

These '78-up exhaust valve springs have 0.13" shorter free height and solid height, and 0.14" shorter installed height than the intake valves.

Notice the accumulation of soot in the bottom of the exhaust ports. This is really a dramatic illustration of the flow path in the exhaust port.

Exhaust gas tends to follow the upper surface of the exhaust port, and the hot exhaust gas flowing continuously across those surfaces keeps them clean and free of deposits. In contrast, exhaust swirls around in the lower part of the port, cooling and lingering long enough to deposit soot and other residues on the lower port surfaces. This problem is more pronounced in 351C 4V exhaust ports.

This is the standard large M-block combustion chamber (78cc), with the standard 2V valves (intake 2.04" diameter, exhaust 1.65" diameter). Because of the depth of these chambers, the first 0.02" to 0.03" milled from the deck face reduces unswept volume by the height of a cylinder.

The angle of the spark plug hole, and obstructions above the hole on the outside of the cylinder head, preclude inserting a rod (even as thin as a coat hanger) in line with the bore axis for an accurate stroke measurement.

On M-block engines equipped with the Thermactor AIR (air injection reaction) system, fresh air is injected directly into the exhaust ports in the cylinder heads. Fresh air is routed to the exhaust ports through AIR system passages in the cylinder head. These passages are drilled through the cast iron head.

The main distribution passage is drilled all the way through the cylinder head, from one end to the other, just above the exhaust ports. The arrow in this picture identifies one end of that main passage.

The pictures on the right illustrate the locations of the drilled AIR system passages in the cylinder head.

Top: From the top of the cylinder head, bosses are visible for the AIR system passages drilled across the head, from the intake manifold mating surface to the main distribution passage.

Bottom: The yellow lines show where the AIR system passages are drilled.

Beginning in MY1973, an AIR “bump” was added to the top of the exhaust port in M-block cylinder heads. The AIR bump was connected to the AIR system main distribution passage above the exhaust port by a hole drilled through the bump.

This picture shows AIR bumps in two adjacent ports. The bump in port #1 is not drilled, while the bump in port #2 is. One exhaust port feeds the exhaust crossover (second from the right when facing the ports), and the AIR bump was not drilled in that port.

1. Thermactor AIR passages at each end of the cylinder head. These passages are drilled from the intake side across to the main distribution passage above the exhaust ports.

2. Exhaust crossover passage. This passage feeds exhaust gas from an exhaust port to the crossover in the intake manifold. The exhaust port connected to the crossover passage is not connected to the AIR system.

Just above the exhaust crossover port on the cylinder head intake face is a blind hole (arrow). It is partially exposed above the edge of the intake manifold when the manifold is installed.

The purpose of this hole is to relieve heat from the cylinder head’s exhaust crossover passage. Some people are tempted to plug it or fill it, perhaps to keep debris from accumulating in it. However, that would defeat the purpose of the hole, and it would aggravate the heat problems inherent in the exhuast crossover design.