1961 to 1966 Nailhead Identification
There are a couple of ways to identify which Nailhead engine you have. One is to check the block casting number. The other is to check the letters stamped at the front/top of the block near the valley pan.
Block Casting Number
The block casting number is found on the bellhousing - just behind the distributor. See pictures below.
1196546 - 401 block casting number for 1961 engines
1349046 - 401 block casting number for 1962 engines
1354714 - 425 block casting number for 1963 engines
1364704 - 425 block casting number for 1963 to 1966 engines
1364705 - 401 block casting number for 1964 to 1966 engines
These displacements were verified by measuring the bore. This list is not meant to be inclusive of all casting numbers!
NOTE! The 1364705 casting is NOT a 425!
1196546 - 401 block casting number for 1961 engines
1349046 - 401 block casting number for 1962 engines
1354714 - 425 block casting number for 1963 engines
1364704 - 425 block casting number for 1963 to 1966 engines
1364705 - 401 block casting number for 1964 to 1966 engines
These displacements were verified by measuring the bore. This list is not meant to be inclusive of all casting numbers!
NOTE! The 1364705 casting is NOT a 425!
Letters Stamped Near The Valley Pan
Just in front of the valley pan at the top of the block (under the water crossover) you will see two letters and a few numbers stamped. These were stamped during machining/assembly to identify which year and displacement engine you have.
Here are some of the codes:
4H - 1961 401 (4 Barrel Carburetor)
4I - 1962 401 (4 Barrel Carburetor)
2I - 1962 401 (2 Barrel Carburetor)
JT - 1963 401
JW - 1963 425
KT - 1964 401
KW - 1964 425
KX - 1964 425 with dual quad carburetors
LT - 1965 401
LW - 1965 425
LX - 1965 425 with dual quad carburetors
MT - 1966 401
MW - 1966 425
MZ - 1966 425 with dual quad carburetors
Here are some of the codes:
4H - 1961 401 (4 Barrel Carburetor)
4I - 1962 401 (4 Barrel Carburetor)
2I - 1962 401 (2 Barrel Carburetor)
JT - 1963 401
JW - 1963 425
KT - 1964 401
KW - 1964 425
KX - 1964 425 with dual quad carburetors
LT - 1965 401
LW - 1965 425
LX - 1965 425 with dual quad carburetors
MT - 1966 401
MW - 1966 425
MZ - 1966 425 with dual quad carburetors
Confusion Over Square Boss On The Bellhousing
Some think that if there is a square casting boss (or bump) on the bellhousing - then you must have a 425. This is not always the case. See the pictures below. This 1966 401 has a square casting boss. The first picture below shows the square boss at the left. The second picture below clearly shows the block casting number. This engine is not a 425! Measuring the bore and using the engine code letters stamped at the front of the block confirm that it is a 401. Note the lifting eye that is cast into the block - this is a 1966 only feature.
1964 425 Casting Boss
In the two pictures below you will see the casting boss on a 1964 425 block. Note the difference between it and the 1966 401 block shown above. 1963, 1964, and 1965 425 blocks will have a boss that looks like the one below.
1966 425 Casting Boss
The 1966 425 block has two bosses adjacent to each other. See both pictures below of a 1966 425. Note in the second picture that the square boss is about half as tall as the rectangular boss. Note that the casting number is 1364704. The lifting eye cast into the block is a 1966 only feature.
1963 425 With Rare Casting Number
See the pictures below showing the bore and casting number of this 1963 425 block. It has an uncommon casting number.
