1966 Chevelle

V8 Block & Head Casting Numbers

The block casting number on small block and big block Chevrolet V8 motors is located on a ledge found at the rear of the block, most often on the driver's side. This ledge is below the deck of the block and the ledge forms the mating surface between the block and the transmission bell housing.

No current information about 6-cyl castings are available.

Casting Number Engine Size Engine Horsepower Main Caps/Information
3834810 283 195 - 220 2-bolt main bearing caps only used by Tonawanda engine plant.
3849852 283 195 - 220

2-bolt main bearing caps. When used with nodular iron crankshaft the block was relieved at the bottom of the cylinders for counterweight clearance. When used with a forged steel crank there are no reliefs.

3857656 327 275 - 350* * March 1966 introduction
3858180 327 275 - 350* * March 1966 introduction
3855961 396 325 - 360 2-bolt main bearing caps
3855962 396 360** - 375

4-bolt main bearing caps high performance casting. Some, if not all, 360hp engines used this block even though it was not call for that application by Chevrolet.

File photo - Block casting #3834810
Block casting 3834810 ~ 283 195/220hp. Also note casting date G35 for July 3, 1965. This block was only used by Tonawanda and this block was also used in 1964 and 1965 applications.

File photo - Block casting #3855961
Block casting 3855961 ~ 396 2-bolt (325/360hp) main caps.

File photo - Block casting #3855962
Block casting 3855962 ~ 396 4-bolt (375hp) main caps. Reportedly used in 360hp applications as well. Due to scheduling and parts being available, 4-bolt blocks were sometimes substituted for 2-bolt applications but never the other way around. Many times during a production run a 2-bolt block was drilled for a 4-bolt application with no documentation to verify the change. The 3855962 block is a good example of this. It is believed the letter "P" indicates the block was destined for passenger car and not a truck, but no documentation has surfaced to confirm/deny this.

** It has been verified that this block was installed in some 1966 Chevelle 396/360hp engines. This caused some problems for Chevrolet Service. See Engine Parts > Block for more information on this block.

Engine Assembly Date/Suffix Code:
The engine assembly code looks similar to T0422ED. In this case the first letter "T" stands for an engine factory Tonawanda, an "F" or "V" for Flint or "K" for McKinnon Industries. The next two digits indicate the month of the year, in this example "04" for the fourth month, April. The "22" indicates the 22nd day of the month. So what do we have so far in this example? The motor was assembled at the Tonawanda factory on April 22. The calendar year itself is not given but remember that the engine assembly had to be manufactured and assembled before the car was built. What we have left is the "suffix code" which in this example is ED, a 1966 396cid with 325hp and coupled to a manual transmission. Chevrolet used two letters to code the type, size, and horsepower rating of the engine and the transmission type to be used with that engine.

The final vehicle assembly plant sometimes stamped a VIN derivative on the same machined area as the engine ID code. This stamping wasn't mandated until the 1968 model year but has been found (and not found) on 1966 models. If yours has the VIN derivative (and hasn't obviously been restamped), you can be sure the engine is original to your Chevelle. The VIN derivative might look like "T0422ED   6A123456" stamped into the pad on the front passenger side of the block. In this example, 6A123456 decodes to 1966 model year, Atlanta final assembly, and the sequence number of the VIN. Sometimes the VIN derivative was stamped into a different area of the block on the flange edge of the rear of the block, near the bell housing mating surface. Sometimes the stamping is found above the timing cover mating surface on the front of the block.

While NOT a hard and fast rule, it has been found that mainly the higher horsepower engines such as the 327/350 RPO L79 and all SS396 engines have a VIN derivative stamping. Mandatory VIN stamping of all engines was not required by federal law until the 1968 model year.

V8 Cylinder Head Casting Numbers

Casting Number Engine Size Engine Horsepower valve Size (intake/exhaust) - combustion chamber volume
3884520 283 195 1.72/1.50 - 60.530 c.c.
Pyramid casting symbol
3782461 327 275
350 (late March)
Double-hump casting symbol produced to about June, 1966.
3890462 327 275
1.94/1.50 - 62.076 c.c.
2.02/1.60 - 62.076 c.c.
Used after January, 1966.
3872702 396 325 - 360 2.06/1.72 - 98.427 c.c.
Oval port, spark plug shield boss, no temp sending boss.
3873858 396 375 2.19/1.72 - 108.989 c.c.
Square/rectangle port


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