Ok, this is what I found out in case anyone is interested.

There are two main codes on a block that can help identify a 4.2L engine

1st. ENGINE DAY BUILD CODE. This is located on the passenger side of the block just below and between #3 and #2 spark plugs holes. It's a flat plate and has this format:

The Engine Day Build Code consists of six characters:

1. Year built code
2 & 3. Number of the month
4. Engine size/type code
5 & 6. Day built

Mine is 012C05

Which comes out to:

0=1980 or 1990
12=Dec
C=2v carb
05=day of the month built

See this site for more info:
http://home.att.net/~farna/amtech/amcengcodes.htm

I was unclear on whether this would be a 1980 or 1990 engine. To narrow it down I located the Engine Serial number (2nd main code) which is located right behind the O2 Sensor on the Drivers side of the block.

Mine reads: EF3235444

When I looked for this number on remanufactured engine sites I see that this block serial number started in 1981 through 1990.

Therefore the best determination I can make is this engine is a 1990 made engine.


HTH,