The engine serial number is usually a ten digit number found on a data plate on the valve cover or stamped onto the side of the block. The serial number found on the block is preferred as it is most reliable (valve covers may have been switched at some time). The model number is a 8-digit code found with the serial number.  Following is the Detroit Diesel's, and Allison Transmission serial number system:

Detroit Diesel

 

Allison Transmission

MODEL S/N Layout   MODEL S/N Layout
Series 60 06Rxxxxxxx   AT 321xxxxxxx
Series 92

 

08VFxxxxxx
The first two digits are the number of cylinders:

06, 08, 12, or 16

 

HD, 4000MH 661xxxxxxx
Series 71 VEE

 

06VAxxxxxx
The first two digits are the number of cylinders:
06, 08, 12, or 16
  HT 243xxxxxxx
Series 71 in-line 06Axxxxxxx   MD, B300, 400, 500, 3000MH, 651xxxxxxx
Series 149

 

16Exxxxxxx
The first two digits are the number of cylinders:

06, 08, 12, or 16
  MT 243xxxxxxx
8.2L 08Gxxxxxxx   1000, 2000, 2400 SERIES 631xxxxxxx
Series 2000
8V
12V
16V

531xxxxxxx

535xxxxxxx
536xxxxxxx
     
Series 4000
12V
16V

526xxxxxxx
527xxxxxxx