Mercruiser Block ID Codes – 4 Cylinder Engine
The model of Mercruiser 3.0L 4 Cylinder Engine can be determined by looking at the last letters of the engine code which is stamped in all complete Marine Longblocks by General Motors at the time of assembly. If the serial number and/or model decals are missing the marine engine code will help to determine the engine model. Refer to the drawing for engine code location.
Information you need from the engine code, example M01187MH. 7=General Motor’s model year in this case 1997, BUT this could also be 1987 or 1977. MH = Type of engine. The 350 cid V8 Mercruiser models 5.7L, 350 Mag Mpi, Black Scorpion Ski, etc.
GM 4 Cylinder 153 cid (2.5L)
Sterndrive Models (LH rotation), flat lifters
| Model | GM Code | Comments | Flywheel Diameter in. (mm) |
| 120 | NB | 1974–1986 | 12–3/4 (324) |
| 2.5L | NB | 1987–1989 | 12–3/4 (324) |
GM 4 Cylinder 181 cid (3.0L) with 2–piece Rear Seal
Sterndrive Models (LH rotation), flat lifters, 3.0L cyl. Head
| Model | GM Code | Comments | Flywheel Diameter in. (mm) |
| 140 | RA | 1974–1986 | 12–3/4 (324) |
| 3.0L | RS | 1987 | 12–3/4 (324) |
| 3.0L | RA | 1987–1989 | 12–3/4 (324) |
| 3.0L | RF | 1989 | 12–3/4 (324) |
GM 4 Cylinder 181 cid (3.0L) with Full Circle Rear Seal
Sterndrive Models (LH rotation), flat lifters
| Model | GM Code | Comments | Flywheel Diameter in. (mm) |
| 3.0L | RM | 1990–1991, 2.5L cyl. head, start full circle rear seal | 12–3/4 (324) |
| 3.0L | RJ | 1992–1994, 2.5L cyl. head | 12–3/4 (324) |
| 3.0L | RX | 1998, 3.0L cyl. head, start stamped rocker arm cover | 12–3/4 (324) |
| 3.0L | RP | 1999–UP start 14” flywheel, staggered starter mount | 14 (356) |
| 3.0LX | RN | 1990–1991, 3.0L cyl. head, start full circle rear seal | 12–3/4 (324) |
| 3.0LX | RK | 1992–1997, 3.0L cyl. head | 12–3/4 (324) |
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