I can see where the confusion comes in on this one because as you start to read the parts catalog headlines of what engine sizes fits what HP or engine liter size (2.5 vs 3.0) ((153 vs 181)), the headline shows one numbering system then while searching the actual parts such as the oil pan, cylinder block, crank/cam, the numbers are reversed and more accurate as seen in the following,
https://www.mercruiserparts.com/120-gm-153-i-l4-1983-1986-6229718-thru
Mercruiser (120 HP / ???2.5L 140 HP???/ 3.0l R / Mr / Alpha) Engine Components
Using your serial number shows the model number to be 0
140200
6 which makes the engine to be a 1986 year model as indicated in this case via the last number of the model number is the year for back then (86)... The second, third, fourth numbers are the engine size of the time (140)... Looking over size details the 140 became more known as/is 3.0 in 1987 all in the 181 flavor.
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) |
Go Directly to Marine Longblocks in the Store……….
3.0L Marine Longblocks
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17-NOV-2003 - Manufacturer Service Bulletin: 3.0L/LX SERVICE REPLACEMENT CRANKSHAFT CHANGE
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