My 325Ci developed rod knock and a bad crank journal. Rebuilding the 2.5 made no sense when the M54B30 out of a 330 or an X5 drops right in, so the real project became sourcing a good donor and making the decisions that determine whether a swap lasts.
Vetting the donor
The donor was a clean local M54B30 with about 140k on it. Before committing I pulled the pan and valve cover and looked for the things that actually predict engine health, in order of what they tell you:
- Under the valve cover: no sludge, no coffee-colored varnish
- Oil pan dropped: no metallic glitter, no bearing material sitting in the bottom
- Oil filter cut open: pleats clean, nothing embedded in them
- Bores through the plug holes: crosshatch still visible, no scoring
That’s the engine equivalent of checking a used car’s service records instead of its paint.
The rod bearing decision
The internet will tell you to do rod bearings on any BMW while you’re in there. That advice comes from the S54 in the E46 M3, which had a genuine bearing defect. The M54B30 doesn’t. The failures in the forums trace back to low oil, overheating, or neglect, and plenty of these engines clear 200k on original bearings.
There’s also a practical catch people skip: the rod bolts are torque-to-yield, single use. You can’t pop a cap for a peek and put it back. Touching one bearing means new bolts all around, and TTY bolts stretch permanently by design, so reusing them is how you make a new problem. A healthy bottom end showing no symptoms earned the right to be left alone.
What got done while the engine was out
Everything you never want to pay labor on twice:
- All external seals and gaskets
- VANOS rebuilt, because the seals harden with age and every M54 needs it eventually
- Timing chain guides, even though the old ones looked fine. They’re brittle plastic and the labor to reach them is the whole job
- Water pump and thermostat
- Flywheel and clutch
- Oil pump nut safety-wired: a known M54 failure point that costs pennies to prevent and an engine to ignore
Making the 3.0 look factory
The swap itself is mix-and-match, and getting the combination right is what makes it factory-reliable instead of a science project. The 325 kept its own oil pan, engine mount brackets, and sensors, because those are chassis-specific. The 3.0 kept its intake manifold and injectors, because those are what make it a 3.0. Then the car got reprogrammed with the M54B30 calibration so the DME fuels and times for the engine it actually has, and it runs like it left the factory with the bigger engine.
Reliability isn’t a parts list. It’s a series of decisions about what to replace, what to leave alone, and why. The photos are on the project page if you want to see the process.