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Main bearings carry the crankshaft inside the engine block. When they wear, the engine makes a dull, heavy knock under load and oil pressure sags when hot. Route 66 Auto Repair in Upland confirms main bearing wear with a pressure test and a listening test, then measures clearance once the bottom end is open. ASE-certified technicians explain whether the crank can be reused. Because most main bearing jobs mean removing the engine, your written estimate shows every step before work begins.
How main bearing wear shows up
- A low, heavy knock that is loudest when accelerating uphill
- Oil pressure light at hot idle after a long drive
- Knock that does not change when a single cylinder is disabled
- Crankshaft end play you can feel at the pulley
- Bearing material in the oil pan or filter
How is main bearing knock different from rod knock?
Rod knock is lighter and sharper, and it often softens when the spark or fuel to that cylinder is cut. Main bearing knock is deeper and tied to load, and cutting one cylinder usually does not change it. We use both tests along with oil pressure readings. The rod side is covered on connecting rod bearing repair.
What about the thrust bearing?
One main bearing also controls how far the crank can slide forward and back. If it wears, you may notice clutch pedal changes or a clunk when shifting. We measure end play with a dial indicator. More detail is on crankshaft thrust bearing repair.
Our main bearing replacement process
Step 1: Confirm the knock
Oil pressure, cylinder cut-out and stethoscope tests point to the mains.
Step 2: Remove and open
On most vehicles the engine comes out and the bottom end is exposed.
Step 3: Measure the crank
Journals are measured for size, taper and out of round, and photos are taken.
Step 4: Install and check clearance
New bearings go in, clearance is verified, and caps are torqued in sequence.
Step 5: Reassemble and test
The engine is primed with oil before start-up, then road-tested.
What's included in main bearing replacement
Included on the work order
- 01Oil pressure and knock diagnosis
- 02Crank journal measurements and photos
- 03New main bearings and thrust bearing
- 04Main cap bolts where the manufacturer calls for new ones
- 05Oil pump pickup inspection
- 06Oil prime before first start and a road test
Need-now or need-later?
Need now: call
- Knock with low oil pressure, since the crank can seize
- Crank end play past the limit
Need later: book
- A rear main seal seep noted while the engine is out, often done at the same time
- Worn motor mounts that are easier to change with the engine out
How pricing and timing work
Engine removal is the largest labor line. If the crankshaft journals are damaged, crank work or a replacement crank is added, and a full engine rebuild may make more sense. Your written estimate shows every step and each option. AAA members get 10% off parts and labor, up to $75, with the card at time of service. Expect several days; we text you if the board changes.
Aftercare
Drive gently for the first few hundred miles and follow the early oil change we list on the invoice. Watch the oil pressure light. Any knock or warning goes to the front of the line.
Frequently asked questions
Can main bearings be replaced without pulling the engine?
On a few engines, yes, with the oil pan off. On most modern cars, the crank and mains are reached only with the engine out. We tell you which applies before quoting.
Why did my main bearings wear out?
Common causes are low oil level, long oil intervals, a weak oil pump and debris in the oil. We look for the cause so the new bearings do not fail the same way.
Is it worth fixing an engine with main bearing knock?
It depends on the crank's condition and the vehicle's value. We give you repair, rebuild and replacement options in writing so you can compare.