Main Bearings

Has your oil pressure dropped, or does the engine need to be stripped and rebuilt? The main bearings support the crankshaft in the engine block and hold the oil film the crankshaft runs on. This category brings together main bearing sets in standard size and for reground crankshafts, both for engine rebuilds and for projects with more power.

What the main bearings do in the engine

The main bearings sit between the engine block and the crankshaft's main journals. The crankshaft rests on a thin oil film in the bearing, and the bearing shell must withstand the load from combustion while also embedding small particles if the oil becomes contaminated. Wear can typically show up as falling oil pressure or scoring on the bearing surface. Depending on the engine, the crankshaft's end play is controlled either by a flanged bearing in the main bearing set or by separate thrust bearings and thrust washers.

How to choose main bearing sets

• Engine code: The sets are made for specific engines, e.g. Toyota 2ZZGE, Mitsubishi 4G63 and Audi CEPA/CEPB. Check the engine code, not just the car model.
• Size: STD fits a crankshaft in original size, while e.g. +0.25mm or 010 is for a reground crankshaft. The correct size depends on what the crankshaft has been ground to.
• Application: ACL offers both High Performance and Race Series sets; the choice depends on load, boost pressure and the desired bearing clearance.

Installation and compatibility

Bearing clearance should be measured during installation, e.g. with Plastigage or a micrometer, and compared with the engine's specification. Clean the block and oil passages thoroughly, lubricate the bearing surfaces with assembly lube, and torque the bearing caps to specification. If you replace the main bearings, it is often relevant to check the connecting rod bearings at the same time. Besides ACL, the category includes brands such as King Engine Bearings, Clevite and Wiseco, and you can find more bearing types under bearings and spacers.

Frequently asked questions

What do sizes like STD, +0.25mm and 010 mean?

STD is for a crankshaft in original size. +0.25mm and 010 denote bearings for a crankshaft that has been ground undersize – stated in millimetres or thousandths of an inch. The correct size depends on the crankshaft measurement after grinding.

Can I reuse old main bearings in an engine rebuild?

It is typically not recommended. The bearings wear and conform to the crankshaft they have run with, and even small scores can affect the oil film. When the engine is stripped, new bearings in the correct size and a measurement of the bearing clearance are the safe choice.

Why do main bearings get scored or wear quickly?

It can be caused by contaminated oil, too low oil pressure, incorrect bearing clearance or dirt during installation. A crankshaft that is not round or straight can also cause uneven wear. Find the cause before you install new bearings.