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Engine bearing information

Here we will go through various questions that come up when talking about engine bearings.

# Different types of engine bearings
# Bearing clearance
# How to measure clearance
# Oil pressure
# Choice of engine oil
# Oversize bearings


Different types of engine bearings

There are different types of engine bearings and they can be briefly divided into three main categories:
Main bearings
Rod bearings
Axial bearings

In addition to these, there are also many different types depending on the engine, such as camshaft bearings and different types of bearings for balance shafts, but the above three varieties are most common.

Why not just use stock bearings with standard clearance or extra clearance?

A so called "race" bearing or aftermarket bearing usually use better material and is initially built to withstand high loads that occur with tuned engines and higher combustion pressure and thus higher stress for the bearing.


Bearing clearance

When choosing a stock, the question immediately arises as to which stock game to choose. Bearing clearance is the space between the bearing and the crankshaft.
Different types of bearing clearance can look like this:

  • H = 0.0125mm (.0005”) - Standard (STD) same or similar clearance as original.
  • HX = 0.025mm (0.001”) - Larger clearance than original

Different types of bearing clearance are named differently from different manufacturers.

Above we can see an example where ACL bearings designate their bearing clearance with H or HX while KING bearings designate theirs with STD for standard and STDX for larger clearance. Car manufacturers usually only print the stock bearing that is used if you buy original bearings.

Are more clearance better?

A common misconception is that more clearance are better on a tuned engine. That is not the case. This is highly individual for how to use the engine and what other components are used.

What bearing clearance should I use?

What bearing clearance e to use depends on many different factors, but further down the page there is information about some key factors that play a big role in the choice of engine bearing clearance.



How to measure bearing clearance

1. Mount the bearings in the engine and tighten the bearing halves with the specified tightening torque. The crankshaft should not be mounted here. Now measure the diameter of the hole with a micrometer or hole indicator.
2. Measure the diameter of the crankshaft pin
3. The difference will be the bearing clearance. This value is then controlled with Flexigauge.

Plastigauge / Flexigauge

Plastigauge and Flexigauge are a type of plastic piece that is placed between the bearing and the crankshaft. The bearing halves are then mounted with the tightening torque to be used. The plastic piece between the crankshaft and the bearing will now be flattened. Then disassemble the bearing halves to be able to measure the width of the plastic piece with the included dipstick that presents the bearing clearance on the scale.


Oil pressure

More bearing clearance = lower oil pressure

A big factor when talking bearing clearance is the oil pressure. If a larger bearing clearance is selected, the oil pressure will drop if other components are kept intact as more oil is allowed to evacuate between the bearing and the crankshaft. If you can accept the lower oil pressure, it is okay, but you should be well aware that lower oil pressure gives less tolerances and a higher risk of mechanical contact.

Thicker oil

If you have chosen larger bearing clearances for various reasons, a thicker oil (other viscosity) is a way to maintain oil pressure. This must be measured with a reliable oil pressure gauge. The measurements must be made at idle up to the highest speed with close monitoring so that the oil pressure is kept within the tolerances.

Upgraded oil pump

For larger bearing clearances, an upgraded oil pump can be a solution to maintaining oil pressure. Therefore, an upgraded oil pump is also a common upgrade for bearings with larger bearing clearances.


Over dimension bearing

Renovated crankshaft dowel

In the event of a engine faliure or otherwise damaged crankshaft, the crankshaft dowel has to be renovated. The crankshaft dowel becomes smaller in diameter. Then the bearing clearance will automatically be larger if the dimension of the corresponding bearing halves is not adjusted upwards. Adjusting the bearing clearance to standard is called oversized bearing. With a renovated crankshaft, oversized bearings are used even if you have larger bearing clearances.

The oversize is printed directly in plain text and can look as follows in both millimeters and inches:

  • 0,025mm
  • 0,25mm
  • 0,50mm and so on.