Bearing Clearance (Full Guide)
Updated: Oct 16, 2021
In this article, we're going to be talking about bearing clearances. I'll explain how to measure and select the correct bearing clearance.
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I'll also explain what bearing clearance is and why it's crucial to proper engine performance and longevity. In this article, I will also show you how to measure bearing clearance.
Bearing clearance in an engine typically refers to the space between the crankshaft rod journal and the rod bearing.
Proper bearing clearance is also required between the crankshaft main journal and the main bearing.
The components inside of your engine should not come into direct contact with one another. Instead, there should be an oil film between these moving parts.
What Is Bearing Clearance?
The bearing clearance determines the thickness of the oil film.
Bearing clearance plays a vital part in your oil pressure and oil temperatures.
If the clearance is too tight, metal to metal contact can occur under loads and during crankshaft flexing.
In most cases, this will lead to damage and can eventually result in engine failure. An example of incorrect bearing clearance causing engine failure is rod knock.
Rod knock occurs when the rod bearings are damaged or worn, causing the rod to shake, move side to side, and vibrate freely on the crankshaft, resulting in a knocking or clicking sound.
Also, excessively loose bearing clearances make it difficult for the engine's oil pump to achieve the proper oil pressure.
A lack of bearing clearance will also result in an excessive amount of oil leaking from the bearings. This inadequate bearing clearance will also result in crankcase windage.
Crankcase windage raises the parasitic drag on the crankshaft and reduces overall engine power and efficiency.
A loose bearing clearance carries less risk, but it's best to aim for a bearing clearance balanced between loose and tight.
How to Select a Bearing Clearance
The correct bearing clearance depends a lot on the type of engine and its modifications.
There is no "one size fits all" formula; however, there is guidance that a proper bearing clearance is 0.001 times your journal diameter.
It doesn't matter if it's inches or millimetres; for example, let's take an engine with 48-millimetre main journals and 40-millimetre rod journals.
This guidance tells us that our main bearing clearance is 48 x 0.001, which equals 0.048-millimetres of main bearing clearance.
Using the same guidance, our rod bearing clearance is 40 x 0.001, which equals 0.04-millimetres.
So this bearing clearance should be a good "rule of thumb".
However, if you check the factory specifications, it may recommend a bearing clearance larger or smaller.
Many modern engines run tighter bearing clearances than what our guidance would recommend.
However, bearing clearances that this "rule of thumb" guidance recommends may better suit significantly modified engines (e.g. adding forced induction, increasing the redline, etc.).
An excellent way to see if a journal bearing is damaged is to run your fingernail across the journal surface.
If you can feel scratches or they snag your fingernail, then the journal bearing fails visual inspection.
You could take it to a machine shop and see if it can be saved by machining.
Some bearings display uneven wear, but this is mild and doesn't usually raise any red flags.
Some engines may have also had a bearing reset.
How to Measure Bearing Clearance
To measure clearances, you will need a micrometre and possibly a dial bore gauge.
You need to zero your measuring tools to accurately measure clearances.
To measure bearing clearances, gently move the micrometre across the journal.
Tighten down the small knob.
The small knob or ratchet ensures that consistent and equal pressure is applied.
This equal pressure results in consistent, accurate and repeatable measurements.
The goal is to make contact with the journal and not over tighten anything.
The micrometre should still be able to slide on and off the journal with minimal effort.
You can measure at any spot as long as it isn't at or near an oiling hole.
After you have the measurement on one side or half of the journal, you can lock your micrometre and measure other sections to check for taper and out of round.
You can also observe the resistance.
If it slides on and off from the following location the same way it did from the previous area, there is no taper in the journal.
Remember not to force the micrometre onto anything if it feels too tight.
All bearings have some eccentricity incorporated in them, as this benefits lubrication and function.
It's best to check your factory service manual to see how much eccentricity is usual for your engine.
Repeat the same measurements on the opposite side of the bearing.
Usually, if a bearing is in good condition and shows no abnormal wear, measurements will be equal at the back and the front of the bearing.
Repeat this whole procedure for every bearing inside the engine.
