0-150MM 0.01mm/0.0004 Inch Outside Micrometer set [ Ссылка ]
50mm-160mm 0.01mm Dial Bore Gauge [ Ссылка ]
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What is up engine heads! Today we-re talking about bearing clearances, we will explain what they are, why they're important and how to measure them.

When we speak about bearing clearance on an engine we're usually referring to the clearance between the crankshaft rod journal and the rod bearing as well as the clearance between the crankshaft main journal and the main bearing. You probably know that the key parts of your rotating assembly are never supposed to make contact with each other. Instead there's a film of oil between your crankshaft journals and your rod and main bearings. The bearing clearance determines the thickness of the oil film and thus plays a key 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 which in most cases results in damage and engine failure. Clearances that are too loose will make it difficult for your oil pump to maintain desired oil pressure and will also result in too much oil coming out from the sides of the bearings which will increase crankcase windage.

The right clearance depends a lot on the type of engine and its modifications and there is no one size fits all formula. However, There is a rule of thumb and it's this – 0.001 times journal diameter, doesn't matter if it's inches or millimetres.
Our engine right here is a second generation Toyota 4AFE 16 valve engine with 48mm main journals and 40 milimeter rod journals.
This means that our main bearing clearance is 48 x 0.001 which equals 0.048mm
And our rod bearing clearance is 40 x 0.001 which equals 0.040mm.

Now let's proceed to measure our bearing clearance. But before we do that we will do quick visual inspection of our crankshaft journals and bearing surfaces. Here we have two used Toyota 4A crankshafts, one which is in good condition and another which is in horrible condition. You can probably tell which is which. Obviously this crankshaft fails visual inspection, it actually spun two rod bearings and as you can see it has very noticeable deep scoring on the rod journals. The same logic applies to the bearing surfaces. Smooth and shiny like this passes visual inspection. Deep scratches and embedded particles fail visual inspection.

No we can start our measuring. To do this we will need two tools – a micrometer and a dial bore gauge. In order to measure things accurately we need to zero our tools. To make measuring easier we can set the crank on four main caps and apply a bit of oil to make turning the crankshaft easier. We are going to start by measuring our rod journals. We do this by gently moving the micrometer across the journal while we tighten down the small knob at the same time. The small knob or ratchet ensures that consistent and equal pressure is applied which results in consistent, accurate and repeatable measurements. The goal is simply to make contact with the journal. The micrometer should still be able to slide on and off the journal with a bit of effort. You will feel a bit of resistance but it will be low.

When it comes to the main journals we will do the same thing as with the rod journals. Measure at both journal halves and then perpendicular to these two locations. Now that we have our journal dimensions we can go ahead and measure our main and rod bearing bores. Once we have zeroed our gauge we will measure at six different locations. We measure by moving the bore gauge gently back and forth and observing the furthest point reached by the needle. The first and most important location is a vertical measurement perpendicular to the bearing parting line.

Now we can proceed to measure our rod bearing bore. Of course we do this with the rod caps torqued down to spec and we measure at the same spots as with the main bearings. Just like with the main bearings we don't measure at or near the parting lines or at or near oiling holes. Instead we measure perpendicular to the parting lines and at around 45 degrees from the vertical axis.

To get our bearing clearance we will deduct our vertical axis bearing bore measurement from our largest journal measurement. The result represents our rod and main bearing clearance.

A special thank you to my patrons:
Daniel
Peter Della Flora
Daniel Morgan
William
Richard Caldwell
Pepe
Brian Durning
Andrew Ruud
Brian Alvarez
Holset90

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