How To Check and Set Main Bearing Clearances

Running the proper clearances for your main bearings are some of the most critical things to control when building an engine. Having the correct clearances means the difference between premature wear, friction, failure and having a long-lasting, power-producing engine.
It is critical to get your main bearing clearance right. Too much clearance and an engine will suffer from low oil pressure. In severe cases, the oil pump won’t be able to keep up with the oil bleeding from the large bearing gap due to centrifugal pumping, the hydrodynamic film will break down. Too little clearance and the crank might bind and make metal-to-metal contact due to runout in the crank. This will result in a lot of friction and damage to the bearings and crank as well.
With modern engines, manufactures have been cutting down on oil pump capacity to reduce pumping losses in order to improve fuel economy. Manufacturers are also spec’ing low viscosity oils with less viscous friction for better fuel economy. These design trends require tighter bearing clearances. Modern manufacturing processes allow for closer tolerances, which enable tighter clearances in mass production. Most modern engines now use aluminum blocks and aluminum expands a lot with heat. Causing the main bearing clearances to increase.
All of these reasons make properly measuring main bearing clearances all the more critical for the engine builder. Gone are the days of 0.003″ clearance on the mains and 0.004″ on the rods working for nearly everything. The typical proper clearance on a modern engine is less than half that nowadays. The proper main clearance can be less than 0.001”, and in some engines, the absolute minimum main bearing clearance is 0.0001. That’s one ten thousand of an inch!
In the old days engine builders relied on plastigauges to measure clearances. Plastigauge was a putty-like material that you would put on a journal, torque down the caps, and measure how much it squished to estimate clearance. With today’s tight clearances, that is no longer an option. Throw your plastigauge in the trash! Let us show you the proper way to measure your main bearing clearances.



The first step is to measure your crank’s main bearing journals. For this, you must always use a high-quality micrometer, not something like calipers. A micrometer can measure down into the sub ten thousands of an inch range, while calipers are only accurate into the one thousands of an inch range. This is not good enough.
When using the micrometer you can use the clutch-driven small knob at the end, which freewheels at a given torque. This makes sure you use an equal amount of pressure when taking a measurement, important for consistency. When learning how to use a micrometer, practice measuring a journal many times until you get the feel right and come out with consistently repeatable measurements. Measure each journal in a couple of places around the circumference and from side to side to see if the journal is out of round or tapered. This is almost never an issue with modern engines. Write the measurements down for each journal diameter.


Next, it is time to measure the inside diameter of the main bearings. For this, you need a high-quality bore gauge. You first need to set up the bore gauge using a micrometer to establish your zero reading. You set your bore gauge with a micrometer to zero it at your recorded main journal diameter.


Most bore gauges come with a set of different size anvils for you to use depending on what size bore you are measuring. Pick the right one for your bearing bore and screw it on before you set your bore gauge to the journal diameter.

Now install your bearings in the main caps and torque them down to the proper torque specs. This is critical as you want to measure the bearings inside diameter once they are crushed. Now measure your bearing inside diameter as shown. You want to measure it here because sometimes the inner bore is tighter at the bearing parting line by design. If you zeroed the bore gauge using the journal diameter, the number you come up with on your bore gauge is your main bearing clearance. For iron blocks, we like to run the clearances from the middle of the factory recommended range to the minimum side of the spec. For aluminum blocks, we like to run close to the factory minimum side of the spec due to heat expansion. To control the clearance you can mix and match different thicknesses of select fit bearing shells. For example you can mix a standard size bearing with a 0.025 bearing. Sometimes you might have to align bore the block to the factory minimum size diameter in order to get more crush to close up the clearances. Or grind the crank journals undersize and use an oversize bearing or a combination of the two.

A little feel is needed when measuring using a bore gauge. Rock the gauge back and forth while watching the dial indicator. The middle of the reading is the correct diameter.

Measure all your main bores and determine your clearances to decide if everything is ok, or if you need to adjust using different sized bearing shells or with machining. As a final check, lubricate the bearings and the crank, put the crank in the block, torque down the caps and spin the crank. If all is right, the crank will spin very freely. If it binds or has tight spots, check your crank for straightness and double check your clearances.
Never just throw a engine together. Most people do this and that’s why many engines don’t last. Take your time, be meticulous and your hard work will pay off.