Running the proper clearances in the main and rod 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, and failure and having a long-lasting, power-producing engine. Our friends at King Bearing wanted us to put together something to show how to correctly measure and control clearances so here is a guide on how it’s done.
Engine Bearings found in automobile engines are hydrodynamic. This means that they are a soft metal shell around the crank journals that depends on a thin film of pressurized oil to prevent metal-to-metal contact between the crank and the bearing to prevent damage. Hydrodynamic bearings are great for this application because for a rotating shaft, they provide the best combination of load-bearing capacity and low friction. They also provide a small level of vibration damping, a factor that is important for high rpm rotating assemblies.
A soft metal is used for the bearings because it can tolerate some touching during startup and other transient low oil pressure situations without damage to the all-important crank journals. A softer metal has the ability to embed contaminates that might end up in the oil to sacrifice itself while saving the harder crank journals. The softer bearing also has rubbing compatibility with the hard crank journal, if it was similar in hardness and composition, galling might occur if any touching of the surfaces was involved. So as you can imagine, although the bearings can take a lot of force they can also be delicate.
Because the delicate balance required to prevent metal-to-metal contact is dependent upon the proper bearing clearance, it is critical to get it right. The proper clearance can be less than 0.001″ and in some engines, the absolute minimum clearance is 0.0001. That’s one ten thousand of an inch!
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, and damage to the bearings, then the crank will result. 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 to improve fuel economy. Manufacturers are also specing low viscosity oil with less viscous friction for better fuel economy. These design trends require tighter bearing clearances and modern manufacturing processes enable 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 bearing clearances to increase.
These factors make the need for precisely controlling bearing clearances when building an engine more important than ever. Bearing clearances are now so tight that when doing a post mortem on other people’s mistakes, having improper clearances is one of the main reasons we find for home-built engine failures. Tight clearances are also much more difficult to control for your local machine shop.
All of these reasons make measuring bearing clearances all the more critical for the hobbyist engine builder. Gone are the days of 0.003″ clearance on the mains and 0.004″ on the rods. The typical proper clearance is less than half that nowadays. Before many home engine builders relied on plastigage to measure clearances. Plastigage 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 clearance.
The first step is to measure your crank 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 but calipers are only accurate into the one thousands of an inch range, 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. We 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. We write our measurements down for each journal.
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.