Running the proper clearances for your rod bearings is a very important factor when building an engine. It is critical to get your rod bearing clearance right. Too much clearance and the spinning crank will fling all the oil out of the area between the bearing and the crank journal making the engine suffer from low oil pressure. This will cause metal to metal contact between the two which will result in spinning a rod bearing, destroying the rod, crank and possibly the whole engine. Too little clearance and the rod bearing might make hard contact with the crank journal and bind, damaging the bearing, causing it to spin, with possible terminal consequences to the engine.
The first step for setting your rod bearing clearance is to measure your crank’s rod 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, which is not good enough for this work. 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 rod bearings. For this, you need a high-quality bore gauge. No, a caliper isn’t accurate enough to do this job either. 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. 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. Like you did with the micrometer, practice by taking the same measurements many times until you can get repeatable results.
Now it’s time to measure your rod bearing bore diameter. We are installing King XPC bearings in a set of racing rods here.
Once the bearings are in place, bring the caps together.
Lubricate the rod bolts before installing using the rod manufactures recommended lubricant.
Now torque the rod bolts to the rod manufactures recommended torque to crush the bearings.
Set up your bore gauge for measuring the rod bearing clearance by zeroing your bore gauge to the measured crank diameter for each rod journal using your micrometer.
Now measure your rod bearing bore diameter, the bore gauge reading is your rod bearing clearance. To adjust the clearance, you can increase crush, to do this you can have your rods resized by a competent machine shop to the tight side of the factory spec. You can also mix and match bearing shell thicknesses. You can also grind your rod journals undersize and use an oversize bearing or use a combination of all three methods. We prefer to set the rod bearing clearance from the middle to the tight side of the factory recommended spec. We do not like to run on the wide side of clearances due to difficulty in maintaining oil pressure, excessive windage which can increase oil consumption and oil temperature as well as wasting power due to windage loss. The fact that the spinning rod end acts like a centrifugal pump to pull oil out of the bearing clearance area makes having the rod clearance right all the more critical.
As a final check, make sure your crank spins freely in the main with the caps torqued down and make sure the rods move freely without binding. Rotate your engine by hand before final assembly and feel for any binding or tight spots. If you have this, double check your measurements and if they were off, take the before mentioned corrective actions to fix them. If they are ok but you still have binding, check your crank for runout. You can straighten your crank with a small sledgehammer, a blunt chisel, and V blocks but this requires some skill and patience. The best automotive machine shops can true a crank to a high degree of precision as well.
Having correct rod bearing clearances is essential for a durable engine and having incorrect engine bearing clearances is probably the second greatest cause of homebuilt (and even some so-called pro-built) engine failures. Now that we showed you how to check your clearances, throw away that Plastigaugeand do it right!
