Properly fitted main and rod...
Properly fitted main and rod bearings can be the difference between a healthy engine that runs hard and one that throws a bearing and puts you out of a race at the worst possible time.
When you get right down to it, many of the tasks involved in building a race engine are mundane. It isn’t too difficult to drop a piston in a hole or bolt up an intake manifold. And neither is checking your valve lash or fitting your main and rod bearings. The trick is knowing how to do it right, taking your time to pay attention to the details and having the proper tools which can make your life a lot easier.
This month we’re going to take an in-depth look at how to measure for the correct clearances between your crank and rod journals, their corresponding bearings and how to install them.
One of the most important factors when it comes to the health and durability of your rotating assembly (crank, rods, and pistons) and the block itself is the amount of open space between the main and rod journals on your crankshaft and the bearing shells they are spinning within. This space is known as the bearing clearance, and it’s critical because that open area is filled with oil which provides a cushion between the journal itself and the bearing. When everything in the engine is right, the oil keeps the steel of the crank journals and the babbit of the bearings separated, and they never touch.
This is a rebuild of the Circle...
This is a rebuild of the Circle Track dyno mule that we’re upgrading for future testing. Both the rod and main bearings are supplied by King Engine Bearings. We chose its XP performance bearing line because they’re designed to withstand high-load endurance applications—in other words, stock car racing. The first step is to make sure that both the housing bores and both sides of the bearings are clean and grit-free. Fit the bearings into the block so that the tangs on the backsides of the bearings fit into the slots in the block. The grooved bearing shells fit into the housing bore half in the block, while the smooth shells fit into the caps.
But problems can enter the picture pretty quickly when the bearing clearances aren’t correct. Too little clearance and you’ll have trouble keeping your oil temperatures in check, and a spun bearing is much more likely. On the other hand, if the tolerance is too loose you hemorrhage oil out from around all the bearings. Obviously, too loose is better than too tight, but hitting the center of the bullseye is always preferable.
As a general rule for a race engine, you want to have approximately 0.001-inch of bearing clearance for every inch of journal diameter. So for a 2.100-inch rod journal, it’s always a good idea to shoot for 0.0021 or 0.0022 bearing clearance. And once again, because too little bearing clearance can mean a spun bearing, we always aim to wind up too high rather than too low.
Adjusting the clearance is easy, but probably a bit more expensive than most of us would prefer. Most quality bearing manufacturers make three versions for all the popular styles. First, there’s the standard bearing. This is where you start. If you find you need more clearance, you can purchase an undersized bearing. This means the bearing shell is thinner, which creates 0.001 more clearance than a standard bearing. An oversized bearing is the opposite. It’s thicker and takes away 0.001 of clearance.
One one-thousandth of an inch is a big jump, but you can actually make adjustments in 0.0005 increments—that’s one-half of one thousandths of an inch. To do this you mix half the shell of a one-over or one-under bearing with half the shell from the standard bearing. This means you may have to purchase a couple sets of bearings to get everything just right in your race engine, but sometimes that’s the price you have to pay. Just remember when mixing shells to always keep the sizes in the same alignment. In other words, if you’re using one-unders with standards for your main bearings, put all the one-unders in the cap with the standards in the block side of the housing bore, or vice versa. So with that said, let’s get on with it.
Install the bearings into...
Install the bearings into the caps the same way. Install and torque the last one in place. For a Chevrolet like this with four-bolt mains, the inner bolts are torqued to 70 ft-lb and the outers to 65 with motor oil lubricating the threads and underside of the bolts.
To do this job right does...
To do this job right does require a few specialty tools. Powerhouse Products offers both micrometers (seen here) and a dial bore gauge that are affordable for non-professional engine builders. Use the micrometer to get the outer diameter (od) of the journal to be fitted.
Use a dial bore gauge (they’re...
Use a dial bore gauge (they’re available from Powerhouse for about a hundred bucks) in the inner diameter (id) of the bearing. Position the gauge so that it’s vertical in the bore and rock it back and forth until you get the smallest reading (that way you know the gauge is truly vertical). Be careful not to measure the groove in the bearing shell. Now zero out the dial indicator.
One of the most important factors when it comes to the health and durability of your rotating assembly and the block itself is the amount of open space between the main and rod journals on your crankshaft and the bearing shells they are spinning within
Lock the micrometer securely...
Lock the micrometer securely in position (you can use a vise, just lay a rag over the jaws to protect the mic) and place your dial bore gauge in it. In this photo we’re only using one hand so you can see what’s going on better, but use two hands to hold the gauge steady. You will know you have the gauge square in the mic when you get the smallest number. This will tell you the clearance between the journal and the bearing.
To help you keep track, mark...
To help you keep track, mark your clearances on the block’s pan rail with a Sharpie. Here, this main has 0.0025 clearance. Later, once everything is done, you can move your data to your more permanent buildsheet.
Now repeat the process with...
Now repeat the process with the rods. Performance rods and bearings have a chamfer to create clearance for the fillet in the crank journals. When installing the rod bearings, make sure the chamfered side of the bearing is in line with the chamfered side of the rod.
With the bolts holding the...
With the bolts holding the main caps in place, light motor oil is normally sufficient for lubricating the threads. But the technology for the rod fasteners is a bit more exacting. Most rod manufacturers recommend ARP’s new Ultra Lube, which doesn’t polish the threads if the rod bolts are tightened and removed multiple times. This keeps the rod bolt stretch more consistent time after time.
These are Eagle’s lightweight...
These are Eagle’s lightweight H beam rods, and Eagle recommends torquing the cap screws to 63 ft-lb for proper loading. We checked the bolt stretch afterward and they were dead on. Bearing crush affects the final id size, so the rod caps must be properly torqued beforehand in order to correctly measure the bearing clearance.
When measuring the crank’s...
When measuring the crank’s rod journals, make sure to measure around the center of where the bearing for that specific rod will sit—not the center of the journal itself. Also, make sure to stay away from the oil gallery holes, which can throw off your readings.
Rod bearings get narrower...
Rod bearings get narrower as you approach the parting lines, so make sure to measure the bearing id perpendicular to the rod cap’s parting line (parallel to the beam of the rod). Also, it’s possible to measure the rod bearing id with the rod sitting on your workbench, but if it wobbles at all it will ruin your accuracy. A much better practice is to secure the rod in a rod vise like you see here.
Once all the bearings are...
Once all the bearings are measured and properly sized, you are finally ready to drop the crank into the block. First, make sure to properly lube with main bearings in the block with some type of assembly lube. We’re using Royal Purple’s synthetic assembly lube. Just make sure it’s thick enough not to wipe away the first time you spin the crank in the bearing saddles.
Besides the clearance between...
Besides the clearance between the main bores and the main bearings, you also need to make sure the crank has proper endplay. The best way to do this is to install a dial indicator over the nose of the crank like you see here.
Also, make sure you have the...
Also, make sure you have the rear main seal installed to avoid having to pull the crank back out once you have realized your mistake. A small amount of silicone or gasket sealer will help lock the seal in place and ensure no oil leaks out from underneath. Make sure the lip of the seal points toward the engine, and clock the seal a bit as you see here so that its parting line doesn’t match up with the parting line of the cap.
When installing the main caps...
When installing the main caps in the block—no matter if you are fitting bearings or final assembly—don’t use the bolts to pull the caps down into the register. This is supposed to be a tight fit and doing it that way can damage both the cap and the register in the block which will lead to cap movement. Instead, slide one side into the register, thread the bolts in a bit to make sure the cap is properly aligned, and use a rubber mallet to tap the other side of the cap into position. Now you’re ready to torque the caps.
You also need to set the thrust...
You also need to set the thrust bearing shells. Do this with the first four main caps installed and torqued down. With the cap containing the thrust bearing (the rearmost on Chevys) in place but the bolts not tightened, use your rubber mallet to smack the nose of the crank to push it backwards in the block. Then hit the rear of the crank to push it back forward. This aligns the two thrust bearing shells. Always align the thrust bearing with as much forward clearance as possible (by hitting the crank toward the front of the block last) because the clutch is always trying to push the crank out the front of the block. Now you’re ready to tighten your final main cap.
Problems can enter the picture pretty quickly when the bearing clearances aren’t correct
Now that all the main caps...
Now that all the main caps are torqued down, check the crankshaft endplay one final time with the dial indicator. You should have between 0.005 and 0.010 of movement.
We had proper clearance using...
We had proper clearance using the King bearings, but if you don’t, you may have to lightly sand the thrust surfaces on the bearing. Place a sheet of very fine sandpaper over the flattest surface you can find—a surface stone is ideal—and sand both thrust faces very gently in a semi-circular pattern. Thoroughly re-clean the bearing and try again. You may need to repeat a few times to get the correct clearance.
Once you have the correct...
Once you have the correct crankshaft endplay, you’re ready to install your pistons on the connecting rods you’ve just sized up and continue on with your engine build.