Measuring Engine Specs - How To Check Bearing Clearances

Above: You will use outside micrometers for most measurements in your engine. Here, QMP’s Rob Bieschke is using a Mitutoyo 2- to 3-inch outside mic to measure main bearing and connecting rod journals of an LS7 engine he is assembling for a customer. Really high-end measuring instruments can read very precise values. The LS7’s rear-most main journal measures 2.55925 inches. That’s to the hundred-thousandths of an inch!

Left: When measuring, be sure to use the proper technique. Hold the micrometer perpendicular to the surface you are measuring. Adjust the ratchet until it is just tight enough that you can pass the mic across the surface with a slight drag.

The same principle applies when measuring pistons. Tighten the ratchet until you feel a slight drag as you pull the micrometer across width of the piston. Also, rock the mic vertically to make sure you are holding it level. Note that pistons aren’t machined to a uniform diameter but are slightly oval shaped, with the widest spot (known as the guide point) being an area of the skirt 90 degrees from the piston pin. To find it, mic the piston from the oil ring to the bottom of the skirt. The guide point will be a couple thousandths wider than the rest of the skirt.

Checking cylinder bores is a similar process, but it is done with a dial-bore gauge—an instrument that uses a dial indicator mounted in an adapter that allows measuring the inside diameters of various parts. A typical dial-bore gauge kit includes several different-length anvils to fit inside spaces ranging from a big-block Chevy’s cylinders to the little end of a connecting rod.

Most of us don’t have QMP’s cool gauge-setting fixture, but you can set the outside micrometer to the exact reading of the piston diameter. To do so, mount the mic in a vice, set up the dial-bore gauge, and read the distance inside the micrometer. Rotate the dial on the face of the gauge to read zero at the diameter of the piston.

To measure piston to cylinder wall clearance, set the bore gauge to the measurements you obtained when measuring the piston diameter with the outside micrometer. QMP Racing Engines has this cool fixture that Bieschke uses to dial in the exact diameter of the piston.

Drop the gauge into the cylinders and measure their diameter. The number read by the needle shows the difference between the outside diameter of the piston and the inside diameter of the cylinder. Honestly, setting the gauge face to zero doesn’t matter, as you’re really looking to see how much the gauge moves. Zeroing it just makes it less confusing. A very general rule of thumb is no more than 0.004 inch piston to cylinder wall clearance. But be sure to check with your piston manufacturer, because clearances change between 4032 and 2618 alloy pistons. Depending on the material and construction, some pistons grow more than others when exposed to the heat of combustion. While you’re at it, check the cylinder in several locations to make sure it is round. Check the service manual for the out-of-round spec. If you really want to be accurate, this is best done with torque plates installed on the block. This is especially important on all aluminum engine blocks.

Checking the bearing clearance is a similar process. For the connecting rods, Bieschke used a smaller-bore gauge and set it based on the measurements he took from the crank journals. With the gauge zeroed at the spec for the journal, the reading indicated by the pointer is the bearing clearance—the difference between the diameter of the crank journal and the inside diameter of the connecting rod with the bearings installed and the rod bolts torqued. Bieschke says he likes to see roughly 0.001 inch of clearance per every inch of journal diameter.

Checking main bearing clearance is done exactly the same way. Set up the bore gauge to read zero based on the crankshaft’s main bearing journal diameter. The difference indicated on the dial is the bearing clearance. Again, the generic 0.001 inch of clearance per inch of main journal diameter applies. More or less will require over or undersized bearings or line honing of the block. Bearing clearances change based mainly on variations in housing bore diameters. Note that to get the most accurate reading, you would have all the main caps installed and properly torqued. Bieschke and the rest of the crew at QMP had thoroughly checked this block before we got there; they just mocked up some of the measurements so we could take the pictures for this article. Thanks, guys! End