How to Gauge the Rod Bearings on a Race Motor

Racing bearings are measured in thousandths of an inch.

Pistons connect to the crankshaft by way of connection rods, which are among the most strained components in a combustion engine. Some racing engines are put through such stress, they are disassembled and rebuilt after every race. The parts in these engines must be carefully evaluated within thousandths of an inch to see if they are still within the race teams specifications or need to be replaced.

Things You'll Need

• Dial calipers
• Dial bore gauge
• Torque wrench
• Sockets

Instructions

1. Work in a temperature-stabilized area. When checking extremely fine clearances, even the difference in temperature from evening to night in an opened door shop can produce expansion or contraction in bearings -- which nullifies the accuracy of your measurements.

2. Make sure your calipers are properly calibrated. If not, calibrate them. Do not switch calipers mid-task as they may vary, throwing off the consistency of your work -- unless you are switching from one type of caliper to another, which is unavoidable.

3. Conduct a rod bolt-stretch on each rod/bearing combination. With your dial calipers, measure the rod bolts prior to torquing them down and after they've been torqued down. The difference in the measurement must be within engineer's specifications.

4. With your bore calipers, measure the specifications of each rod to double-check it against the manufacturer's specifications. You must ensure it is machined to the proper specs or, if it has already been used, check that it has not suffered any damage or alteration.

5. Measure each rod bore. Because the rod bearings fit in the rod bores, you must know the size of the bore to isolate the dimensions of the bearings.

6. Measure the bearing-to-radius clearance of each rod and bearing. This measurement confirms the specification of the crankshaft radius to the rod bore, which includes the clearances of the bearings.

7. Check the side clearance of each rod. Measure the width of the rod journal, then two rod journals as they sit in the crankshaft. Subtract the single measurement from the double measurement to get the distance between them, ensuring it's within specification.

References

• Circle Track: Tips and Tricks for Crankshaft and Rod-Bearing Installation