6.6 Bearing Fit, Clearance and Crankshaft Endplay

Select-Fit Bearings, Crush/Spread & Plastigauge Oil Clearance (Task C.8)

Select-Fit Standard Bearings vs. Reground Undersize Bearings

• OEM Select-Fit Production Grading: Many modern truck engines (such as Ford 6.2L/7.3L and GM V8 platforms) machine standard main housing bores and crankshaft journals within three tight production tolerance bands stamped as codes (1, 2, 3 or A, B, C) on the cylinder block oil pan rail and the crankshaft counterweight. By cross-referencing the block bore stamp with the crankshaft journal stamp in the service manual chart, the technician selects color-coded standard bearing upper and lower halves graded in 0.0004 to 0.0005 in. wall-thickness increments to hit an exact 0.0018 in. target oil clearance.
• Undersize Bearings for Reground Crankshafts: When a damaged crankshaft journal is ground smaller on a crankshaft grinder (for example, 0.010 in., 0.020 in., or 0.030 in. smaller than standard), it requires corresponding 0.010 in., 0.020 in., or 0.030 in. undersize (US) bearings (0.25 mm, 0.50 mm, 0.75 mm). Undersize bearings have a thicker bearing wall to compensate for the metal ground off the crankshaft journal.

Bearing Spread and Bearing Crush

Two mechanical interferences keep a thin-wall split bearing shell alive inside a running engine:

1. Bearing Spread: The free, uninstalled diameter across the parting faces of a bearing half-shell is manufactured 0.005 to 0.020 in. wider than the housing bore diameter so the shell must be snapped firmly into the saddle or cap and will not fall out when the cap is turned upside down.
2. Bearing Crush (Radial Interference): When two bearing half-shells are seated in the block saddle and bearing cap, their parting edges stand 0.001 to 0.002 in. proud above the machined split line of the housing. When the bearing cap bolts are torqued to specification, those protruding parting edges are forced together, creating powerful radial outward pressure (hoop stress) that presses the steel backing of the bearing tightly against the iron housing bore.

Plastigauge Oil Clearance Measurement & Worked Scenario

To verify main or connecting rod bearing oil clearance (typical truck V8 spec: 0.0015 to 0.0030 in.) using green Plastigauge (0.001 to 0.003 in. range):

1. Wipe all oil off the bearing inside diameter and crankshaft journal (oil dissolves the wax thread).
2. Lay a strip of Plastigauge axially across the full width of the journal about 1/4 in. away from the oil hole.
3. Install the bearing cap and torque the bolts to full specification. Never rotate the crankshaft while the Plastigauge is clamped, or the wax strip will smear.
4. Remove the cap and match the widest and narrowest flattened sections of the wax stripe against the paper scale and record the minimum and maximum indicated clearance on the envelope. Remember: a wider flattened stripe means smaller (tighter) oil clearance (0.001 in. is wider than 0.003 in.).

Worked Journal Undersize & Plastigauge Calculation

• Standard Factory Rod Journal Diameter: 2.0990 to 2.1000 in.
• Specified Rod Bearing Oil Clearance: 0.0015 to 0.0030 in.
• Measured Crankshaft #4 Rod Journal: 2.0794 in. near the front edge and 2.0795 in. near the rear edge (journal taper is 0.0001 in., well under the 0.0005 in. limit).
• Undersize Determination: 2.0995 in. (Nominal Standard) - 2.0795 in. (Measured) = 0.0200 in.. The crankshaft has been ground 0.020 in. undersize and requires 0.020 in. US rod bearings.
• Plastigauge Verification: With 0.020 in. US bearings installed and torqued, the crushed green Plastigauge strip measures matching widths equivalent to 0.0020 in. on the paper gauge scale across both ends—supporting clearance within the example range; use micrometers for a precise taper measurement.

Checking and Correcting Crankshaft Thrust Endplay (Task C.9)

Crankshaft fore-and-aft movement is controlled by a single flanged thrust main bearing (whose location must be identified from the engine diagram). During assembly, snug the thrust main cap bolts finger-tight, then tap the crankshaft sharply rearward and forward with a dead-blow mallet to align the upper and lower thrust bearing flanges perfectly flat against the crankshaft thrust cheeks before applying final main cap torque.

Mount a magnetic-base dial indicator with its plunger parallel to the crankshaft axis against the crankshaft snout, pry the crankshaft fully rearward, zero the indicator, and pry the crankshaft fully forward. Normal crankshaft axial endplay is 0.004 to 0.008 in. (0.10 to 0.20 mm). Insufficient endplay (< 0.003 in.) causes the thrust flanges to wipe and seize as the crankshaft expands thermally; excessive endplay (> 0.009 in.) indicates worn crankshaft thrust cheeks (often caused by a bad torque converter or riding a manual clutch pedal) and requires an oversized-flange thrust bearing or crankshaft replacement.

Select-Fit Assemblies Beyond Bearings

A production grade can apply to a piston and its bore or to another matched assembly as well as to bearing-shell thickness. Use the engine's code chart and final measurements instead of assuming a color means the same size across manufacturers. Replacement connecting rods must match the specified length, pin and journal fits and any required weight or balance grade. Verify the selected piston's gauge diameter and clearance in its intended cylinder. A nominal part label does not replace the assembled dimensional check.