6.3 Cam Bearings, Lobe Lift and Endplay

In-Block Camshaft, Cam Bearings, Lifters & Timing Chain/Gear Service

Many medium- and heavy-duty gasoline truck engines—including the GM 6.0L/6.6L Vortec/L8T and Ford 7.3L V8—use an in-block (cam-in-block) pushrod architecture because it packages high displacement, massive low-RPM torque, and a compact external width inside a rigid deep-skirt cast-iron block. Because the camshaft tunnel sits deep inside the cylinder block valley and shares pressurized oil galleries with the crankshaft main bearings and hydraulic roller lifters, precision inspection of the camshaft bearings, lobe geometry, lifter guides, and timing drive is essential.

In-Block Camshaft Bearings & Camshaft Metrology (Task C.4)

One-Piece Sleeve Camshaft Bearings: Stepped ODs & Oil Hole Alignment

Unlike two-piece split main and rod bearings, in-block camshaft bearings are one-piece babbitt-lined steel sleeves pressed into machined tunnels in the cylinder block webs using a long mandrel bar, expanding collet/cone, and driver handle:

1. Progressively Stepped Outer Diameters (OD): Even when all five camshaft journals share the exact same diameter, the outer diameters of the block cam bearing bores on many truck V8s are stepped progressively larger from rear (#5) to front (#1) (or grouped by specific journal position). This stepped tunnel design allows the smaller rear cam bearings to slide cleanly through the front bearing bores during installation without scratching the block housings. Before driving any cam bearing into the block, lay out the full bearing set in numerical order and verify each bearing's part number and outer diameter with a micrometer.
2. Cam Bearing Oil Hole Alignment & Clocking: Every cam bearing has one or more drilled oil feed holes that must register with the vertical oil passages connecting the main bearing saddles to the camshaft tunnel and upper lifter galleries. On blocks without an annular groove behind the cam bearing, misaligning the oil hole by even half a hole diameter restricts oil flow to the journal and can starve the entire bank of hydraulic lifters. On blocks that incorporate a 360° circumferential oil groove machined into the block bore behind the cam bearing, OEM procedures specify clocking the bearing oil hole at a precise angle—typically the 2 o'clock or 5 o'clock position looking from the front of the engine—so incoming oil enters the unloaded hydrodynamic zone just ahead of the downward valve-spring load vector at the 6 o'clock bottom dead center of the bearing.
3. Rear Camshaft Expansion Plug: Always install a new cup-type rear camshaft bore plug at the back of the block sealed with a light film of anaerobic or non-hardening sealant, driven straight to the specified recess depth so the rear of the camshaft neither rubs the plug nor leaks engine oil into the bellhousing.

Measuring Camshaft Lobe Lift, Journal Wear & Runout

Camshaft lobes on heavy-duty truck engines can be measured either in the engine block prior to teardown or on the bench after removal:

• In-Block Dial Indicator Method: Remove the rocker arms and mount a magnetic-base dial indicator on the cylinder head deck with a long probe extension resting directly in the pushrod seat of the lifter (or on the tip of the pushrod) strictly parallel to the lifter bore axis. Rotate the crankshaft slowly in the normal direction of rotation until the lifter rests on the lowest point of the cam lobe (base circle / heel), zero the dial indicator, and continue rotating the crankshaft until the lobe reaches peak lift (nose). The maximum reading is camshaft lobe lift.
• Out-of-Block Outside Micrometer Method: Using a 1 to 2 in. or 2 to 3 in. outside micrometer, measure across the cam lobe from the tip of the nose to the opposite heel (Major Diameter, A), then rotate the micrometer 90° to measure across the round base circle (Minor Diameter, B):
  • Camshaft Lobe Lift = Nose-to-Heel Major Diameter (A) - Base Circle Diameter (B)
  • Theoretical Gross Valve Lift = Camshaft Lobe Lift × Rocker Arm Ratio

Worked Example: Camshaft Lobe Lift & Gross Valve Lift Calculation Scenario

A technician removes the camshaft from a 6.6L truck V8 that uses 1.7:1 ratio pedestal rocker arms. The stated example specification calls for an intake lobe lift of 0.295 in. (0.5015 in. gross valve lift) with a maximum allowable lobe wear loss of 0.0025 in.. Measuring the #3 intake lobe with a calibrated outside micrometer yields:

• Nose-to-Heel Major Diameter (A): 1.7660 in.
• Base Circle Minor Diameter (B): 1.4755 in.
• Actual Lobe Lift: 1.7660 in. - 1.4755 in. = 0.2905 in.
• Lobe Wear Loss: 0.2950 in. (Spec) - 0.2905 in. (Actual) = 0.0045 in. (exceeds the 0.0025 in. limit by 0.0020 in.).
• Actual Gross Valve Lift: 0.2905 in. × 1.7 = 0.49385 in. (a loss of 0.00765 in. at the valve tip).
• Diagnosis: Inspection of the #3 intake lobe nose reveals subsurface Hertzian fatigue spalling (flaking pits)—the classic failure mode of a hardened steel roller camshaft lobe—caused by a seized needle bearing in the #3 intake roller lifter. Both the camshaft and the lifters must be replaced.

Camshaft Thrust/Retainer Plate & Axial Endplay

In-block roller camshafts are held axially by a hardened steel camshaft thrust/retainer plate bolted to the front face of the cylinder block behind the camshaft timing sprocket. Check camshaft endplay (0.0015 to 0.0060 in.) with a dial indicator mounted on the front of the block while prying the camshaft sprocket gently rearward and forward.