5.2 In-Block Camshaft, Crankshaft, Select-Fit Bearings, and End Play

In-block camshafts are a pickup-diesel job, not a TDI job

Cam-in-block (overhead-valve) light diesels put the camshaft in the block: Ram 6.7 Cummins, GM Duramax, and Ford 6.7 Power Stroke. Cam bearings are usually full-round bushings pressed into the block tunnels. 3.0 EcoDiesel and many TDI engines are overhead-cam; you will not find in-block cam bearings there. On the test, match the hardware to the engine family.

This section is independent OpenExamPrep teaching of published A9 cam, crank, and select-fit skills. It is not an ASE course and not a claim of official approval.

Inspect in-block cam bearings and replace only with oil holes clocked to the galleries

With the cam out, inspect each cam bearing for scoring, wiped overlay, spun shells, and blocked oil holes. A spun cam bearing on a Duramax or 6.7 Power Stroke often follows oil aeration, a starved pickup, or a plugged gallery after a sludged EGR-soot oil change interval.

If replacement is required:

• Use the correct bearing set for that block (position-specific bearings exist on some engines).
• Clock oil holes to the feed galleries. A 180° error starves a journal even though the bearing “went in.”
• Seat bearings with the proper driver so the oil hole is not smeared shut.
• Confirm the cam turns by hand after install, before followers and gears go on.

Do not reuse a bearing that has turned in the block. Repair the housing bore or replace the block; a “deeper knurl” is not a diesel fix.

Measure the cam, set end play, and inspect followers

Inspect and measure the camshaft:

• Journals: diameter, taper, out-of-round, and scoring
• Lobes: heel-to-toe lift remaining versus spec; pitting and edge wear from roller skid
• Thrust surfaces that control cam end play

Cam end play is not “taken care of by the timing gears.” Gears locate rotational timing; thrust is usually a plate, a flanged rear bearing, or a specified bearing shoulder. Pry the cam (or use the OEM method) and read a dial indicator. Excessive end play pounds the thrust face and can walk injection-pump or gear timing on gear-driven engines. Insufficient end play binds the cam when hot.

Cam followers (roller tappets / roller lifters on 6.7 Cummins, Duramax, and 6.7 Power Stroke) get their own inspection: roller roundness, pin looseness, flat spots, and body scoring. A collapsed or skidded roller will destroy a new cam in minutes. Keep followers in order if they will be reused; never mix a used roller onto a different lobe. If the cam is replaced, follow OEM policy on new followers—many diesel roller cams are a cam-and-follower set.

Worked example: a 2015 LML Duramax with a tick and low oil pressure after a neglected CCV and sludge has scored cam journals. Replacing the cam without new bearings, clean galleries, and new rollers repeats the failure.

Crankshaft: clean, measure, and inspect every mounting surface

Clean the crankshaft as an oiling part, not as a shiny club. Brush oil passages, chase any gallery plugs or drilled-passage plugs the crank uses, and verify plugs are present and tight. Magnaflux a crank that had a rod knock, an overspeed, or a spun bearing. Polish only as the OEM allows; do not roll the fillet radii down, or you raise stress at the journal edge.

Measure journal diameters with an outside micrometer:

• Main journals and rod journals at two axes, front and rear of each journal
• Taper along the journal and out-of-round between axes
• Thrust-face width where the thrust bearing rides

Compare to standard, undersize, and select-fit grades. A diesel crank that is 0.001 in out-of-round is not “close enough for Plastigage later.”

Inspect mounting surfaces:

• Snout: keyway, damper register, and seal journal (wear ring if grooved)
• Flywheel or flexplate flange: cracks at bolt holes, raised metal, and runout
• Reluctor or tone ring if it is crank-mounted—bent teeth create crank-sensor faults that look like a fuel problem

A 6.7 Cummins, Duramax, and 6.7 Power Stroke crank is a heavy forging or casting. Handle it so you do not nick a fillet. Store it standing or in a rack, not rolling on a concrete floor.

Select-fit pistons, rod bearings, and main bearings

Select-fit means the plant (and the technician on a rebuild) matches component grade to measured size so oil clearance lands in a tight diesel window. GM Duramax is the textbook color-code example: main and rod bearing shells and sometimes pistons are graded and painted. Ford and Cummins also publish grade or color charts on many years. The chart—not the leftover assortment in the bearing drawer—picks the part.

Process:

1. Measure each main housing bore (block plus cap, bolts at spec, bearings out) if you are diagnosing a spun bearing or a resized cap.
2. Measure each crank journal.
3. Measure each connecting-rod big end (rod plus cap, bolts at spec, bearings out) and each piston skirt at the OEM gauge point.
4. Open the OEM chart: journal size plus housing size equals a color pair or grade.
5. Install that pair. Do not mix a “red” upper with a random “blue” lower because they were in the same box.

Installing the thickest undersize set “to tighten it up for diesel load” is how you spin a bearing on first fire. Too little clearance kills the oil film; too much clearance knocks and dumps oil pressure at idle.

Pistons are select-fit the same way: cylinder grade plus piston grade. An oversize piston is not a color-code substitute unless the bore was actually machined to that oversize.

Main bearings, cap fit, clearances, and crankshaft end play

Inspect and replace main bearings as a set when any shell is wiped, embedded with debris, or worn through the overlay. Read the wear pattern (covered with pistons in the next section) before you throw the shells away—the pattern tells you whether the crank, the bore, or the cap is the cause.

Cap fit:

• Caps are numbered and usually arrowed to the front. They do not swap.
• Duramax and 6.7 Power Stroke use cross-bolted main caps (side bolts). Torque the vertical bolts and side bolts in the published sequence; a cap pulled in only one plane is out-of-round.
• Some EcoDiesel architectures use a bedplate/ladder. Treat the bedplate as the “caps”: clean dowels, no extra RTV in oil passages, correct sequence.
• Check that the cap sits on its register without a “step.” A filed cap from a previous amateur rebuild is a block or cap replacement, not more sandpaper.

Oil clearance is housing bore with bearings installed minus journal diameter (or a calibrated bore-gauge reading on the bearing ID minus the journal). Typical light-diesel main clearance is on the order of 0.001–0.003 in depending on journal size—use the OEM min/max, not a gasoline rule of thumb.

Plastigage versus micrometer and bore gauge

Plastigage is a rough assembled check. It is not the rebuild method on a crank that sees more than 2,000 psi cylinder load. It will not catch a barrel-shaped journal or a mixed select-fit pair that still “looks like 0.002 in” in one spot. Use micrometers and a bore gauge; use Plastigage only as a confirmation after the measurements already agree.

Crankshaft end play is controlled by the thrust bearing (usually a designated main). Pry the crank fore and aft (or use the OEM lever points) and read a dial indicator on the snout or flange. Excessive end play comes from worn thrust faces—torque-converter ballooning, a clutch that is riding the throwout bearing, or towing with a slipping converter. Insufficient end play binds the crank and overheats the thrust. Correct with the specified thrust bearing, not by grinding the block thrust wall in the bay.

Traps: ignoring cam end play because “it is geared”; painting all mains one color; Plastigage-only rebuilds; swapping main caps; leaving side bolts loose on a Duramax; skipping snout and flange inspection until the damper or flywheel is already on.