5.6 Lifter Preload, Valve Lash and OHC Service
Key Takeaways
Set lash or preload with the lifter on the base circle under the specified thermal condition.
Measure preload after machining; rocker geometry prevents a simple universal depth-addition rule.
Cam lobe lift is major lobe diameter minus base-circle diameter.
Note
Lifter Preload, Valve Lash and OHC Service uses stated teaching examples. Actual pin assignments, dimensions, thresholds and procedures must come from the identified engine, component and diagnostic tool.
Hydraulic vs. Mechanical Lifters and Valve Lash / Preload (Task B.7)
| Valvetrain Component / System | Critical Inspection & Measurement Standard | Adjustment / Corrective Action Specification |
|---|---|---|
| Hollow Tubular Pushrods | Roll on surface plate / V-blocks (≤ 0.002 in. runout); blow air through oil hole | Replace bent or plugged pushrods; never straighten; keep intake/exhaust lengths matched |
| Flat-Tappet Lifter Base | Check foot with straightedge (0.001–0.002 in. convex crown in center) | Flat or concave/dished lifter base stops rotation and wipes cam lobe; replace cam & lifters |
| Hydraulic Roller Lifter | Check roller wheel for pitting/flat spots, needle bearing play, and dog-bone / tray guides | Replace pitted rollers or cracked alignment trays (prevents roller turning sideways on lobe) |
| Non-Adjustable Pedestal Preload | Lifter on base circle; torque pedestal bolt (e.g., 22 lb-ft); 0.040–0.080 in. preload | Valve job raises stem tip and increases preload; install shorter select-fit pushrods |
| Adjustable Hydraulic Lifter | Piston at TDC compression; tighten nut to zero lash (no vertical pushrod play) | Turn nut 1/2 to 1 full turn tighter past zero lash to center plunger in travel |
| Mechanical / Solid Valve Lash | Measure gap with flat go/no-go feeler gauge between rocker/cam and stem tip | Exhaust and intake lash are design-specific due to greater exhaust stem heat expansion |
| OHC Camshaft Lobe & Journals | Lobe Lift = Major Dia − Base Circle; journal oil clearance 0.0010–0.0030 in. | Check cam tunnel alignment with straightedge; measure axial endplay (0.0015–0.0060 in.) |
When inspecting lifters (Task B.7), distinguish between flat-tappet and roller designs:
- Flat-Tappet Lifters: Hold the blade of a precision steel straightedge across the bottom cam-contact foot of the lifter against a light source. A serviceable flat-tappet lifter foot is ground with a slight convex crown (
0.001 to 0.002 in.higher in the center than at the outer edges). This convex crown mates against a slight taper (~0.5° to 1.0°) machined into the camshaft lobe so the lobe contacts the lifter slightly off-center, forcing the lifter to spin continuously in its bore and distribute wear evenly. If the lifter foot wears completely flat or becomes concave (dished), light shows under the center of the straightedge; a dished lifter stops rotating and rapidly wipes the lobe off the camshaft. - Hydraulic Roller Lifters and Roller Finger Followers: Inspect the hardened steel roller wheel for micro-pitting, spalling, or flat skidding marks, and check the roller axle needle bearings for radial play. Always inspect the lifter alignment guides (steel "dog-bone" retainer plates and hold-down spider, or composite plastic lifter guide trays) for cracks or worn flats—if an alignment tray fails and allows a roller lifter to twist even a few degrees in its bore, the roller wheel skews sideways across the camshaft lobe and causes catastrophic engine failure.
Important
How a Valve Job Changes Hydraulic Lifter Preload on Non-Adjustable Valvetrains: Most modern pushrod truck engines (such as the Ford 7.3L and GM 6.0L/6.6L V8) use non-adjustable net-lash pedestal rocker arms. With the lifter resting on the camshaft base circle (heel), torquing the rocker pedestal bolt to specification (e.g., 22 lb-ft) automatically depresses the hydraulic lifter plunger 0.040 to 0.080 in. (1.0 to 2.0 mm) below its internal retaining clip—this distance is hydraulic lifter preload. When a cylinder head is resurfaced or its valve seats and faces are ground during a valve job, the valve stem rises higher toward the rocker arm. That extra stem height pushes the pushrod down and increases hydraulic lifter preload. If preload exceeds the maximum limit, the lifter plunger bottoms out solidly inside the lifter body and holds the valve slightly off its seat, causing a rough idle, low compression, and burned valves. Because the pedestal bolt is non-adjustable, technicians restore proper 0.040–0.080 in. preload by installing shorter select-fit pushrods (or by correcting valve stem tip height within case-hardening limits).
On engines with adjustable hydraulic lifters, rotate the crankshaft to bring the target cylinder to TDC on the compression stroke (confirming both intake and exhaust lifters sit on the camshaft base circle). Back off the rocker adjusting nut until play is felt, spin or gently move the pushrod up and down with your fingers while tightening the nut just until all vertical free play disappears (zero lash), and then turn the adjusting nut an additional 1/2 to 1 full turn (per OEM specification) to center the hydraulic plunger in its travel.
On engines with mechanical (solid) lifters or solid OHC followers, measure physical valve lash with a flat feeler gauge (using a go/no-go step blade, where the thinner GO step slides smoothly through the gap and the 0.002 in. thicker NO-GO step will not enter). Exhaust valve lash is often wider, but the specified intake and exhaust values govern (for example, 0.012 in. intake vs. 0.018 in. exhaust) because the hotter exhaust valve stem elongates significantly more from thermal expansion during heavy-duty operation.
Overhead Camshaft (OHC) & Bearing Inspection and Endplay (Task B.8)
On overhead camshaft (OHC) commercial engines (Task B.8), inspect camshaft lobes for scoring or pitting and measure camshaft lobe lift with an outside micrometer by subtracting the base-circle diameter (across the low heel of the lobe) from the major lobe diameter (from heel to nose tip):
Camshaft Lobe Lift = Major Lobe Diameter − Base-Circle Diameter
Measure camshaft journal diameters with an outside micrometer for taper and out-of-round, and measure camshaft journal-to-bearing/saddle oil clearance (typically 0.0010 to 0.0030 in. / 0.025–0.076 mm) using Plastigauge laid across each journal (with all roller followers removed so valve spring load does not push the camshaft down against the lower saddle and give a false reading) or inside/outside micrometers. Remember that many aluminum OHC truck cylinder heads run the camshaft journals directly inside precision-machined aluminum head saddles without replaceable bearing inserts—if oil starvation (often caused by a plugged oil feed passage or neglected oil changes) galls the aluminum cam saddles, the cylinder head must be replaced unless the OEM permits a verified bearing/tunnel repair.
Tip
Preventing OHC Camshaft Breakage During Cap Removal and Installation: Chilled cast-iron and hollow-tube overhead camshafts are extremely brittle in bending. Before installing an OHC camshaft, check camshaft tunnel bore alignment across the lower head saddles with a straightedge, verify every bearing cap number (I1–I5 / E1–E5) and directional arrow (pointing toward the front timing drive), and tighten the bearing cap bolts gradually in 1/2-turn crisscross stages across all caps. Tightening one cam cap all the way down while adjacent lobes are pressing against stiff valve springs will snap the camshaft in half.
Finally, measure camshaft axial endplay by mounting a dial indicator on the front of the cylinder head (or engine block) with its plunger parallel to the camshaft centerline resting against the camshaft snout or sprocket hub. Gently pry the camshaft fully rearward, zero the dial indicator, and pry the camshaft fully forward. Typical camshaft axial endplay is 0.0015 to 0.0060 in. (0.038 to 0.152 mm), controlled by a machined thrust flange on the camshaft riding between a thrust saddle in the cylinder head or a bolt-on front camshaft thrust plate. Excessive camshaft endplay causes erratic Camshaft Position (CMP) sensor signals, timing chain wear, and an audible front-cover knock at warm idle.
Worked Example: Valve Preload & OHC Lobe Lift Scenario
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Measured Hydraulic Preload Example: The stated acceptable range is 0.045–0.075 inch. After head and valve work, a correctly set measuring procedure finds 0.095 inch preload, which exceeds the maximum by 0.020 inch. An adjustable pushrod checker establishes the length that restores 0.060 inch preload. Do not simply add head-milling depth to valve recession: rocker geometry changes the motion ratio. Confirm length, contact pattern and preload directly before ordering a select-fit pushrod.
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OHC Camshaft Lobe Lift Check: On a Ford 6.8L V10 OHC camshaft, Cylinder #2's intake lobe measures
1.682 in.across the major lobe diameter (heel to nose) and1.410 in.across the base circle. Subtracting gives1.682 in. − 1.410 in. = 0.272 in.lobe lift, matching the0.270–0.274 in.specification.
A commercial V8 engine with non-adjustable pedestal-mount hydraulic roller rocker arms has a rough idle and low compression on Cylinder #4 immediately after a cylinder head valve job. During the valve job, the machinist ground the Cylinder #4 exhaust valve face and cut the exhaust valve seat by a combined 0.030 in., and milled 0.010 in. off the head deck, but reinstalled the original pushrods without grinding the valve stem tip. What is the most likely cause of the low compression?
The exhaust pushrod oil hole became enlarged during the valve job, bleeding off oil pressure and increasing mechanical valve lash.
Grinding the valve face and seat moved the valve stem tip lower toward the combustion chamber, leaving excessive clearance between the rocker arm and valve stem tip.
The exhaust valve lash is too loose because exhaust valves contract slightly as they heat up to normal operating temperature.
Sinking the valve into the seat and milling the head deck decreased the distance between the camshaft and rocker arm pad, increasing hydraulic lifter preload beyond specification so the bottomed lifter plunger holds the exhaust valve slightly off its seat.
A technician is inspecting the overhead camshaft (OHC) assembly on an aluminum V10 truck cylinder head (Task B.8). Which procedure correctly measures camshaft journal oil clearance and camshaft axial endplay?
Leave all roller followers and valve springs installed so spring pressure holds the camshaft tight, place Plastigauge under the lower journal saddle, and measure axial endplay with a flat feeler gauge between the cam lobes.
Remove the roller followers so valve spring load does not push the camshaft down, lay a strip of Plastigauge across each camshaft journal and torque the numbered directional bearing caps in gradual sequence to check 0.001 to 0.003 in. oil clearance, and mount a dial indicator inline with the camshaft nose while prying the camshaft fore and aft to measure 0.0015 to 0.0060 in. axial endplay.
Tighten each camshaft bearing cap completely one at a time from rear to front against full valve spring pressure, then measure lobe lift by adding the base-circle diameter to the major lobe diameter.
Measure camshaft journal oil clearance by prying the camshaft forward and rearward with a dial indicator, and measure axial endplay by rolling the camshaft across V-blocks on a surface plate.
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