4.3 Valve Lash Adjustment & Hydraulic Lash Adjuster Diagnostics

Valve Lash Adjustment & Hydraulic Lash Adjuster Diagnostics

Engine valvetrain operation requires precise control over the clearance between the camshaft profile and the valve stem tip. Depending on engine design, valvetrains utilize either mechanical (solid) valve lash systems or hydraulic lash adjusters (HLAs / hydraulic lifters). Technicians must master mechanical lash setting, shim math for overhead camshafts, and diagnostic procedures for hydraulic lifter noise and oiling faults.

Mechanical vs. Hydraulic Valvetrain Comparison

Mechanical Valve Lash Adjustment Procedures

Mechanical valvetrains must have clearance (lash) when cold to ensure that as the valve stem and valvetrain components expand with engine heat, the valve is not held off its seat.

• Consequences of Incorrect Lash:
  • Too Tight (Insufficient Lash): As the engine reaches operating temperature, thermal expansion holds the valve slightly open off its seat. This prevents proper heat dissipation through the seat, leading to burned valves, low compression, and engine misfires.
  • Too Loose (Excessive Lash): Causes noisy valvetrain clatter, reduces total valve lift and opening duration, degrades engine volumetric efficiency, and subjects valve stem tips and rocker arms to destructive hammering impact loads.

Setting Mechanical Lash (Pushrod & Screw-Adjustment OHC)

1. Engine Operating Temperature: Verify whether specs require cold or hot engine adjustment. Exhaust valves always require wider clearance than intake valves due to higher thermal expansion rates.
2. Cylinder Position: Rotate the crankshaft until the target cylinder is at Top Dead Center (TDC) on its Compression stroke. At TDC compression, both intake and exhaust cam lobes are facing completely away from the lifters (on the base circle).
3. Feeler Gauge Measurement: Insert the specified feeler gauge between the valve tip and rocker arm pad (or cam base circle and follower). Use the "Go/No-Go" gauge technique: the specified gauge must slide with a slight smooth drag, while a gauge 0.002 in thicker must not enter the gap.
4. Locking Mechanism: Hold the adjustment screw stationary using a screwdriver or Allen wrench while tightening the locknut with a box wrench. Re-check feeler gauge drag after tightening the locknut, as torquing can alter the setting.

Overhead Camshaft (OHC) Bucket Lifter Shim Calculations

Many OHC and DOHC engines utilize bucket-type followers with selective adjustment shims located either over the bucket (shim-over-bucket) or under the bucket (shim-under-bucket).

The OHC Shim Calculation Formula

$ext{New Shim Thickness} = ( ext{Measured Clearance} - ext{Specified Clearance}) + ext{Present Shim Thickness}$

Overhead Camshaft Shim Calculation Example Table:

+-------------------------------------+-----------------------+-----------------------+
| Parameter Description               | Imperial Specification| Metric Specification  |
+-------------------------------------+-----------------------+-----------------------+
| Factory Specified Clearance         | 0.010 in              | 0.25 mm               |
| Actual Measured Clearance           | 0.014 in              | 0.35 mm               |
| Measured Clearance Deviation        | +0.004 in (Excessive) | +0.10 mm (Excessive)  |
| Present Shim Thickness Removed      | 0.100 in              | 2.54 mm               |
| Calculated New Shim Thickness Needed| 0.104 in              | 2.64 mm               |
+-------------------------------------+-----------------------+-----------------------+
Result: Installing a 0.104 in (2.64 mm) shim reduces lash by 0.004 in, restoring 0.010 in spec.

• Procedure for Under-Bucket Systems: Under-bucket shims require removing the camshafts to access and exchange shims. Always measure and record all valve clearances before removing camshafts to minimize teardown cycles.

Hydraulic Lash Adjuster (HLA) Operation & Preload Setup

Hydraulic lash adjusters automatically maintain zero valvetrain lash using engine oil pressure.

                      +-----------------------------+
                      |   Pushrod / Rocker Contact  |
                      +--------------+--------------+
                                     |
                         +-----------+-----------+
                         |   Plunger Assembly    |
                         |   (High-Pressure      |
                         |    Oil Chamber)       |
                         +-----------+-----------+
                                     |  <--- Internal Check Ball & Spring
                         +-----------+-----------+
                         |   Lifter Body & Base  |
                         +-----------------------+
                                     ^
                         Engine Oil Gallery Feed

• Operating Principle: Low-pressure engine oil enters the lifter body oil groove and passes into the inner plunger. When the valve is closed, the plunger spring expands to eliminate all valvetrain play. As the cam lobe pushes against the lifter, oil inside the lower high-pressure chamber pushes upward against the check ball, sealing the chamber. Liquid oil cannot compress, transforming the lifter into a solid hydraulic column that opens the valve.

Setting Hydraulic Lifter Preload (Pedestal/Stud Rockers)

On adjustable pushrod engines, hydraulic lifters require setting plunger preload:

1. Rotate engine to TDC compression for the cylinder being adjusted.
2. Loosen the rocker arm nut, then slowly tighten it while rotating or wiggling the pushrod up and down with your fingers.
3. The exact point where vertical pushrod end-play is completely eliminated is Zero Lash.
4. From Zero Lash, tighten the rocker adjusting nut an additional specified turn (typically 1/4 to 1 full turn). This depresses the internal lifter plunger into the center of its operating travel range (preload).

Hydraulic Lifter Diagnostic & Troubleshooting Guide

Hydraulic Lifter Bleed-Down Testing

A lifter leak-down (bleed-down) tester measures the time required for a hydraulic lifter plunger to collapse a specified distance under a constant load (e.g., 50 lbs) while submerged in testing oil.

• Fast Bleed-Down: Indicates a worn check ball seat or excessive plunger-to-body clearance. Fast bleed-down causes lifter collapse and valvetrain clatter under heavy load.
• Slow / No Bleed-Down: Indicates varnish deposits trapping the plunger. A stuck plunger acts like a solid lifter and can hold the valve open when hot.