Cummins Valve Lash Adjustment: When & How

What Valve Lash Is and Why It Matters on the Cummins

Valve lash (or valve clearance) is the gap between the rocker arm and the top of the valve stem when the lifter is on the base circle of the cam lobe (valve closed). Cummins ISB 5.9L and 6.7L engines use a solid lifter, push-rod, rocker arm valvetrain with mechanical lash adjustment -- there are no hydraulic lifters that self-compensate. Thermal expansion of the valve, valve seat wear, pushrod stretch, and rocker arm wear all change the effective lash over time. Too much lash causes a noisy top end, reduced valve lift, and reduced engine output. Too little lash (or zero lash) prevents the valve from fully closing during the compression stroke, burns the valve seat and valve face from hot combustion gases, and eventually drops compression or holes a valve. The Cummins recommended service interval -- every 150,000 miles or at major engine work -- exists specifically to catch wear before it damages valves.

Symptoms That You Need an Overhead

Loose lash: audible ticking or tapping from the valve cover area at idle, slightly louder cold than hot, often worse on one side of the engine than the other. Gentle top-end noise that gets louder with RPM. Slightly reduced fuel economy. Tight lash: progressively harder cold starts, rough idle, a cylinder that seems weak on a cylinder cutout test, low compression on one or more cylinders, hissing from the exhaust manifold on compression stroke (audible at the manifold with the valve cover off), and eventually white smoke at startup from incomplete combustion in the affected cylinder. Blown head gasket and burned valve damage are the expensive end states if tight lash is ignored; a $30 valve adjustment prevents a $3,000 head job.

Tools and Specs You Will Need

Service manual spec for the 5.9L ISB (2003-2007) and 6.7L ISB (2007.5+) is the same: intake 0.010 inch (0.254 mm), exhaust 0.020 inch (0.508 mm). Feeler gauge set with 0.010 and 0.020 leaves is required -- a 0.015 leaf is useful for go/no-go checks (it should not fit into the intake gap). The adjusting screw takes a flat screwdriver or hex key, with a 9/16-inch (14mm) lock nut on common-rail ISBs. Torque spec on the lock nut after adjustment: 24 N-m, which is 18 ft-lb (212 in-lb) -- the SAME spec for both the 5.9L and 6.7L. (Do not confuse the newton-meter figure for a foot-pound value; 24 ft-lb would over-torque every rocker.) Bar-over tool or a socket on the damper bolt to rotate the crankshaft -- never use the starter to bump the engine. Valve cover gasket is reusable on most years (inspect and replace if cracked or compressed). New valve cover bolts are not required but the seals on the hold-down bolts should be inspected; replace if leaking.

The Procedure Step by Step

Let the engine cool to ambient temperature -- lash is measured cold on Cummins ISB. Remove the intake air tube and any accessories in the way of the valve cover. Remove the valve cover bolts and lift the cover straight up. Bring cylinder 1 to TDC compression by rotating the crankshaft until the damper mark aligns and both rocker arms on cylinder 1 are loose (valves closed). Set cylinder 1 intake to 0.010 inch with the feeler gauge -- the gauge should slide with light drag. If out of spec, loosen the lock nut, turn the adjusting screw, tighten the lock nut to 24 N-m (18 ft-lb), and re-check (torqueing the lock nut often shifts the clearance slightly so always re-check after). Set cylinder 1 exhaust to 0.020 inch. Rotate the crank 120 degrees to cylinder 5 TDC compression. Set cylinder 5 valves. Continue in firing order: 1-5-3-6-2-4. After all six cylinders are adjusted, rotate the engine through two full revolutions and spot-check a couple of cylinders to confirm nothing drifted. Install valve cover with proper bolt torque (18 ft-lb typical for M8 valve cover bolts). Reinstall accessories.

Common Mistakes to Avoid

Adjusting a hot engine -- Cummins spec is cold (at ambient, under 90 F head temp). Hot numbers are different and you will end up with tight lash when the engine cools. Forgetting to torque the lock nut before checking final clearance -- the adjustment moves when the lock nut tightens. Setting both valves at the same time instead of waiting for TDC compression of each cylinder. Mixing up intake and exhaust specs -- intakes are always tighter than exhausts on the 24-valve ISB. Using a fresh bare feeler gauge without wiping oil off -- residual oil film makes the gauge feel tighter than it is. Reusing crushed or oil-soaked valve cover gaskets (they leak). Over-torquing the valve cover bolts (strips the aluminum threads on the head). Running the engine without verifying all valves were actually adjusted -- skip cylinder 3 by accident and you will hear it within an hour.

When to DIY vs When to Pay a Shop

A competent mechanic with the tools can do a 5.9L or 6.7L ISB overhead in 2-3 hours. Shop labor runs $250-$500 depending on region. The job is accessible for a patient DIYer -- no specialty Cummins tools are required, just good wrenches, a torque wrench, and a feeler gauge. Budget a weekend for your first time. Pay the shop if: you do not own a torque wrench, you have never pulled a valve cover, the truck is under Cummins warranty, or you are adjusting an engine with known damage (a tight valve that was run for miles may need a compression test and leak-down to verify the valve itself is not burned already). DIY if: you have done cylinder head work before, you want to know exactly how it was done, or you are on a commercial truck schedule where $400 labor every 150,000 miles gets old.

Frequently Asked Questions