Camshaft and Valve Train Noise After Head Work
A shop replaced a cracked cylinder head on a 12 L diesel and reused the original camshaft, lifters, and pushrods. The engine starts and runs but has a rhythmic top-end tick that is loudest at idle and fades above 1500 rpm. Oil pressure is good. The technician removed the valve cover and found one rocker with noticeably more lash than the rest, and the corresponding pushrod is straight. The camshaft lobe for that cylinder looks polished but not scored. The engine has 180,000 miles and the head was replaced because of a coolant leak at the injector cup, not because of a valve train complaint. The customer is upset and wants the engine fixed today.
1. The technician wants to simply adjust the lash on the noisy rocker and release the truck. Evaluate that plan.
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Adjusting lash and releasing the truck is risky. A rhythmic tick that appears right after head work, with one rocker showing excess lash and a straight pushrod, points to a change in the valve train geometry or a worn component that was disturbed during the job. Simply taking up lash may mask the cause and can lead to a dropped valve or a bent pushrod if the real problem is a worn cam lobe, a collapsed lifter, or a mis-seated rocker.
Before adjusting:
- Compare the cam lobe lift on the noisy cylinder to a good cylinder using a dial indicator on the pushrod or rocker. A polished lobe can still be worn.
- Check the lifter face for spalling and the lifter bore for wear.
- Verify the rocker shaft and bushings, and check that the correct head gasket and pushrod lengths were used.
- Confirm the valve stem height and installed height match the new head; a head with different valve seat recession changes lash.
If the geometry and components check out, adjust lash to spec and recheck hot. If cam lift is low or a lifter is worn, the repair is a cam and lifter replacement, not a lash adjustment.
2. Cam lift on the noisy cylinder measures 0.020 in less than a good cylinder. What does that tell you, and what is the correct repair and break-in procedure?
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A 0.020 in lift deficit indicates a worn cam lobe (or a worn lifter face), even though the lobe looks polished rather than scored. Polishing with material loss is classic lobe wear. Because the engine has 180,000 miles and the cam was reused, the correct repair is to replace the camshaft and all lifters as a set, and inspect the pushrods, rockers, and cam bearings.
Repair and break-in:
- Replace cam and lifters together; never install new lifters on a worn cam or a used cam with new lifters unless the cam is known good.
- Check cam bearing clearance and lobe taper/orientation.
- Use the specified break-in oil or an oil with the correct anti-wear additive for a flat-tappet cam if applicable; many heavy-duty diesels use roller lifters, in which case break-in is less critical but cleanliness still matters.
- Pre-lube the valve train, set lash to spec, run at a fast idle per the manufacturer's break-in schedule, then retorque and readjust lash hot.
- Change oil and cut the filter after break-in to check for debris.
Document the lift measurements before and after so the customer understands why a lash adjustment was not the fix.