Yesterday, 06:51 PM
The NTC-350’s Role in Heavy-Duty Diesel History
The Cummins NTC-350 was a cornerstone of American diesel engineering in the 1970s and 1980s. Built on the NH/NT block architecture, the NTC series featured turbocharged inline-six engines with mechanical fuel systems and robust cast iron construction. Rated at 350 horsepower, the NTC-350 powered countless Class 8 trucks, construction equipment, and oilfield rigs. Cummins, founded in 1919, had by then become a global leader in diesel innovation, and the NTC series helped solidify its dominance in long-haul and vocational markets.
The “C” in NTC stands for “Controlled,” referring to the use of a PT (pressure-time) fuel system with calibrated injectors and governed fuel delivery. These engines were known for their longevity, rebuildability, and straightforward mechanical layout. Valve adjustment was a critical part of their maintenance routine, directly affecting power output, fuel efficiency, and engine life.
Terminology Notes
Valve lash affects how long and how far valves open during combustion. Too tight, and valves may not seat properly, leading to burnt valves and loss of compression. Too loose, and the valve timing becomes erratic, reducing efficiency and increasing wear. On the NTC-350, proper valve adjustment also ensures the Jake Brake functions correctly, as it relies on precise valve actuation.
Cummins recommended valve adjustments every 100,000 miles or annually, depending on duty cycle. In high-load applications like logging or mountain hauling, more frequent checks were advised.
Step-by-Step Valve Adjustment Procedure
A 1974 Kenworth hauling timber through the Cascades began losing power on climbs. The operator suspected fuel issues but found no injector faults. After performing a valve adjustment, several exhaust valves were found with lash over 0.040 inches. Once corrected, the truck regained torque and fuel economy improved by 8%. “It was breathing through a straw,” the mechanic said. “Now it’s back to full lungs.”
Recommended Parameters
A snowplow crew in Maine adjusted valves on their NTC-350 after noticing hard starts in cold weather. The intake valves were too tight, reducing cylinder fill. After correction, cold starts improved and idle stabilized. “It’s not just about power—it’s about balance,” the foreman noted.
In Texas, a fleet manager added valve adjustment logs to each truck’s maintenance binder. Over time, they noticed that trucks with consistent lash settings had fewer breakdowns and better fuel mileage. “It’s the cheapest horsepower you’ll ever find,” he said.
Parts Availability and Support
Valve adjustment on the Cummins NTC-350 is a ritual of mechanical stewardship. It’s where precision meets performance, and where a few thousandths of an inch can make the difference between sluggish and strong. For operators who still run these vintage powerplants, valve lash isn’t just a spec—it’s a statement of care. In the world of diesel, the NTC-350 still roars when its valves are set right.
The Cummins NTC-350 was a cornerstone of American diesel engineering in the 1970s and 1980s. Built on the NH/NT block architecture, the NTC series featured turbocharged inline-six engines with mechanical fuel systems and robust cast iron construction. Rated at 350 horsepower, the NTC-350 powered countless Class 8 trucks, construction equipment, and oilfield rigs. Cummins, founded in 1919, had by then become a global leader in diesel innovation, and the NTC series helped solidify its dominance in long-haul and vocational markets.
The “C” in NTC stands for “Controlled,” referring to the use of a PT (pressure-time) fuel system with calibrated injectors and governed fuel delivery. These engines were known for their longevity, rebuildability, and straightforward mechanical layout. Valve adjustment was a critical part of their maintenance routine, directly affecting power output, fuel efficiency, and engine life.
Terminology Notes
- Valve Lash: The clearance between the rocker arm and valve stem, allowing for thermal expansion.
- Jake Brake: An engine compression brake that uses the valve train to slow the vehicle.
- PT Fuel System: A Cummins-designed injection system using pressure and timing to meter fuel.
- Rocker Arm: A pivoting lever that transfers camshaft motion to the valve.
- Crosshead: A bridge component that connects dual intake or exhaust valves to a single rocker arm.
Valve lash affects how long and how far valves open during combustion. Too tight, and valves may not seat properly, leading to burnt valves and loss of compression. Too loose, and the valve timing becomes erratic, reducing efficiency and increasing wear. On the NTC-350, proper valve adjustment also ensures the Jake Brake functions correctly, as it relies on precise valve actuation.
Cummins recommended valve adjustments every 100,000 miles or annually, depending on duty cycle. In high-load applications like logging or mountain hauling, more frequent checks were advised.
Step-by-Step Valve Adjustment Procedure
- Preparation
- Warm engine to operating temperature
- Remove valve cover and Jake Brake housing if equipped
- Rotate engine to Top Dead Center (TDC) on cylinder 1 using barring tool
- Warm engine to operating temperature
- Adjustment Sequence
- Use Cummins firing order: 1-5-3-6-2-4
- Adjust intake and exhaust valves on cylinders at TDC
- Intake Lash: 0.014–0.016 inches
- Exhaust Lash: 0.027–0.029 inches
- Use feeler gauge and lock nut wrench
- Torque lock nuts to spec (typically 25–30 ft-lbs)
- Use Cummins firing order: 1-5-3-6-2-4
- Jake Brake Adjustment
- Set lash between actuator and valve crosshead
- Typical spec: 0.020 inches
- Ensure solenoid function and wiring integrity
- Set lash between actuator and valve crosshead
- Final Checks
- Reinstall valve cover with new gasket
- Start engine and listen for valve chatter or misfire
- Monitor exhaust for smoke and idle smoothness
A 1974 Kenworth hauling timber through the Cascades began losing power on climbs. The operator suspected fuel issues but found no injector faults. After performing a valve adjustment, several exhaust valves were found with lash over 0.040 inches. Once corrected, the truck regained torque and fuel economy improved by 8%. “It was breathing through a straw,” the mechanic said. “Now it’s back to full lungs.”
Recommended Parameters
- Intake Valve Lash: 0.014–0.016 inches
- Exhaust Valve Lash: 0.027–0.029 inches
- Jake Brake Lash: 0.020 inches
- Valve Cover Torque: 18–22 ft-lbs
- Adjustment Interval: Every 100,000 miles or 1,000 hours
- Feeler Gauge Accuracy: ±0.001 inch
- Always adjust valves with engine warm for accurate clearance
- Replace valve cover gasket during each service to prevent leaks
- Inspect rocker arms and crossheads for wear or pitting
- Clean valve train components before reassembly
- Use torque wrench on lock nuts to prevent thread damage
- Record lash measurements for trend analysis
A snowplow crew in Maine adjusted valves on their NTC-350 after noticing hard starts in cold weather. The intake valves were too tight, reducing cylinder fill. After correction, cold starts improved and idle stabilized. “It’s not just about power—it’s about balance,” the foreman noted.
In Texas, a fleet manager added valve adjustment logs to each truck’s maintenance binder. Over time, they noticed that trucks with consistent lash settings had fewer breakdowns and better fuel mileage. “It’s the cheapest horsepower you’ll ever find,” he said.
Parts Availability and Support
- Valve cover gaskets, rocker arms, and crossheads available through Cummins legacy parts dealers
- Feeler gauges and barring tools stocked by diesel tool suppliers
- Technical manuals include firing order diagrams and torque specs
- Jake Brake components supported by Jacobs and aftermarket vendors
- PT fuel system calibration tools available for full tune-ups
Valve adjustment on the Cummins NTC-350 is a ritual of mechanical stewardship. It’s where precision meets performance, and where a few thousandths of an inch can make the difference between sluggish and strong. For operators who still run these vintage powerplants, valve lash isn’t just a spec—it’s a statement of care. In the world of diesel, the NTC-350 still roars when its valves are set right.
