Why NTA855 overhauls should be planned as a set
The NTA855 is a 14-litre inline-six that powers a large number of marine mains, auxiliary sets and industrial gensets. When an overhaul comes back with blow-by, high oil consumption or a knocking noise, the cause is usually one of three things: liners and rings from different batches, bearing clearances not matched to the measured journal, or seals that have been on the shelf for years.
Parts to replace as a matter of course
- Cylinder kit — liner, piston, rings, pin and retainers supplied per cylinder, so every hole is built from the same batch.
- Main and connecting rod bearings — select standard or oversize from the measured journal, then verify clearance before assembly.
- Head gasket and rocker cover gaskets — single-use items; do not reinstall after teardown.
- Valves and seats — check valve recession after grinding and replace seats when out of spec.
- Oil and water seals — front and rear crankshaft seals, water pump seal; prefer the newest rubber batch available.
Parts that are often overlooked
- Tappets and pushrods — wear here keeps changing valve clearance.
- Camshaft bearings and the camshaft itself — first to suffer when oil is dirty.
- Oil pump and pick-up tube seals — a common source of low pressure after overhaul.
- Damper and pulleys — rubber ageing is hard to see, so replace by hours.
- Injector sleeves and seals — a hidden path for coolant into the fuel system.
Assembly and acceptance checks
- Measure liner protrusion after pressing and keep all cylinders within the same tolerance.
- Stagger ring gaps at 120 degrees and clean ring grooves completely.
- Coat bearings with clean oil and tighten rod bolts to the specified torque plus angle.
- Bar the engine over at least two full turns by hand before cranking.
- After first start, recheck oil pressure, exhaust temperature, blow-by and coolant level.
Buying advice
Order the overhaul set per cylinder in one batch, then top up with single items only after teardown. Mixed batches are the most common reason a fresh overhaul does not perform as expected. For NTA855 liners, pistons, bearings, gaskets and valves, see our Cummins Engine Parts category or contact us with your engine model and part numbers.