Cummins NT855 overhaul kits support top-end repair, in-frame rebuilding and complete engine overhaul for the established N855 engine family. Available configurations can be prepared for NT855, NTA855, NH855 and related application-specific engines used in marine vessels, generator sets, oilfield equipment, industrial drives and stationary power systems.
The 855 platform is built around a 14.0L inline six-cylinder architecture with replaceable wet cylinder liners and the Cummins Pressure Time fuel system. However, the N855 family has been produced across multiple generations, aspiration arrangements, camshaft revisions, ratings and applications.
An overhaul kit should therefore not be ordered only by the “NT855” name, an old compression-ratio table or a broad statement such as “Big Cam.” ZEB Power matches Cummins NT855 engine parts and overhaul packages using the Engine Serial Number, CPL, complete engine suffix, current part numbers, installed components and workshop measurements.
- Top-end, in-frame, complete and customized repair packages
- Six-cylinder piston, ring, wet-liner and bearing groups
- Support for NT855, NTA855, NH855 and identified N855-family variants
- Small Cam and Big Cam configuration checks
- Cummins PT fuel-system component identification
- Standard and approved repair-size bearing options
- Genuine Cummins and documented replacement alternatives
- Individual components, workshop bundles and complete kits
NT855 Product and Selection Overview
| Selection Item | NT855-Family Information | Purchasing Significance |
|---|---|---|
| Engine architecture | 14.0L inline six-cylinder diesel engine | A complete power-cylinder rebuild normally requires matched components for six cylinders. |
| Cylinder construction | Replaceable wet cylinder liners | Liner reference, seal arrangement and installed protrusion must be verified. |
| Fuel system | Cummins Pressure Time system with camshaft-actuated injectors | Pump, injector, governor and timing components depend on the installed engine configuration. |
| Repair options | Top-end, in-frame, complete or customized | The repair scope should be based on inspection results rather than on operating hours alone. |
| Family variants | N855, NH855, NT855, NTA855, NTC and other identified configurations | Family similarity does not guarantee component interchangeability. |
| Production generations | Small Cam, Big Cam and application-specific revisions | Camshaft, cylinder-head, fuel-system and internal component differences must be checked. |
| Primary applications | Marine, generator, oilfield, industrial and stationary power | Cooling, aspiration, speed and duty rating affect the final BOM. |
| Primary selection method | ESN-specific parts information supported by CPL and physical verification | Reduces the risk of receiving mismatched pistons, rings, liners, bearings or gaskets. |
NT855, NH855 and NTA855 Are Not One Universal Configuration
The different 855-family designations describe related engines, but they should not be treated as interchangeable product numbers. Aspiration, aftercooling, fuel settings, camshaft generation, cylinder-head configuration and application hardware may differ.
| Family Designation | General Identification | Kit-Selection Risk |
|---|---|---|
| N855 | Broad base-family designation used across different historical applications | The exact aspiration, rating and production generation must still be identified. |
| NH855 | Historical N-series configuration found in industrial, marine and stationary installations | Do not assume that an NH855 uses the same piston, head or gasket package as an NT855. |
| NT855 | Turbocharged member of the 855 family | Aftercooling, piston cooling, rating and camshaft generation can vary by complete suffix. |
| NTA855 | Turbocharged and aftercooled configuration | Piston, turbocharging, cooling and fuel-system requirements may differ from an NT855. |
| NTC and related Big Cam engines | Related production generations with camshaft, fuel-system and component revisions | “Big Cam” alone is not a complete parts reference. |
| Generator-drive suffixes | Fixed-speed engines configured for generator duty | Rated speed, frequency, governor and cooling package must be confirmed. |
| Marine suffixes | Propulsion or auxiliary engines with marine-specific systems | Cooling, exhaust, rating and certification requirements may change the supply scope. |
Some parts may be shared between individual engines when the current part numbers are identical. This must be demonstrated through the ESN-specific catalog rather than assumed from the family name.
Small Cam and Big Cam Identification
The 855 platform includes multiple production generations commonly described as Small Cam and Big Cam. These terms are useful for preliminary identification, but they do not replace the complete engine model, ESN and CPL.
| Identification Point | Possible Difference | Required Check |
|---|---|---|
| Camshaft generation | Camshaft diameter, bushings, followers and related block features may differ | Confirm the installed camshaft and current catalog references. |
| Cylinder-head configuration | Head casting, injector arrangement, valve components and gasket groups may vary | Record the casting or part numbers and inspect each head. |
| PT fuel-system version | Pump, injector, governor and calibration arrangements may differ | Provide the fuel-pump dataplate and injector numbers. |
| Piston design | Crown, bowl, compression height, ring grooves and cooling arrangement may vary | Confirm the piston number or an approved supersession. |
| Lubrication system | Oil routing, cooling nozzles and pressure-regulating components may differ | Inspect the installed system before adding supporting parts. |
| Previous rebuild history | An older engine may contain later replacement components | Compare ESN data with the parts physically removed from the engine. |
A visual description such as “Big Cam 3” or “old NT855” may help begin the search, but it is not sufficient for releasing a piston, liner or complete overhaul package.
Choose the Repair Scope from the Engine Condition
Top-end, in-frame and complete overhaul kits serve different repair objectives. The appropriate option depends on which engine areas remain within the applicable service limits.
| Repair Program | Main Objective | Typical Work Area | Approval Requirement |
|---|---|---|---|
| Top-End Repair | Restore cylinder-head sealing and valve-train condition | Head gaskets, upper gasket sets, valve seals and selected head components | Pistons, liners, bearings and the lower engine must remain serviceable. |
| In-Frame Rebuild | Restore the six-cylinder power assembly while the crankshaft remains installed | Pistons, rings, wet liners, liner seals, connecting-rod bearings and gasket sets | Crankshaft journals, main-bearing area, connecting rods and block must remain repairable without complete removal. |
| Complete Overhaul | Inspect and rebuild the power-cylinder and lower-engine systems | In-frame components plus main bearings, thrust parts and other lower-end components | Required when the crankshaft, block, camshaft system or main-bearing area needs full disassembly or machining. |
| Customized Component Package | Replace only the components identified during teardown | Selected piston kits, liners, bearings, gasket groups or supporting-system components | Requires a documented inspection and approved part-number list. |
The fact that an engine can be serviced inside the equipment does not prove that an in-frame rebuild is sufficient. The crankshaft, bearing areas, connecting rods, block and lubrication system must still be inspected.
A complete overhaul kit also does not automatically include cylinder heads, fuel-system components, turbochargers, pumps, controls or every fastener required for the finished engine.
Identification Hierarchy for Correct Parts Selection
| Reference | Information Provided | How It Should Be Used |
|---|---|---|
| Engine Serial Number | Individual as-built engine configuration | Use as the primary reference for the applicable parts catalog and current supersessions. |
| Complete model and suffix | Family variant, aspiration and application | Use to distinguish NH855, NT855, NTA855, generator, marine and industrial configurations. |
| CPL | Controlled component grouping used for the engine build | Use as an important supporting reference, particularly for historical engines and kit cross-references. |
| Compression ratio | Supporting information about the piston and combustion configuration | Use after the engine build has been identified, not as the sole ordering method. |
| Piston and liner numbers | Components currently installed in the power-cylinder assembly | Compare with the ESN-specific catalog and approved supersessions. |
| Bearing markings | Previously installed standard or repair size | Use as a preliminary reference and verify by measuring the journals. |
| Fuel-pump and injector data | PT-system version and installed fuel components | Use when fuel-system or timing components are included in the inquiry. |
| Workshop measurements | Actual condition after wear, machining and previous repair | Use for final bearing, liner, piston and machining decisions. |
Compression Ratio Is Not the Kit Part Number
Historical NT855-family documentation and aftermarket cross-references may group engines under compression ratios such as 14.0:1, 14.5:1 or 17.0:1. These groups can help narrow the piston configuration, but they are not a complete substitute for the engine-specific catalog.
The piston crown, combustion bowl, compression height and ring-groove design can change between engine builds. The corresponding ring set and piston pin must match the selected piston. However, the liner should not automatically be assumed to change solely because the published compression ratio changes.
| Ordering Method | Reliability | Recommended Treatment |
|---|---|---|
| NT855 name only | Insufficient | Identifies only the general engine family. |
| Equipment model or horsepower | Insufficient | Use as supporting application information. |
| Compression ratio only | Insufficient | Use as a secondary piston-family reference. |
| Historical CPL list | Useful but potentially incomplete | Check for duplicate, obsolete or superseded references. |
| ESN-specific parts catalog | Primary reference | Use to identify the as-built components and current replacement numbers. |
| Removed components and measurements | Final physical verification | Required when the engine has previously been rebuilt, modified or machined. |
A CPL missing from an old table should not automatically be labeled as an unsupported special variant. Similarly, a CPL appearing in more than one historical compression-ratio group must be resolved through ESN-specific records and physical verification before ordering.
Six-Cylinder Power-Component Package
The N855 platform uses six inline cylinders. A complete power-cylinder rebuild therefore normally requires matched components for six cylinder positions, although every quantity and included part must be stated in the final quotation.
| Component | Typical Full-Engine Quantity Logic | Selection Requirement |
|---|---|---|
| Piston assemblies | One approved piston assembly per cylinder | Confirm crown, bowl, compression height, cooling design, pin and part number. |
| Piston-ring sets | One ring set per piston | Ring dimensions must match the piston grooves and approved liner. |
| Wet cylinder liners | One liner per cylinder | Confirm liner reference, sealing grooves, block fit and current supersession. |
| Liner seals and O-rings | A complete seal group for each replaced liner | Confirm seal material, position and cooling-system compatibility. |
| Connecting-rod bearings | A bearing group for all six connecting rods when included | Select the size from actual journal measurements. |
| Piston pins and retainers | Supplied according to the selected piston assembly | Do not assume that every NT855-family piston uses the same pin. |
| Piston-cooling nozzles | One inspection position per cylinder | Check the installed engine configuration, flow and physical condition. |
| Head-gasket groups | Quantities corresponding to the six individual cylinder-head positions | Confirm the head configuration and current gasket references. |
A matched six-cylinder package is generally preferable when wear is distributed across the engine. Individual cylinder components may still be supplied when the remaining positions have been inspected and confirmed suitable for continued service.
Typical Contents by Repair Scope
The terms “top-end,” “in-frame” and “complete overhaul” are not standardized across every supplier. The approved quotation should list every individual component rather than relying only on the kit name.
| Component Category | Top-End | In-Frame | Complete Overhaul |
|---|---|---|---|
| Upper gasket and seal groups | Typically included | Typically included | Typically included |
| Head gaskets and valve-stem seals | Typically included | Typically included | Typically included |
| Valves, guides, seats and springs | Optional or quoted separately | Optional or quoted separately | Optional or quoted separately |
| Lower gasket and sealing group | Normally not included | Typically included | Typically included |
| Pistons, pins and rings | Not included | Typically included as a matched six-cylinder group | Typically included as a matched six-cylinder group |
| Wet liners and liner seals | Not included | Typically included as a matched six-cylinder group | Typically included as a matched six-cylinder group |
| Connecting-rod bearings | Not included | Typically included according to measured size | Typically included according to measured size |
| Main bearings | Not included | Normally quoted separately | Typically included according to measured size |
| Thrust components | Not included | Normally quoted separately | Included when identified in the confirmed BOM |
| Camshaft bushings, bearings and followers | Not included | Normally quoted separately | Included only when required and correctly identified |
| Crankshaft front and rear seals | Normally not included | May be included or quoted separately | Typically included when specified |
| Piston-cooling nozzles | Not included | Inspection item or optional replacement | Inspection item or optional replacement |
| Filters and commissioning consumables | Optional | Optional | Optional |
The quotation should identify each part number, quantity, manufacturer, bearing size, component source, included hardware and exclusion.
Application-Specific Configuration Checks
| Application | Operating Characteristics | Information Required Before Quotation |
|---|---|---|
| Marine Propulsion | Variable speed, sustained load, corrosion exposure and installation-specific cooling | Complete marine suffix, rating, rated speed, cooling arrangement, exhaust system and certification requirements |
| Marine Auxiliary Power | Fixed-speed operation, vessel cooling and restricted service access | Frequency, rated speed, auxiliary duty and vessel documentation |
| Industrial Generator Sets | Fixed-speed service, load acceptance, standby periods and prime or continuous duty | G-drive suffix, 50 or 60 Hz speed, governor, cooling package and duty classification |
| Oilfield and Drilling Equipment | Long operating periods, variable loading, contamination and remote maintenance | Driven equipment, original rating, fuel quality, shutdown history and spare-parts strategy |
| Stationary Industrial Drives | Fixed installation, coupling alignment and process-related load cycles | Driven machine, speed, alignment records and previous overhaul configuration |
| Fire-Pump Drives | Emergency duty, regulatory requirements and application-specific controls | Complete fire-pump model, rating, certification, control system and installed accessories |
Inspection Required Before BOM Release
A new overhaul kit cannot correct an unmeasured crankshaft, damaged block, distorted connecting rod, defective oil pump, blocked piston-cooling nozzle or contaminated cooling system.
| Inspection Area | Required Checks | Possible Effect on the Order |
|---|---|---|
| Crankshaft | Journal diameter, taper, out-of-round, scoring, fillets and surface condition | Determines bearing size, machining requirements or crankshaft replacement. |
| Connecting rods | Big-end bore, alignment, pin bushings, cracks and fastener condition | May require rod repair, replacement, bushings or new fasteners. |
| Cylinder block | Deck condition, cracks, liner counterbores, cavitation and fretting | May require machining before new liners can be installed. |
| Wet liners | Bore wear, polishing, scoring, cavitation and installed protrusion | Confirms the liner-replacement scope and installation requirements. |
| Pistons and rings | Crown damage, skirt wear, pin bore, ring lands and stuck rings | Confirms the required piston and ring configuration. |
| Cylinder heads | Flatness, cracks, valves, guides, seats, injector bores and sealing surfaces | May require machining or separately supplied head components. |
| Camshaft and valve train | Lobes, journals, bushings, followers, rocker components and timing parts | May identify Small Cam, Big Cam or other generation-specific requirements. |
| Lubrication system | Oil pump, pressure regulation, passages, piston-cooling nozzles and contamination | May require additional components beyond the basic kit. |
| Cooling system | Water pump, thermostat arrangement, coolant treatment, corrosion and contamination | Cooling repairs may be required to protect the replacement liners and seals. |
| PT fuel system | Fuel pump, injectors, governor, timing, contamination and installed references | Fuel components must be identified separately from the mechanical overhaul package. |
| Turbocharging and air system | Turbocharger condition, oil leakage, intake restriction and aftercooler condition | May require separate turbocharger or air-system repairs. |
Workshop Measurements to Record
- Main-journal and connecting-rod-journal diameters
- Journal taper and out-of-round
- Main and connecting-rod bearing oil clearances
- Crankshaft end play
- Liner protrusion
- Piston-to-liner clearance
- Piston-ring end gap
- Connecting-rod bore and pin-bushing condition
- Cylinder-head flatness
The applicable limits, torque values and component-reuse criteria must be taken from the service information corresponding to the actual ESN and engine generation. Generic limits from another 855-family engine should not be treated as universal assembly specifications.
Operating Conditions That Justify Further Diagnosis
- Increasing lubricating-oil consumption
- Excessive crankcase blowby
- Progressive power loss under load
- Uneven cylinder temperature or power contribution
- Persistent blue, white or abnormal black smoke
- Recurring low-oil-pressure warnings
- Coolant entering the lubricating oil
- Oil entering the cooling system
- Wet-liner cavitation or unexplained coolant loss
- Abnormal metals or particles in oil-analysis reports
- Knocking, bearing noise or abnormal vibration
No single symptom determines the final repair scope. Low oil pressure, for example, may be related to bearing wear, oil-pump condition, pressure regulation, oil viscosity, instrumentation or another lubrication-system fault.
Overhaul planning should consider duty cycle, load factor, maintenance history, oil analysis, blowby, oil consumption, cooling-system condition and physical measurements. One universal operating-hour interval is not suitable for every marine, generator, oilfield or stationary engine.
Components Commonly Quoted Separately
| Component | Why It Is Usually Separate | Identification Requirement |
|---|---|---|
| Cylinder heads and valve-train parts | Condition, casting and machining requirements vary by engine generation | ESN, head casting or part number and inspection results |
| PT injectors and fuel pump | Calibration, governor and timing arrangements vary | ESN, CPL, pump dataplate and injector numbers |
| Turbocharger and aftercooler components | Aspiration and cooling differ between NH855, NT855 and NTA855 configurations | Complete suffix, turbocharger dataplate and application |
| Oil and water pumps | Wear condition must be evaluated independently | Installed part numbers and inspection findings |
| Piston-cooling nozzles | Not every configuration uses the same components or arrangement | ESN-specific parts information and physical inspection |
| Thermostats and temperature switches | Opening or switching specifications depend on the application | Installed part number, cooling system and control requirements |
| Oil-pressure and temperature instruments | Mechanical, electrical, voltage and sender requirements vary | Panel type, voltage, thread and operating range |
| Connecting-rod and cylinder-head fasteners | Reuse or replacement depends on the applicable service procedure | ESN-specific service information and inspection results |
| Filters and commissioning fluids | Selected according to the installed filtration and maintenance program | ESN, application and destination requirements |
Quality Options and Component Traceability
NT855-family overhaul parts may be supplied as Genuine Cummins components, documented replacement parts or general aftermarket alternatives. The source and manufacturer of every critical component should be disclosed before order approval.
| Supply Option | Description | What the Quotation Should Confirm |
|---|---|---|
| Genuine Cummins | Components supplied with Cummins packaging, current numbers and applicable traceability | Part numbers, packaging, source, documents and warranty terms |
| Documented Replacement Parts | Independent components supplied against declared dimensional, material and performance requirements | Manufacturer, inspection reports, materials, dimensions, tolerances and warranty |
| Aftermarket Parts | Third-party components whose construction and controls vary by manufacturer | Manufacturer, hardness, dimensions, surface finish, quality records and claim terms |
| Mixed-Brand Package | A BOM containing components from more than one approved source | The manufacturer of each piston, liner, ring, bearing and critical gasket |
The term “OEM-equivalent” does not independently prove that a component was manufactured by a Cummins-authorized supplier, on the same production line or from identical raw materials. Such statements should only be used when supporting documentation is available.
Information Required for an NT855-Family Quotation
- Clear photograph of the engine dataplate
- Complete engine model and suffix
- Engine Serial Number and CPL
- Small Cam, Big Cam or other known generation information
- Equipment, generator-set or vessel model
- Marine, generator, oilfield, fire-pump or industrial application
- Rated speed, power and duty classification
- PT fuel-pump dataplate and injector numbers where relevant
- Required top-end, in-frame, complete or customized scope
- Current piston and liner part numbers
- Existing bearing markings
- Crankshaft-journal measurements
- Previous crankshaft, block or cylinder-head machining
- Required component quality level
- Quantity and destination
When the identification plate is unavailable, photographs of the engine, fuel pump, cylinder heads, removed pistons, liners and bearings can support identification. They should not replace an ESN-specific catalog check when the serial number can still be recovered.
Supply, Export Packing and International Shipping
| Supply Item | Available Arrangement | Confirmation Point |
|---|---|---|
| Order format | Single kit, workshop batch, fleet project or component package | Subject to final BOM and stock status |
| Repair package | Top-end, in-frame, complete or customized | Exact numbers and quantities must be approved before ordering |
| Component source | Genuine, documented replacement, aftermarket or mixed-brand | The source of each critical component must be declared |
| Internal packing | Separated, protected and labeled components | Depends on the final component combination |
| Export packing | Protected cartons and wooden case where required | Depends on total weight, shipment route and destination |
| Corrosion protection | Protective oil, sealed bags or extended-storage packing | Confirm for sea freight and long-term storage |
| Shipping method | Courier, air freight, LCL or FCL sea freight | Select according to dimensions, weight, urgency and destination |
| Trade terms | EXW, FOB, CFR, CIF, DAP or other agreed terms | Subject to the destination and final quotation |
| Export documents | Commercial invoice, packing list and agreed supporting documents | Bill of lading, certificate of origin and customs documents depend on the transaction |
Lead time, gross weight, packing dimensions and freight costs should only be stated after the exact BOM, component source, order quantity and destination have been confirmed.
Warranty and Installation Documentation
Warranty coverage depends on the component manufacturer, supplied BOM, machining work, installation quality, operating conditions and final sales contract. A loose overhaul-parts package should not automatically be described as carrying the same warranty as an engine rebuilt through an authorized service program.
| Required Record | Why It Matters |
|---|---|
| Final supplied BOM | Defines exactly which components were included in the order. |
| Crankshaft and block measurements | Demonstrate that the selected components correspond to the repaired engine. |
| Bearing-clearance records | Confirm the selected bearing sizes and final assembly condition. |
| Liner-protrusion and ring-gap records | Support correct installation of the six-cylinder power assembly. |
| Torque and assembly records | Show that the procedure for the actual engine generation was followed. |
| Fuel, lubrication and cooling checks | Help distinguish a component defect from a supporting-system failure. |
| Commissioning records | Document initial pressure, temperature, smoke and operating observations. |
Frequently Asked Questions
Can I order an NT855 overhaul kit using only the engine model?
No. The complete suffix, Engine Serial Number and CPL should be provided because the NT855 name covers multiple generations and applications.
Are NT855, NH855 and NTA855 overhaul kits identical?
Not automatically. Aspiration, aftercooling, pistons, cylinder heads, fuel settings and supporting components may differ.
Does “Big Cam” identify the correct overhaul kit?
No. It helps identify a production generation, but the exact engine suffix, ESN, CPL and installed components are still required.
Can a historical compression-ratio table identify every NT855 kit?
No. Historical lists may be incomplete, duplicated or based on superseded components. They should only be used as preliminary references.
How many pistons, rings and liners are required?
The N855 platform is an inline six-cylinder engine, so a complete power-cylinder package normally covers six cylinder positions. The quotation should state every quantity explicitly.
Can components from different compression-ratio groups be mixed?
Components should not be mixed unless the piston, ring, liner and related references are confirmed as an approved combination for the actual engine.
What is the difference between an in-frame and complete overhaul?
An in-frame rebuild normally replaces the six-cylinder power components and connecting-rod bearings while the crankshaft remains installed. A complete overhaul includes full lower-engine inspection and the parts identified after disassembly.
How is the correct bearing size selected?
The crankshaft journals must be measured and the existing bearing markings reviewed. Bearing size should not be selected from the model or CPL alone.
Does low oil pressure automatically require a complete overhaul?
No. The oil pump, regulation system, oil condition, instruments, internal leakage and bearing clearances should be diagnosed first.
Are repair-size bearings available?
Standard or approved repair-size options may be supplied when they correspond to the measured crankshaft condition and the required part references. Availability should be confirmed in the final quotation.
Are thermostats and temperature switches universal across the 855 family?
No. Temperature settings, thread sizes, electrical voltage and control functions can differ by application and installed system.
Are cylinder heads, injectors, pumps and turbochargers included?
These components are not normally assumed to be included in a standard mechanical overhaul package. They should be inspected and listed separately when required.
Is there one recommended overhaul interval for every NT855?
No. Planning should consider the application, load factor, maintenance history, oil analysis, blowby, oil consumption, cooling condition and physical measurements.
Can individual components be ordered?
Yes. Pistons, rings, wet liners, bearings, gasket groups, heads, fuel-system parts and supporting components can be supplied separately after identification.
What should the quotation include?
It should state the exact part numbers, quantities, manufacturers, bearing sizes, component sources, included hardware, exclusions, warranty terms, packing method, lead time and trade terms.
Request an NT855-Family BOM Check
A configuration-specific BOM helps prevent mismatched pistons, rings, wet liners, bearings, gaskets, camshaft components and PT fuel-system parts. Previous machining, Big Cam or Small Cam conversions and mixed component revisions should be disclosed before ordering.
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