Cummins NT 855 Service Manual – 693 Pages of OEM Repair Data for Big Cam III & IV

Description

I’ve seen too many guys chase their tails because they used a reprint manual with torque values that came from a different engine family. That costs you time and can ruin a head gasket. This is the real deal: the Cummins NT 855 Service Manual—693 pages of OEM data from Cummins itself. It covers the Big Cam III and Big Cam IV configurations for the NT 855 engine. If you are working on a truck, generator, or industrial equipment with this engine, you need this on your bench.

Page 2 of the foreword states that this manual is organized for logical troubleshooting steps. It does not waste space on vehicle bodywork—it is all engine. Page 3 shows you the full table of contents: seven systems sections—Cooling, Lubricating Oil, Combustion Air, Compressed Air, Fuel, Electrical, and Base Engine Components. Each of those sections has step-by-step procedures, flow charts, and specifications.

Cooling System: Factory Procedures for Big Cam III and IV

The cooling section alone is worth the price. You get the correct thermostat testing procedure for both Big Cam III and Big Cam IV—in-chassis and removed tests. The manual tells you exactly which special tools you need (list of service tools at the start of the section). It covers water pump replacement, fan clutch checks (air engaged and disengaged types), and radiator cap testing. No generic “drain and fill” instructions—Cummins gives you the exact venting procedure and the DCA4 additive specs.

Troubleshooting 50+ Engine Symptoms

Section T is a massive troubleshooting guide. It lists symptoms from coolant temperature above normal to excessive engine noise. Over 50 specific conditions, each with a cause-and-correction chart. For example: low lubricating oil pressure—the manual walks you through checking pressure regulator, oil pump wear, bearing clearance. Refer to page 3 for the full symptom list. This is not a typical “check this, check that” list; it is Cummins’ own diagnostic logic. You follow the tree and you find the root cause.

Safety warning: before you dig into any system, read the symbols and general repair instructions in Section I. I have seen injuries from compressed air releases and hot coolant sprays. That section covers the correct depressurization steps and tool handling.

Why You Need the OEM Torque Specifications

Aftermarket manuals often copy torque values wrong. I have personally seen a cylinder head spec off by 15 ft-lbs because someone transcribed it from a different model. This manual has the OEM-specified torques for every fastener on the NT 855—head bolts, main bearing caps, connecting rods, injection pump, and all accessories. Torque sequences are included. Using the wrong spec on a 12-point head bolt will cost you a gasket and a tow. Don’t guess.

Engine Identification Diagrams and Specs

Section E gives you complete engine identification: serial number location, engine specifications in a table (bore, stroke, displacement, compression ratio, firing order), and detailed diagrams of the engine—front, rear, and cross-sections. If you need to confirm whether your engine is a Big Cam III or Big Cam IV, those diagrams are the quickest way. The specs table includes valve lash settings and injection timing data.

Lubricating Oil System: Pressure and Consumption

Oil problems are common on the NT 855 if you don’t follow the right service intervals. The Lubricating Oil section covers oil pump replacement, pressure regulator valve adjustment, and oil cooler service. The troubleshooting symptoms include excessive oil consumption and sludge buildup. The manual gives you the acceptable oil pressure ranges at different RPMs—something most aftermarket books leave out.

Factory specs matter. That is why I trust only the original Cummins manual.

For related documentation: the Cummins NT 855 Big Cam IV 1989 Technical Manual – Factory Specs covers that specific variant. Need more diagnostic depth? The Cummins NT 855 Troubleshooting Manual – 693 Pages of OEM Diagnostic Data mirrors sections of this manual. If you are working on a military M915 or M920, the Cummins M915-M920 Big Cam III 2001 Service Manual (940 pgs) gives you chassis integration details.

This manual focuses on the engine only. For drivetrain repairs, check external resources like Manual Transmission rebuild guides or Manual Transmission maintenance procedures. Those are separate from this engine manual, but having both ensures a complete overhaul.

Stop chasing bad data. Access the Factory Manual and keep it on your tablet. That is the only way to be sure.

Does this manual cover the Big Cam IV?

Yes. Both Big Cam III and IV are covered.

What about the PT fuel system adjustments?

The Fuel section covers the PT fuel system, including pump adjustments, injector timing, and the variable speed governor. Procedure includes disassembly of the AFC and throttle linkage. Torque values for fuel lines and fittings are given. If you’ve ever seen a PT pump die from wrong adjustment, you know why OEM data matters.