Model Spotlight

Case 780C Construction King Backhoe Loader: Specs, Parts & Common Issues

The engine-architecture watershed of the 780 line. Produced 1981 – 1988, the 780C dropped the 4-cylinder A336BDT used on the 780CK and 780B in favor of the Case 6T-590 — a turbocharged 5.9-liter six-cylinder diesel sharing Cummins B-series block geometry. 124 HP gross, 370 lb-ft of torque, and a switch from 24V to 12V electrical.

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Broken Tractor Editorial Team• Updated May 2026• 9 min read

The Case 780C is the engine-architecture watershed of the 780 line. It came to market in 1981, sold alongside the carryover 4-cylinder 780B for the first four years of its run, and continued through 1988 as the primary 780 nameplate. The defining 780C change is the engine: Case retired the Case A336BDT four-cylinder turbo used on the 780CK and 780B, and replaced it with the Case 6T-590 — a turbocharged 5.9-liter six-cylinder diesel sharing the 4.02 × 4.72 in bore-stroke geometry of the Cummins B-series industrial engine.

Output went up: 124 HP gross and 112 HP net vs. the 780B's 120 HP / 111 HP. More importantly, peak torque climbed substantially — from 333 lb-ft at 1,500 rpm on the 780B to 370 lb-ft at 1,200 rpm on the 780C. That's both a 37 lb-ft increase in peak torque and peak arriving 300 rpm earlier in the rev range, which gives the 780C a noticeably stronger working feel during digging cycles.

At-a-glance specifications

The 6T-590 engine and Cummins B-series lineage

The Case 6T-590 introduced on the 780C is the same engine that arrived on the 680L in 1989 and continued onto the 780D and several other Case industrial machines through the early 1990s. The block shares its bore-stroke geometry (4.02 × 4.72 in) and lower-end architecture with the Cummins 6BT industrial six-cylinder — a B-series engine that powered everything from Dodge Ram pickups to industrial generator sets, and which gives the 780C an unusually wide parts-availability network for a machine of this age.

• Substantially more torque — 370 lb-ft at 1,200 rpm vs. the A336BDT's 333 lb-ft at 1,500. Peak torque arrives 300 rpm sooner and is 11% higher in magnitude.
• Higher compression (17.5:1 vs. 15.8:1) — better thermal efficiency, slightly improved fuel economy under load.
• Six-cylinder smoothness — half the power pulse interval of a four-cylinder at the same rpm, producing less engine vibration.
• Cummins B-series parts ecosystem — pistons, rings, bearings, head gasket sets, water pumps, and many valve train components cross-source through B-series channels.

The 780C's larger-frame chassis and loader iron mean the same 6T-590 engine drives a substantially larger physical machine than the smaller 680L sibling — Case got more working capacity from the same engine by sizing the rest of the machine up.

Hydraulics and cab

The 780C's open-center hydraulic system uses the D127917 main hydraulic pump that crosses to the 680K, 780D, and 780C — a three-machine cross-reference covering the bulk of the 6T-590 backhoe loader lineup. The combined radiator and transmission oil cooler (A172138) extends that cross-reference to four machines (680K, 680L, 780C, 780D).

Cab equipment was updated with the York-type A/C compressor (A141060) as an option — cross-fits 580D, 680K, 780C, and the W14 through W36 wheel loader family.

Parts we carry

The 780C is over 35 years old. Parts availability is good — better than most 780-line predecessors because of the Cummins B-series engine lineage and the cross-fit network with the 680K, 680L, and 780D. The items below are available at Broken Tractor with verified 780C fitment.

Common issues and what to expect

6T-590 engine wear patterns

The Cummins B-series block is durable, with good parts availability through both Case sources and Cummins B-series channels. Common items on machines of this age: head gasket failure (typically at the front of the block between cylinders 1-2 or 5-6), valve seat recession on heavy-duty service, turbo bearing wear, and injection pump wear on machines with long fuel-system service intervals.

Turbocharger wear

The 6T-590's wastegated turbocharger is a routine replacement item. Symptoms of wear: blue smoke at startup or cold throttle response, oil consumption, audible whine outside the normal range, or visible shaft play. Replace at the first sign of bearing wear — a failed turbo can ingest its own compressor wheel and damage the engine.

Cooling system corrosion

The A172138 combined radiator is the most-replaced item on 780C machines. Internal corrosion in the radiator cores is the most common cause of overheating complaints. The dedicated hydraulic oil cooler should be inspected alongside the radiator.

12V electrical aging

The 780C's switch from 24V to 12V did not eliminate electrical-system maintenance — corroded battery cables, marginal connections, and aged starter solenoids are routine on a 35-plus-year-old machine. Inspect at every service.

Hydraulic cylinder seal failure

Every cylinder on a 780C has seals that are 35-plus years old. The G109503 seal kit covers the boom and bucket cylinders; the G103073 lift cylinder is a common starting point. Bores are typically still good — most cylinders only need reseal kits.

Where it sits in the 780 family

The 780C bridges the 4-cylinder 780CK/780B generation and the 780D that closed out the line. The Cummins B-series 6T-590 introduced at the 780C continued onto the 780D essentially unchanged, so the 780C and 780D share most service parts. Below the 780C in Case's lineup sat the 680K and 680L (smaller-frame Construction Kings on the same 5.9-liter engine block); above and beside it sat the 850-series dozers and W14 wheel loader on related industrial powertrains.