GM Truck Generations: 1973-1987 Square Body


GM Truck Generations: 1973-1987 Square Body



1973 Chevrolet Squarebody Advertisement

An all-new clean-sheet redesign of General Motors’ Chevrolet and GMC Truck C/K-Series pickups debuted in 1972 for the 1973 model year. Development of the new third-generation trucks began in 1968, four years prior to the model's launch in late 1972, with vehicle components undergoing simulated testing on computers before the first prototype pickups were even built for real-world testing. The redesign was revolutionary in appearance at the time, particularly the cab, departing from typical American pickup truck designs of the era. Aside from being near twins, the Chevrolet and GMC pickups looked like nothing else on the road.The third-generation trucks are officially known within GM as the “Rounded-Line” generation. Most enthusiasts, however, know them as the "Squarebody", given that the trucks appear square-like when compared to more modern automotive design standards.

GM’s design engineers fashioned the “Rounded-Line” exterior in an effort to improve aerodynamics and fuel efficiency, using wind tunnel technology to help them sculpt the body. Third-generation design traits include “double-wall” body construction, sculpted bodywork, a flared secondary beltline, an aerodynamic cab that featured rounded doors cutting high into the roof and a steeply raked windshield featuring an available hidden radio antenna embedded into the glass.

There were two types of pickup boxes to choose from. The first type, called Fleetside by Chevrolet and Wideside by GMC, was a “double-wall” constructed full-width pickup box and featured a flared secondary beltline to complement the cab in addition to new wraparound tail lamps. Both steel and wood floors were available. The second type, called Stepside by Chevrolet and Fenderside by GMC, was a narrow-width pickup box featuring steps and exposed fenders with standalone tail lamps. Initially, only wood floors were available.

The wheelbase length was extended to 117.5 in (2985 mm) for the short-wheelbase pickups, and 131.5 in (3340 mm) for the long-wheelbase pickups. A new dual rear wheel option called “Big Dooley” was introduced on one-ton pickups, along with a new Crew Cab four-door option on the 164.5 in (4,178 mm) wheelbase. Crew Cabs were available in two versions: a “3+3” which seated up to six occupants and a “bonus cab” which deleted the rear seat and added rear lockable storage in its place. The fuel tank was moved from the cab to the outside of the frame, and a dual tank option was available which brought fuel capacity to 40 US gallons. 1980 was the first year that a cassette tape player could be purchased, along with a CB radio.

The Rounded-Line generation ultimately ran for a lengthy 15 model years (1973–1987). For 1988, not to confuse buyers with the GMT400-platform C/K series, the Rounded-Line trucks were renamed R/V and the half-ton model was dropped. The 3/4-ton would continue production until 1990, while 1-ton trucks, the Blazer/Jimmy sport-utility vehicles and the Suburban wagons would continue until 1991 before being replaced with GMT400-based versions. 1991 Blazer and Jimmy are the last GM full-size SUVs with a removable hardtop.

Brothers Truck Show 2022 Blazer

Interior and Safety

Jaws Blazer interior

Soft-touch materials were used throughout the passenger cabin, like the dashboard, arm rests on the doors, steering wheel, and shift levers. Subtle-grained interior panels and bright metal work were used on the inside with high-quality materials also used on the outside, like chrome, aluminum, and polished stainless steel, particularly on top-of-the-line luxury Silverado or Sierra Classic trim levels. Vinyl or soft "Custom Cloth" cloth and velour seating surfaces were used, along with fabric headliners, door inserts, and plush carpeting, depending on the trim level. Upper-class trim levels also used acoustic deadening materials for quieter ride comfort.

Other safety features included soft-padded interior panels for appearance and safety, 3,329 square inches of tempered and laminated safety glass, a prismatic rearview mirror, six turn-signal indicator lamps with asymmetrical flash, four-way hazard function, and lane departure function.

Chassis and Powertrain

1987 Chevrolet Big Dooley

Third generation Rounded-Line C/K-Series pickups gained an all-new, high tensile strength carbon steel ladder-type frame with a “drop center” design. Steering controls included variable-ratio recirculating ball steering gear with optional hydraulic power assist. Braking controls included front self-adjusting disc brakes with rear finned drum brakes and optional four-wheel hydraulic Hydra-Boost or Vacuum-Boost power assist. Engines choices initially consisted of six or eight-cylinder engines with either manual or Turbo Hydra-Matic automatic transmissions.

C-Series pickups included two-wheel drive and featured an independent front suspension (IFS) system with contoured lower control “A” arms and coil springs. GM’s new Load Control rear suspension system took up residence in the back. The Load Control rear suspension system consisted of a rear live axle with dual-stage Vari-Rate multi-leaf springs and asymmetrical (offset) shock absorber geometry, to help sort out any “wheel hop” under heavy loads or hard acceleration.

K-Series pickups included either Conventional, Permanent, or Shift-on-the-move four-wheel drive. The latter system was introduced in 1981. Regardless of the type of four-wheel-drive system equipped, all K-Series pickups featured four-corner Vari-Rate multi-leaf springs, a front live axle with symmetrical (inline) shock absorber geometry, and the Load Control rear suspension system. K-Series pickups also featured an off-road oriented design, with the transfer case bolted directly to the transmission and running gear tucked up as high as possible under the vehicle to reduce the chances of snagging vital components on obstacles, as well as to achieve a low silhouette and optimal ground clearance. Exposed brake lines wrapped in steel were standard, with underbody skid plate armor optional for further protection.

Conventional four-wheel drive pickups featured manual locking hubs and a two-speed dual range New Process 205 transfer case with four drive modes: Two High, Four High, Neutral, and Four Low. Two High gave a 0:100 torque split, while Four High yielded a locked 50:50 torque split. Four Low applied reduction gearing. The front and rear propeller shafts were locked at all times in Four High and Four Low. Neutral allowed for flat towing, or use of the power take off (PTO).

Permanent four-wheel-drive pickups featured a two-speed dual range New Process 203 transfer case with planetary center differential and lock. Five drive modes were provided: High,Low, Neutral, High Loc, and Low Loc. In High the center differential was unlocked and allowed the front and rear propeller shafts to slip as needed for full-time operation. The system could be manually shifted into High Loc which locked the center differential for a locked 50:50 torque split. Low and Low Loc applied reduction gearing with or without lock, depending on the mode selected. Neutral was also available for use of the PTO.

A new Eaton Automatic Differential Lock (ADL) was introduced in 1973 as an optional extra on the Rounded-Line C/K-Series pickups, for the rear hypoid differential. The new automatic locking differential was offered under the G86 code, replacing the Eaton NoSpin differential, and eventually replacing the old Positraction limited-slip differential in 1974, at which point it assumed the G80 code. The Eaton ADL featured intelligent differential control via an internal governor which monitored vehicle speed and wheel slip to know when to automatically lock and could lock up 100 percent at or below 20 mph (32 kph) increasing tractive effort. The differential lock would unlock and deactivate at speeds above 20 mph for safety reasons, such as the vehicle being on dry pavement.

Towing and payload capacity ratings for Rounded-Line C/K-Series pickups varied, depending on how they were configured. Factors such as engine and transmission combination, differential gear ratio, curb weight, and whether the pickup was the two-wheel drive or four-wheel drive decided how much the pickup could safely tow or haul.

A properly equipped C-Series half-ton class pickup could tow up to 8,000 lbs (4 tons) of braked trailer, while a properly equipped C-Series three-quarter-ton or one-ton class pickup could tow up to 12,000 lbs (6 tons) of braked trailer. Adding four-wheel drive reduced towing capability due to increased curb weight, which resulted from additional driveline components (transfer case, front axle, front differential, front propeller shaft, and so on) needed to facilitate four-wheel drive. A properly equipped K-Series half-ton or three-quarter-ton class pickup could tow up to 6,500 lbs (3.25 tons) of braked trailer, whilst a properly equipped K-Series one-ton class pickup could tow 500 lbs more, up to 7,000 lbs (3.5 tons) of braked trailer. The decreased towing capability of K-Series pickups, when compared to their C-Series pickup siblings, is a valid tradeoff for all-weather, all-terrain capability.

Heavy-duty towing equipment was available for both C and K-Series pickups, such as the Trailering Special package (included power steering, uprated battery, and uprated generator), 7-pin trailer electrics connector, heavy-duty engine oil cooler, heavy-duty transmission oil cooler, and a weight distributing trailer hitch.

Year-To-Year Differences

Brothers Trucks 2022 1975 Blazer

  • 1973 Chevy C10/GMC K10 Differences

The third-generation pickups were offered in several equipment level packages or trim packages. Chevrolet/GMC used various names for the trim levels throughout the vehicle’s life cycle and some were rearranged in their class order. For the 1973 and 1974 model years, the base (standard) trim level was Custom/Custom, mid-range trims were Custom Deluxe/Super Custom, luxury trims were Cheyenne/Sierra, and top-of-the-line luxury trim levels were Cheyenne Super/Sierra Grande.

  • 1975 Chevy C10/GMC K10 Differences

For the 1975 model year, the trim levels were revised and the base trims were now Custom Deluxe/Sierra, mid-range trims were Scottsdale/Sierra Grande, luxury trims were Cheyenne/High Sierra, and the top-of-the-line luxury trim levels were now known as Silverado/Sierra Classic. They remained in this configuration up to the 1987 model year.

The third safety advancement was the introduction of dual front lap-and-shoulder safety belts with emergency locking retractors for outboard occupants in 1975 for the 1976 model year. These replaced the outdated and inadequate lap belts previously used. A center lap safety belt with slack adjustment was provided for the center occupant. Ford and Dodge would follow one model year later adding lap-and-shoulder safety belts to their pickups.

  • 1976 Chevy C10/GMC K10 Differences

A new gauge to show voltage replaced the ammeter in 1976, and the engine size decals were removed from the grille during this model year.

  • 1977 Chevy C10/GMC K10 Differences

For 1977 models, power windows and power door locks were introduced as an optional extra. There was another round of new grilles, revised inner door panels that left less metal exposed, a four-wheel drive, full one-ton chassis was added to the lineup, and a Dana 60 was used for the front axle, as well as an electric oil pressure gauge replacing the mechanical unit. Trucks with an optional trim level, but without an additional wheel upgrade, received flatter stainless steel hubcaps, still with painted accents. This was also the only year with yellow painted trim instead of black.

  • 1978 Chevy C10/GMC K10 Differences

The addition of the first diesel engine of the three American automakers in a light-duty pickup the 125 hp 350 cu in (5.7 L) Oldsmobile diesel V-8 began in 1978. All models got new, flatter dash trim panels, black on the lower two trims and aluminum-look on the fancier two. Base models received the flatter stainless hubcaps, and Stepsides got new squared-off taillights with built-in backup lights and side markers, while the rear fenders were smoothed out where the old side markers were.

The wood grain inserts were replaced by bright brushed aluminum.

  • 1979 Chevy C10/GMC K10 Differences

The 1979 models got a new grille surround that incorporated the turn signals; inside there was a new full-width “houndstooth” seat trim on base models and a (rare) fifth interior color option on the higher series called “oyster” by Chevrolet and “Mystic” by GMC (mostly white with a gray dash, carpeting, and cloth).

The wood grain inserts were replaced by bright brushed aluminum.

Brothers Trucks Show 2022 1980 Chevrolet

  • 1980 Chevy C10/GMC K10 Differences

For the 1980 model year, permanent four-wheel drive was discontinued on K-Series, leaving only conventional four-wheel drive. Some pickups gained a new grille, others did not; high-trim Chevys had both a new surround that incorporated near-flush square headlights and revised turn signals with a new, squarer grille pattern, while a GMC base model was entirely carryover, base Chevys had the new center section in the 1979 surround while GMCs with uplevel trims or the separate RPO V22 option had the new square-light surround with the main grille introduced in 1977. Blue interiors were a darker shade than before.

The wood grain inserts were replaced by bright brushed aluminum.

  • 1981 Chevy C10/GMC K10 Differences

A mid-life cycle cosmetic facelift and mechanical refresh were carried out for the 1981 model year. In response to the recent 1979 energy crisis, the 1981 rework featured several fuel-saving techniques to help make the Rounded-Line C/K-Series pickups more fuel-efficient. Again, engineers turned to wind tunnels to re-sculpt the front end with new sheet metal, reducing areas that could hinder airflow and cause drag. A sleeker front bow-like look emerged, similar to a ship’s bow with the front end being gently swept back from the center. New dual-tier halogen headlamps became available with the Deluxe Front Appearance package. Mechanical updates included more anti-corrosion techniques, reduced weight, and a new 5.0 L 305 cubic inch V-8 with electronic spark control. The 5.7 L 350 cubic inch pushrod V-8 was dropped from the half-ton class pickups, except in California where it was offered in place of the new 5.0 L 305 engine with electronic spark control, which did not meet California’s emissions requirements.

A new Shift-on-the-move four-wheel-drive system with a two-speed dual range New Process 208 aluminum transfer case was introduced on K-Series pickups for the 1981 model year. It replaced the permanent four-wheel-drive system, on pre-1980 models. The shift-on-the-move four-wheel drive system featured new automatic self-locking hubs and synchronized direct high-range planetary gearing, such that the truck could be shifted from two-wheel drive, to fully locked four-wheel drive at speeds of up to 25 mph. Once the shift from two-wheel drive to four-wheel drive was made, the vehicle could be driven at any forward or reverse speed. Four drive modes were offered: Two High, Four High, Neutral, and Four Low. Two High gave a 0:100 torque split, with Four High yielding a locked 50:50 torque split through directly synchronized gearing. Four Low applied reduction gearing. The front and rear propeller shafts were locked at all times in Four High and Four Low. Neutral was provided for the disengagement of both propeller shafts. Conventional four-wheel drive was still available with manual locking hubs.

A new four-speed Turbo Hydra-Matic 700R4 transmission with overdrive gearing became available in 1981 for the 1982 model year. The 151 hp 379 cu in (6.2 L) Detroit Diesel V-8 was added to replace the LF9 Oldsmobile diesel. Chrome front bumpers were now standard on base models.

  • 1982 Chevy C10/GMC K10 Differences

A new four-speed Turbo Hydra-Matic 700R4 transmission with overdrive gearing became available.

  • 1985 Chevy C10/GMC K10 Differences

1985 saw the new 262 cu in (4.3 L) LB1 introduced to replace both inline-six engines. Hydraulic clutches were introduced. Also, a new grill was used. The most expensive radio was the AM/FM stereo seek/scan with cassette tape at $594. A variation of the C/K series was introduced in 1985 in Brazil, replacing the locally-produced C10, introduced in 1964.

  • 1987 Chevy C10/GMC K10 Differences and 1987-1991 R/V Series

For the 1987 model year, the last model year for the conventional cab pickups, the Rounded-Line C/K-Series was renamed the R/V-Series. R-Series now designated two-wheel drive, while V-Series represented four-wheel drive. The name change is also found in the vehicle identification number. This was done in preparation for the next generation GMT400 trucks, which were produced concurrently with the older line. The new 1988 model trucks entered production on December 8, 1986, at Pontiac East, Oshawa, and the new Fort Wayne plant. The 1987 models continued to be built at Janesville, St. Louis, and Flint.

Along with the name change, came other major improvements and tweaks for the final model year of the conventional cab pickups. Single-point electronic throttle-body fuel injection (TBI) was introduced on GM’s full-size pickups, with new electric fuel pumps and high-pressure fuel lines. In addition, a “smart” power train control module (PCM) was also introduced, which controlled the fuel injection system, fuel-to-air burn ratio, engine ignition timing, and (if equipped with an automatic transmission) the Turbo Hydra-Matic’s turbine torque converter clutch. The 5.7 L 350 cubic inch pushrod V-8 was reintroduced to the order books for R-Series and V-Series half-ton class pickups, with the new TBI fuel injection system. Horsepower and torque output were increased to 210 hp, and 300 lb-ft of torque.

After 1987, R/V remained in use for the Rounded-Line one-ton crew cab pickups through 1991 (built at Janesville), and the Rounded-Line utilities (Chevrolet K5 Blazer and Suburban, built at Flint) through 1991. From the 1988 model year and onward, C/K was re-used for the fourth generation “GMT400” design.


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