C10

Why the square body exists

Development of the third-generation C/K began in 1968, and Wikipedia notes that components were tested in computer simulation before the first prototypes were built. Hagerty adds that the body spent time in the wind tunnel, which is why the corners are rounded and the side glass is curved on a truck everyone calls square. The truck it replaced was the Action Line, covered on its own page. The job was not more payload. Drivingline says Chevrolet and GMC were ready to push toward comfortable, capable trucks that could double as daily drivers, and High Flow Fuel lists larger glass, curved side windows and a roomier cab. The new frame let the cab sit lower, the front springs were softened, a front anti-roll bar was added and front disc brakes became standard.

1973: what was new

Five engines were offered at launch: 250 and 292 cu in sixes and 307, 350 and 454 cu in V8s. The 307 lasted one year; Wikipedia says it was offered only for 1973, Hagerty says it was dropped for 1974, and RM Sotheby's sold a documented 1973 C10 with a 307 and its factory build sheet at Auburn in 2012. Short beds rode a 117.5 in wheelbase and long beds 131.5 in. A four-door crew cab arrived for one-ton trucks, along with the dual-rear-wheel Big Dooley. The fuel tank moved out of the cab to a sidesaddle position outside the frame rail, a decision that would return twenty years later. Motor Trend's 1973 pickup test found the Cheyenne's 350 labored and judged that the 1973 cars handled their smog equipment better than the trucks did.

1974 through 1980

Catalytic converters arrived for 1975 on trucks under about 6,000 lb GVWR, and Chevrolet answered with the BIG 10, a half-ton rated at 6,050 lb that sat just over the line and ran on leaded fuel. The 400 V8 joined the K trucks the same year. For 1977 the grille was revised again and a 305 V8 arrived for half-ton C trucks. The 1978 Oldsmobile 350 diesel, at 120 hp and 220 lb-ft, made GM the first major American maker to sell a light-duty diesel pickup. The 454 was dropped from trucks under 8,501 lb GVWR for 1980, the year of the one-off egg-crate grille.

The diesel and the 1981 refresh

The diesel shared its head bolts and bolt pattern with the gasoline engine, and The Autopian and Wikipedia describe stretched bolts, blown head gaskets and, per The Autopian, a period when GM could not sell it in California. Sources disagree on whether the half-ton kept it after 1978. For 1981 the trucks got new sheet metal, a new dashboard and up to 300 lb less weight, the fuel tank moved to the driver's side, and the 350 disappeared from the C10 order sheet until 1987.

1982 through 1987, and after

For 1982 the 379 cu in (6.2 liter) Detroit Diesel replaced the Oldsmobile engine and the four-speed 700R4 overdrive automatic arrived; the Cheyenne trim was dropped. The 4.3-liter V6 replaced the 250 six for 1985. For 1987 GM renamed the line R/V and fitted throttle-body injection to the gasoline engines, and its dealer kit rates the injected light-duty 350 at 210 hp. Drivingline says the R/V trucks were sold alongside the GMT400; by 1990 only the three-quarter ton remained, and after 1991 the square bodies were gone. Those later trucks are a note here, not part of this page.

The best-documented fault is the 1978 Oldsmobile 350 diesel. It kept the gasoline engine's head bolts and bolt pattern, and Hagerty, Wikipedia and The Autopian describe head bolt failures and blown head gaskets; GM also left out a water separator to save money, and The Autopian reports the engine could not be sold in California in 1979 and early 1980. The 6.2-liter Detroit Diesel that replaced it from 1982 is described by Hagerty as highly regarded. NHTSA's recall database, queried for each C10 model year, adds the factory-level record: 77V178000 (1976-1977) for an accelerator pedal rod that could catch in the mat and hold the throttle open, 77V171000 (1977) for trucks built with a standard front suspension where heavy-duty was specified, 81V148000 and 81V118000 (1981) for substandard brake fluid and an oversize spare wheel, 82V128000 and 83V045000 (1982) for brake drum weights loosening aluminum wheels and a differential pinion shaft locking screw, and 87V043000 (1985-1986) for wrong-size rear brake assemblies. The sidesaddle fuel tanks drew a staged Dateline NBC crash in 1992, lawsuits and a 1994 GM settlement with NHTSA of $51 million for safety programs; Wikipedia reports GM's investigators found the tested tanks had not ruptured. Rust is the gap: owner comments describe 1973-1975 trucks rusting over the rear wheel wells after one salted winter, but no specialist rust survey or repair-cost figure was retrievable, so none is given.

The first question on a square body is what year it is, because the years look alike and the drivetrains changed. One parts-supplier guide gives a grille key: a recessed egg-crate grille for 1973-1974, a bolder flush grille from 1975, more horizontal lines for 1977, a one-year egg-crate for 1980, a new front end from 1981, another grille for 1983 and a further change for 1985. Square headlamps on a 1980 truck mean Silverado; round ones were the common setup. The engine should match the year. The 307 was a 1973-only engine, the 454 left trucks under 8,501 lb GVWR after 1979, and the 350 was not offered on the C10 from 1981 through 1986, which the EPA's 1986 listings also show, so a 350 in a 1981-1986 C10 is a later swap unless the paperwork says otherwise. Swaps are common: the one-family 1978 truck sold on Hagerty Marketplace in July 2026 carried a crate 350 installed in 2000. A diesel truck still running its Oldsmobile engine is rare. A BIG 10 badge on a 1975 or later half-ton means a 6,050 lb GVWR truck built to sit above the catalytic converter line. Two recalls are worth matching to the VIN: the 1976-1977 accelerator pedal rod that could cut into the floor mat, and the 1982 campaign for brake drum balance weights contacting aluminum wheels. The sidesaddle tanks got plastic shields for 1984. On rust, no specialist survey was retrievable; owner comments describe early trucks rusting over the rear wheel wells, and the 1978 Hagerty truck showed rust-through on the tailgate.

As of September 2026 classic.com lists the average price of a third-generation C10 at $23,924, with 98 trucks listed and a lowest recorded sale of $1,300 for a 1973 Cheyenne/Super long bed on June 13, 2026. A classic.com dealer listing for a 1978 Scottsdale short bed asked $33,100 in August 2026 and shows a comparable-sales range of $14,787 to $37,011; that is an asking price, not a sale. Two Hagerty Marketplace auctions show where ordinary trucks land: a one-family 1978 C10 with a replacement 350 crate engine, a 1990s repaint and rust-through on the tailgate sold for $9,898 in July 2026, and a lowered 1985 C10 Custom Deluxe with its 305 and 700R4 on 20-inch wheels sold for $15,783 in September 2026, the page showing a last bid of $14,750. SlashGear, undated in the text consulted, counts 1,326 square body sales over five years worth $34.5 million, an average near $26,000, and a record of $228,250 for a 1983 C10 Custom at Mecum in January 2023; its figures cover all square body trucks, and Mecum's premium is not separated from the hammer. For an older reference point, RM Sotheby's sold a documented 27,444-mile 1973 C10 with a 307 and its build sheet for $13,750 at Auburn in fall 2012. No Hagerty valuation or Bring a Trailer figure is quoted because those pages could not be retrieved.