Chevy Small Blocks –Number-9

A hidden VIN code separates GM’s last true first-generation small block from any other 350, and most owners never learn where to look for it. The Vortec 5700 L31 quietly powered Escalades, Suburbans, and full-size trucks through the late 1990s, while its oversized cousin, the 400 cubic inch small block, carried its own quirks and a notorious overheating flaw. Read on to find out what separates these two small blocks from the rest of the Chevy family, and why one of them is still available brand new today.

Pop the hood on a 1990s GM truck and you might be staring at the final chapter of an engine family that started in 1955, and you’d have no way of knowing it just by looking. There’s no badge that says so, no chrome script announcing the lineage. The only proof is buried in the VIN, an eighth-digit code that separates a common truck motor from something with real small-block pedigree. Meanwhile, a few blocks over in GM’s parts catalog sits a strange, oversized cousin, a 400 cubic inch small block that shares almost nothing with its 350 siblings except the family name. Both engines have stories worth knowing before you buy, swap, or walk away.

The VIN Code That Tells the Real Story

The engine in question is the Vortec 5700, GM’s L31, and the giveaway is the eighth digit of the VIN, which reads “R” on anything built around this powerplant. That single letter is the difference between a garden-variety 350 and the last true first-generation small block Chevy ever built in volume. It shares head design and intake port geometry with the earlier LT1, but it dropped the LT1’s reverse-flow cooling system in favor of a simpler, more serviceable layout. The tradeoff mattered to swappers: L31 heads bolt onto earlier blocks and can wake up an old 350, but only with an L31-specific four-bolt intake manifold, since older blocks used a six-bolt pattern to secure the manifold to the heads.

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Where the L31 Actually Lived

This wasn’t a niche engine. The L31 shipped in the Cadillac Escalade, the Chevy Express and Tahoe (including the Z71-trim models), full-size pickups, and the GMC CK, Suburban, and Yukon lineup. It was the default workhorse under millions of hoods through the late ’90s and into the early 2000s, quietly doing the job while flashier engines got the magazine covers, and it earned a reputation among owners for shrugging off high mileage with minimal drama.

Numbers That Still Hold Up

Output for the Vortec 5700 ranged from 255 horsepower (190 kW) up to 350 horsepower (261 kW) at 4,600 RPM, with torque spanning 330 lb-ft (447 N-m) to 350 lb-ft (475 N-m) at a low, truck-friendly 2,800 RPM. Those numbers made it a legitimate performer for its era, especially in a platform not chasing quarter-mile bragging rights. The L31 can trace its bloodline all the way back to the 1955 265 cubic inch small block, the engine that started the whole small-block Chevy dynasty, before finally being displaced in regular production by the 5.3 liter LM7 in 2002.

A Small Block You Can Still Buy Brand New

Here’s the detail most owners never learn: the L31 didn’t fully die when GM stopped installing it. A crate version, sold as the RamJet 350, has continued in production for marine and automotive hobbyists through companies like Volvo Penta and Mercury Marine. The marine-spec manifold on that crate engine is built to comply with automotive emissions standards, which makes it a legitimate option for anyone rebuilding a vehicle around a fresh, warranted small block instead of a decades-old core. The very last production vehicle to roll off the line with an L31 under the hood was the 2005 Kodiak/Topkick heavy-duty truck, a quiet exit for an engine that spent a decade as GM’s default V8.

Meet the 400: The Oversized Outlier

Move over to the 4.125-inch bore family and there’s only one member: the 400 cubic inch (6.6 liter) small block, built from 1970 to 1980. It combined that big bore with a 3.75-inch stroke, and unlike every other small-block Chevy, it used 5.565-inch connecting rods instead of the standard 5.7-inch length across the rest of the family. The main bearings measure 2.65 inches with a 2.10-inch rod bearing journal, specs unique enough that the 400 is often treated as its own subspecies rather than a bored-out 350.

The 400’s Design Flaw You Need to Know Before You Buy

The 400’s bore is so large relative to the block’s water jackets that GM had to add steam holes just to keep it from developing hot spots at the head gaskets and heads. Skip that detail during a rebuild and the engine is prone to overheating or, in the worst cases, seizing entirely. Main bearing caps switched from a four-bolt design in 1970-72 to a two-bolt design from 1973 through the end of production in 1980, which matters if you’re chasing durability for anything beyond mild street use.

Not a Performance Engine, But an Honest Workhorse

The 400 was never marketed as a high-performance small block, and its factory ratings back that up: 265 horsepower gross when it landed in 1970, sliding to roughly 150 to 180 horsepower once net ratings and emissions gear took over by the mid-1970s, with a two-barrel carburetor as standard equipment and a four-barrel added as an option starting in 1974. Where it earned its reputation was torque, not top-end power, which made it the engine of choice for full-size GM and Chevy trucks, the K5 Blazer, the Jimmy, and everything from half-ton to three-quarter and one-ton and larger medium-duty trucks. It also found its way into passenger cars on both the A-body and B-body platforms from its introduction through the 1976 model year.

A Legacy Still Working Today

Blocks show either two or three frost plugs per side, with provision for a third even on castings that only used two, a small manufacturing detail that tells you GM built in flexibility from the start. Decades later, both the L31 and the 400 remain fixtures in racing circles, on and off road, proof that a well-engineered small block doesn’t need a badge or a magazine spread to earn its keep.

What This Means If You’re Buying One Today

For anyone shopping a 1990s-2000s GM truck or SUV built around these engines, that VIN check is worth doing before any other inspection. An L31-equipped truck is generally the safer buy of the two small blocks discussed here, since parts availability remains excellent and the factory reliability record is well documented across millions of miles of fleet and personal use. A 400-powered vehicle from the ’70s demands more caution, specifically verifying that a rebuild addressed the steam-hole cooling issue properly, since a poorly executed repair can leave the overheating problem exactly where it started. Either way, knowing which small block variant you’re actually looking at, rather than assuming every 350 badge tells the same story, is the difference between a smart purchase and an expensive lesson.

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