Chevy 2 Bolt vs 4 Bolt Main | Years, How to ID & Is It Worth It?

Chevrolet V8 engines have long been the backbone of hot rodding, and one topic that often puzzles newcomers is the difference between 2-bolt main and 4-bolt main engine blocks. In simple terms, this refers to how many bolts secure each main bearing cap which holds the crankshaft in the block. The number of bolts might sound trivial, but it has big implications for strength, durability, and performance. In this article, we’ll break down the fundamentals of 2-bolt vs 4-bolt mains, how to identify them, which Chevy engines had them and when, and whether a 4-bolt main is truly necessary for your build. Let’s dive in!

From the factory, Chevrolet typically reserves 4–bolt main blocks for engines expected to see higher loads—think Corvette high-performance small-blocks, Camaro Z/28 engines, and big-blocks in muscle cars or 1-ton trucks. The added bolts keep the crankshaft secure during sudden, high torque moments (like hard acceleration) and sustained high RPM use. A 2-bolt main block has fewer bolts and slightly less bottom-end rigidity, but it shouldn’t be underestimated—it was used in countless daily drivers and can handle moderate power without issue. In fact, a stock 2-bolt 350 block is generally considered to be safe for builds up to around 450–500 horsepower. You can check out our build recommendations for 350 small blocks to see what it takes to get between 300 to 600 horsepower. Chevy even pushed some factory 2-bolt 350s to roughly 300-plus horsepower in the muscle car era without failures.

One interesting nuance: Because 2-bolt main blocks don’t have the extra bolt holes, the block’s main web (the area around the main bearings) is actually a bit thicker. This means a 2-bolt block isn’t a “weak” block but has plenty of meat in the main webs. The 4-bolt main design, meanwhile, adds strength by sheer clamping force and spreading the load across four fasteners. It serves to distribute the strain, whereas two bolts concentrate it in fewer spots. For high-output or race engines, the 4-bolt mains are insurance for the bottom end staying intact at 6000-plus RPM or with power adders (nitrous, superchargers, etc.). For a street engine or mild build, a 2-bolt main is often sufficient and can be strengthened with aftermarket main studs if needed.

If you’re inspecting an engine that’s out of the car, flipping it over to see the caps is the direct approach. But if the engine is still together, there are a few clues and codes that can hint at the main cap count without a teardown.

Bottom line, if you must know (say you’re scouting an engine at a swap meet), try to get a peek under the oil pan or research the casting number for its specs. Use the codes or pop that pan off for certainty. A hand-held inspection camera is also a great alternative to bring with you, as often you can pass the camera through the oil drain hole in the pan and see inside with the camera’s display screen.

1969 350 Engine: The first factory 350s with 4-bolt mains appeared. For example, the 300 horsepower L48 350 used in the 1969 Camaro SS and Corvette came with a 4-bolt main “010” block. This was a departure from earlier 350s (1967–68) which were 2-bolt.

1970–1972 350 Engine: 350ci high-performance engines like the LT-1 (370 hp solid-lifter 350 in the 1970 Z/28 Camaro and Corvette) featured 4-bolt mains from the factory. Other 350s in Corvettes and muscle cars during 1970–72, such as the L46 and L48, also commonly had 4-bolt blocks, as Chevy was targeting the high-performance market. Regular passenger car 350s could be 2-bolt, but anything with higher horsepower in this era generally got the 4-bolt bottom end for durability.

1973–1980 350 Engine: During the mid-1970s, as emissions and lower compression took over, not all car 350s needed 4-bolt mains. Many standard car 350s in this period were 2-bolt. However, truck and van 350s often retained 4-bolt mains for heavy-duty service. Additionally, performance-oriented models (like the Corvette’s L82 350 in the late ’70s) were equipped with 4-bolt main blocks for added strength. Chevy’s production philosophy was to use 4-bolt mains where necessary and 2-bolt everywhere else to save cost.

1981–1986 350 Engine: The 350 lived on mostly in trucks by the 1980s. One noteworthy engine is the LT9 5.7L (often called the “M-code 350”), which was a heavy-duty 350 used in 3/4-ton and 1-ton trucks, vans, and school buses. The LT9 (1981–1986) was generally built with 4-bolt main caps to endure the stresses of towing and commercial use. If you find a 1980s truck 350 with VIN code “M” or with a casting number indicative of a truck block, chances are it’s a 4-bolt main. Police package Caprices in the 80s (9C1 cars) also sometimes had 4-bolt main 350s since they were essentially using truck-spec engines for durability.

1987–1995 350 Engine: With the introduction of TBI fuel injection and the Gen I small-block’s last hurrah, 350s were produced in both configurations. Most 1/2-ton trucks (pickups, Blazers, and Suburbans) with the 350 TBI in this era got 2-bolt main blocks (since their power was modest at approximately 210 hp). However, many 3/4-ton and 1-ton trucks (and especially those with heavy towing packages) came with 4-bolt main 350s. A common casting in this era is #638 or #880 for late ’80s/early ’90s 350 blocks, which could be 2 bolt or 4-bolt main. It often depended on the vehicle’s GVW rating. In general, by the early 90s Chevy was leaning toward 4-bolt mains in more of the truck blocks for longevity.

1996–2002 350 Vortec Engine: The last production first-generation small-block was the L31 5.7L Vortec 350 (not to be confused with the LS1 5.7 motor), was used in trucks and SUVs from 1996 up to 2002. These Vortec 350s came in both 2-bolt and 4-bolt flavors. Typically, 3/4-ton and 1-ton trucks with the Vortec 350 got 4-bolt mains, while many 1/2-ton trucks and SUVs (Tahoes, etc.) got 2-bolt mains. There is no way to tell externally on a Vortec engine either. The same casting (10243880 a.k.a. “880 block”) was used for both, and you’d have to drop the pan to be sure. Generally, the heavier-duty the application is, the more likely it’s a 4-bolt. If you have a Vortec engine from a 3500-series truck or an express van, odds are it’s a 4-bolt. Light-duty 1500-series engines are often 2-bolt

However, it is interesting to note that all Goodwrench factory 350 crate engines up to the 1990s were usually 4-bolt mains by default, as GM advertised them for performance use.

1970 454 Engine (Introduction of 454): The 454 Chevy was introduced for 1970, and the highest-performance version that year was the LS6 454 in the Chevelle SS. The LS6 was a 450-horsepower beast, and it featured a beefed-up bottom end with 4-bolt main caps from the factory. The LS6 used the same block casting as the lower-output LS5 454, but with solid lifters and other upgrades, including 4-bolt mains to handle the 500 lb-ft of torque. The LS5 454 (390–365 hp hydraulic-cam version) in 1970 is a bit trickier. Most LS5 engines were 2-bolt main blocks, and only a few rare cases of LS5s came through with 4-bolt caps. Both were high compression, high performance engines, but Chevy clearly reserved the 4-bolt blocks for the absolute top dog.

Early 1970s 454 Engines (1971–1972): By 1971 compression ratios dropped thanks to EPA mandates. The LS6 was gone after 1970, and the LS5 continued as the sole 454 in Corvettes and full-size cars. It was rated around 365 hp in ’71 and 270 hp (net) in ’72. These early ’70s LS5 454s in Corvettes and Chevelles were typically 2-bolt main engines, although the basic block castings were still capable of 4-bolt. Chevrolet did produce some 4-bolt 454 blocks for marine and performance use, but in passenger cars the need wasn’t as high once power dropped.

1973–1990 454 Engines (Mark IV truck): After 1972, the 454 big-block largely became a truck engine except for some big cars in the early/mid-’70s. Throughout the late 1970s and 1980s, 454s were found in C/K pickups, Suburbans, and heavy-duty trucks. Most of these 454s were 2-bolt main blocks from the factory. They made around 240–275 hp and GM was keeping costs down. However, certain high-GVWR truck 454s and industrial/marine blocks could be a 4-bolt big block Chevy. Many 454s in one-ton trucks (especially with oil-cooler provisions) had 4-bolt main caps, whereas 1/2-ton and 3/4-ton truck 454s were usually 2-bolt big blocks. There isn’t an easy year-by-year distinction here; it was more about application. Unless you have casting numbers or you open it up, it can be hard to know on these older Mark IV 454s.

1991–1995 454 Engines: In 1991, GM introduced the Gen V big-block—an updated version of the 7.4L for trucks and marine use with some updates like a one-piece rear main seal. Notably, starting in 1991 the Gen V 454s were all designed with 4-bolt main caps from the factory. This was a design decision to improve strength and standardize the block for performance. So, any 454 engine from 1991 onward should be a 4-bolt main block.

1996–2000 454 Big Block Vortec Engines: The Gen VI 454, also known as the Vortec 7400 (RPO L29), came out in 1996 for pickups and Suburbans. These are 4-bolt main engines as well. Essentially, all production 454 engines in the late ’90s (and the 8.1L Vortec 8100 that followed in 2001) had four-bolt mains from the factory as part of their robust design. GM knew these engines were going into heavy trucks and would see hard use, so they didn’t skimp on the bottom end.

A good rule of thumb is a well-prepped 2-bolt Chevy small-block can handle around 450–500 horsepower and about 6,000 RPM safely, especially if you use high-quality main studs instead of the factory bolts. The main caps themselves are sturdy; the difference is the clamping force. Upgrading to ARP studs on a 2-bolt block can improve strength by better securing those caps and is a common trick if you’re approaching the upper limits of a 2-bolt block’s capacity. But you should note that swapping from stock bolts to high-strength studs can warp the main caps so it’s always a good idea to have the block line honed with the new studs torqued in place. Also note that SBC torque specs and sequence vary between 4-bolt and 2-bolt mains.

Where the 4-bolt main shines is when you go beyond that moderate power range. If you want 500-plus horsepower, 7,000 RPM, or you’re adding boost/nitrous, the 4-bolt main provides extra insurance. The additional two bolts per cap reduce the chances of the main caps walking or distorting under extreme load. In a 2-bolt block if it is pushed too far, you might see evidence of cap walk (shiny fretting marks where the cap has moved) or even main web cracking in worst cases. The 4-bolt’s extra clamping helps prevent that by literally doubling the fastening points on the critical center three caps (on a V8, the #2, #3, and #4 mains usually see the most stress).

So, is a 4-bolt main worth the upgrade? If your goals exceed 400hp or you plan to rev the engine very high, a 4-bolt main block is a smart upgrade for peace of mind. It becomes even more important for endurance (like road racing) or heavy vehicles that put constant load on the engine. However, if you have a good 2-bolt block already, you might not need to chase a 4-bolt.

For a big-block Chevy (454), the considerations are similar. A factory 2-bolt 454 block can handle quite a bit of torque. Remember, many big-blocks made 500 lb-ft of torque in stock form and lived in trucks pulling heavy loads. Generally, a 2-bolt Mark IV 454 is fine up to about 5,500–6,000 RPM and 500hp. Pushing beyond that (like a 600-plus hp drag engine or a big-block with forced induction) and you’d certainly want 4-bolt mains or an aftermarket block.

The reason is the 327 just came along too soon. The engine was largely phased out by 1969, and 4-bolt mains in small-blocks didn’t become common until 1969-1970. In fact, 1969 was the first model year Chevy started using 4-bolt mains on small-blocks (with the 302 and 350 engines). By that time, the 327 was being replaced by the 350 as Chevrolet’s go-to performance small-block.

• 3/4-ton and 1-ton trucks (2500/3500 series pickups, heavy-duty Suburbans, cargo vans) generally got the 4-bolt main Vortec 350s. The assumption is that these vehicles might tow or haul heavy loads, so the extra strength was added insurance.
• 1/2-ton trucks and SUVs (1500 series Silverado, Tahoe, etc.) often came with 2-bolt main Vortec 350s. Since these vehicles were lighter duty and the engine was making around 255hp stock, GM likely deemed a 2-bolt sufficient and created significant cost savings over millions of units.

The casting number 10243880 was used for virtually all Vortec 350 blocks. That casting had the provisions for 4-bolt caps, but not all were drilled for the extra bolts. Unfortunately, there is no external marking or code on a Vortec that definitively says “4-bolt” or “2-bolt.” Even the casting number “880” itself only tells you it’s a Vortec block, not the bolt count. But if you can determine the original vehicle the engine came from it can give you a clue. If the engine came out of a 3/4 or 1-ton, there’s a good chance it’s 4-bolt block; if it came from a 1/2-ton, it’s likely 2-bolt block.

It’s worth mentioning that all GM Goodwrench crate 350s in the Vortec era (and HT383 crate, etc.) were 4-bolt mains. So, if you have a service replacement Vortec 350, those are often 4-bolt block regardless of application.

By and large, no mainstream Chevy 305 came with 4-bolt mains. All the F-body, B-body, and pickup 305s were 2-bolt. If you stumble on a 305 with 4-bolt caps, it likely means someone has modified it, either by drilling a 2-bolt block for splayed caps or simply misidentifying a 305 vs 350 block. Given the 305’s intended purpose, Chevy had no reason to add cost with 4-bolt caps.

To convert, you’ll need to purchase a set of 4-bolt main caps (aftermarket steel caps are recommended) and have a machine shop perform the conversion. The machinist will do the following:

• Machine the block for the new caps: The mating surface where the cap fits (the register) may need to be cut to fit the new caps.
• Drill and tap new bolt holes in the block for the additional bolts: In a typical Chevy small-block, the two inner bolts thread vertically. The conversion adds two outer bolts per cap. Many kits use splayed caps, meaning the outer bolts are angled outward into the beefier part of the block’s web for added strength. The machinist is responsible for drilling at the correct angle and location for these threaded bolt holes.
• Install new caps and align-bore or align-hone the main bearing bores: This step is crucial. Once the new caps are on, the main bores might not be perfectly aligned. The bores in the new main caps are purposely undersized, so the machine shop can cut and then hone the combined block-and-cap assembly, so that they ensure that each main bore is perfectly straight and properly sized.

The conversion, when done with quality splayed 4-bolt caps, can actually result in a bottom end stronger than a factory 4-bolt block because the angled outer bolts anchor into a thicker section of the block and spread the load better. Aftermarket caps are typically made of nodular iron or steel, which are stronger than the factory cast iron caps. So, you’re upgrading both the fastener count and the cap material.

For many hot rodders, an easier “halfway step” is simply to install ARP main studs in your 2-bolt block. You can potentially get away without paying to have a machine shop line-hone the mains after installing main studs, but it’s not advisable. Still, installing 4-bolt main caps requires a lot of involved machining steps, so swapping in main studs and line honing the block can still be a significantly cheaper option.