2 Bolt Main vs. 4 Bolt Main: Which Is Better for You?

Do you want to build a high-performance engine and don’t know the difference between a 2-bolt main and a 4-bolt main? They refer to the design of an engine block that houses the crankshaft and determines the horsepower an engine can handle.

A 4-bolt main is arguably the better option for high-power engines. The main caps are held by 4 bolts instead of two in a 2-bolt main.

Your decision on which type of main block to choose depends on the power output, budget, and to some extent, availability. Read on to find the information you need.

How Important Is A Main?

2 bolt main cap
A 2-bolt cap makes a 2-bolt main engine

An internal combustion engine has a crankshaft, which is driven by pistons. The crankshaft is fixed into the engine block at the mains. This part of the engine is visible after removing the oil pan. And to hold the crankshaft in place, you will need the main caps.

In simple terms, the main cap holds the crankshaft and bearings onto the mains. Depending on the number of bolts that the main cap has, it can be a 2-bolt cap or a 4-bolt cap. A 2-bolt cap makes a 2-bolt main engine, while a 4-bolt cap makes a 4-bolt main engine. 

4 bolt main cap
a 4-bolt cap makes a 4-bolt main engine. 

The main is an area within the engine block that houses the crankshaft, and the main cap prevents the crankshaft from flying out.

What’s The Difference Between A 2-Bolt Main and A 4-Bolt Main?

The difference between a 2-bolt main and a 4-bolt main is as the name suggests. As hinted in the previous section, a 2-bolt main requires two-bolt caps to help the crankshaft in place. Similarly, you would also need 4-bolt caps to hold a crankshaft in a 4-bolt main.

The main caps are not interchangeable unless you modify the engine block, as we will see when converting a 2-bolt main to a 4-bolt main. So, to tell which main you have, just count the number of bolts on each cap or holes drilled in the engine block for fixing each cap.

Also, you can distinguish the two from the sizes. 4-bolt main engine blocks tend to be larger. However, you can also find an equally big 2-bolt engine block. It may be inconclusive to differentiate the two based on the engine size.

A video about 2 bolt vs 4 bolt main.

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Which Is Better? Why?

2-bolt and 4-bolt mains are different designs for the intended applications. Determine your power needs before you decide on what to buy. A 2-bolt main engine is better for power below 450 hp. If you are building for higher power, you will need a 4-bolt main.

The higher the power you need to draw from an engine, the higher the load on the main caps. More bolts on the main caps are likely to hold on for long at higher output power and torque.

2 Bolt Main and 4 Bolt Main Pros And Cons

2 Bolt Main Pros And Cons

2 bolt main
2-bolt mains are popular because they are easy to find and are cheaper.

2-bolt mains are popular because they are easy to find and are cheaper. Many people have preferred them because it is cheaper to upgrade a 2-bolt main to a 4-bolt main than buying a new one.

However, this type is prone to cap-walk and will not withstand high power and torque.

4 Bolt Main Pros And Cons

4 bolt main
A 4-bolt main is strong and can withstand high torque and power builds.

A 4-bolt main is strong and can withstand high torque and power builds. However, it is expensive and rare to find. That is why many people are just fine with upgraded 2-bolt mains.

In fact, you will rarely find an old 4-bolt main block just lying in the garage. The limited ones are already in use.

Read more: GM 4.8 Vs 5.3 Vs 6.2 Engine (With Comparison Chart)

Can You Convert A 2 Bolt Main To A 4 Bolt Main?

Yes. A 2-bolt main can be converted to a 4-bolt main using specialized aftermarket kits.

But why would you need the conversion? If you are building for strength, power, and bottom-end survival, a 4-bolt main is not optional. Also, 4-bolt-main blocks are not easy to find, but 2-bolt ones are almost everywhere.

To convert a 2-bolt block to a 4-bolt block, you would need a machine workshop and an aftermarket kit. If you have experience working with metal, this should be a 2-minute hack. You only need to drill additional holes in the main block for the two extra bolts on each cap.

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2-Bolt Main Vs. 4-Bolt Main Comparison Chart

 2-Bolt Main4-Bolt Main
CostMore affordableMore expensive
AvailabilityReadily availableNot easy to find
Power ratingLess than 450 hpStrong enough to handle more than 450 hp
Number of bolts on each cap2 bolts per cap4 bolts per cap

FAQs

1. How do you make a 2-bolt main stronger?

You can significantly improve the strength of a 2-bolt main by using studs instead of bolts. A 2-bolt main with aftermarket studs can handle up to 600 hp.

2. How much horsepower can a 2-bolt main Chevy 350 handle?

A 2-bolt main Chevy 350 engine is expected to handle 450 hp. You can strain it a bit higher, but that will reduce its service life. If you need to get more than that, go for a 4-bolt main.

A video about how much power can a 2 bolt main cast iron SBC block hold.

Read more: Chevy 350 (SBC and BBC) Firing Order [With Diagram]

3. How much horsepower can a 4-bolt main Chevy 350 handle?

A factory-made 4-bolt main Chevy 35o engine can handle 600 hp. However, a custom-built engine with splayed 4-bolt main and center studs can withstand double the power.

4. What are Chevy 350 4 bolt main years?

Chevy 350 4-bolt mains were manufactured and widely marketed between 1969 and 1972. You will find them in most Chevy and GMC cars and trucks made after these years.

5. What are Chevy 350 2 bolt main years?

2-bolt-main Chevy 350 engines were in production between 1973 and 1980. That is why they are more readily available than the 4-bolt main blocks.

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