2 vs. 4-Bolt Main? Diving Deeper Into the Myth.

by | Oct 10, 2018

**The information below applies to seasoned blocks, also known as re-manufactured blocks. If the manufacture does not list the block as “new,” then it is seasoned. **

When it comes to comparing a 2 and 4-bolt main, it’s hard to find the right information since there’s so much falsehood floating around the internet. Have you found the right source? Do you know where to look?

What is a 2 and 4-Bolt Main Anyway?

Mains are what hold your crankshaft in place on the bottom of the engine. Some engines have 2 bolts or 4 bolts on each main securing the crankshaft to the engine. The good-old American theory: MORE IS BETTER!! Right?… in this case, NO.

Where the Theory Came From…

GM started making the 4-bolt main in the 1970’s because their theory was that as power increases the crankshafts would shoot out the bottom of the engine. They believed that when you increased the load on the crankshaft, you increased the load on the main caps. While this is true, as blocks evolved they became thinner and having more bolts on the bottom of the engine wasn’t the best solution.

Well, GM’s theory wasn’t as factual as we assumed because their strategy didn’t evolve with the times. Over the years, GM used the 4-bolt main as a marketing strategy to convince consumers that it was the right choice and it was for blocks made in the 1970’s. As consumers fell in love with the idea, engineers understood the concept, and GM’s car sales improved.

Everyone was happy until the 1980’s rolled around and here’s what happened…

The History

The Environmental Protection Agency, also known as the EPA drives stringent requirements that are federally mandated to regulate vehicles and make sure their safety ratings, fuel economy, smog and luxury features are up to standards. To stay in business, car manufacturers needed to build vehicles lighter and quieter with more safety features and a smoother ride. Hence why engines are now becoming smaller and lighter with the same horsepower and better fuel economy.

What does this have to do with engine blocks? Well, starting in the mid 1980’s, the EPA began a stronger enforcement with things such as Noise Vibration Harshness (NVH), which was the turning point for the aftermarket performance industry. GM, Chrysler, Ford and other car manufacturers began molding thinner cast iron engine blocks making them lighter to gain better EPA ratings for achieving more safety and better fuel economy.

What they didn’t take into consideration is that when the cast iron blocks became thinner and removed the nickel (a high strength metal material that gave the block more integrity), the blocks would flex as horsepower increased which weakened the 4-bolt main design.

In return, the 4-bolt mains continually had hairline fractures over time varying on the abuse the engine took. This caused issues that the 2-bolt mains didn’t experience since there were less bolts to cause a flaw in how the engine flexed. We discovered through our own research and development that adding aftermarket studs on a 2-bolt design is much stronger and reliable.

Now, this is not to say that the blocks that were manufactured in the late 80’s and 90’s were poor. That’s not true. They’re a better design, lighter, the rear mains are stronger and less prone to leak, the cam designs are better. Overall, it’s the block we prefer to build our engines with that has brought us the most success and almost a zero-failure rate.

Here’s the facts:

  • The block you want is from 1987-2002. This is also known as a 1-piece rear main block or a full round seal block. Stay away from the 2-piece rear main!
  • A 2-bolt main with aftermarket studs is stronger than a 4-bolt main block, less prone to crack, and can take up to 600 HP on a seasoned block.
  • NEVER buy a new block that does not have the manufacture name on it such as “Dart” or “GM Performance” because it’s made overseas and it is CHEAP. These blocks are brittle and will crack.
  • Do not fall into the “4-bolt main” marketing trap. Most of the engine builders online, including some big brands, will drill extra holes on their 2-bolt main blocks and it makes the mains weaker and more prone to cracking.

New or Seasoned Block

As you digest all the information we’ve provided, we’d like for you to keep in mind that all these facts apply to seasoned blocks ONLY, also known as remanufactured blocks. When you start to dive into NEW blocks that manufacturers are offering you really must be careful.

Most new engine blocks are cheaply made in Mexico or overseas, have weak mains, poor machining, and are built by underpaid employees who don’t really care. These blocks are more prone to crack due to lack of quality control.

In ANY industry, the people creating the product is senior to the process. If you don’t have quality people, it’s hard to have a quality product. It’s proven that when you have the right people who care about what they do, you will have a solid product because it matters to them. What a concept, right?

Therefore, when you hear people say, “just buy a crate motor from GM” it gets consumers into a lot of trouble. GM has multiple facilities all around the world and their Chevy small block engines get the worst reputation (also known as target engines or target master engines) because it has the lowest quality control in their entire operation and is based out of Mexico. We’ve seen hundreds of GM crate engine customers come to us out of devastation when their block fails, and they have a warranty with so much fine-print stating it doesn’t cover it.

Cheaper Isn’t Better

We hope you’ve learned some valuable tips that we love sharing with our consumers! Buying an engine isn’t a cheap purchase, but sometimes people turn to cheap and consider it convenience. You’re wasting your money and we don’t want that! Always do your homework and you can always count on West Coast Engines to provide you with quality engines, but also the quality content you’re looking for when doing your research.

Other Related Posts:
Read our expert guide on what a crate engine is here.
Expert Tips: Buy A Crate Engine Or Build One?
What You Need To Now Before Buying A Cheap Engine


  1. Les Cormier

    Why do you recommend a one piece rear main seal?

    • West Coast Engines

      Hey Les!

      Great question! The reason why we recommend and use a one piece rear main seal is because they can use a roller cam. They’re also less prone to leaks and are a great build!

    • Jacob Dalton

      Less prone to leaks, instead of having to get 2 seals to fit and not leak it’s alot easier getting 1 big deal to seat properly

      • West Coast Engines

        Yes for sure! 1 pieces are less prone to leak.

        • West Coast Engines

          Heck yes 🙂

      • West Coast Engines


      • Lin

        Is the 2 piece seal block a stronger block

        • West Coast Engines

          Good question. Yes the 2-piece seals had higher quality metals back in the day. GM stopped using those metals for various reasons but mainly because they were considered hazardous to the environment.

  2. yamaha sea scooter

    Howdy! I simply would like to give you a big thumbs up for the
    excellent information you have here on this post. I will be returning to your website for more soon.

    • West Coast Engines

      Thank you!!

  3. Sergio Montenegro

    What year did gm go to serpentine belt engines?

    Any block prior non serpentine or single rear main seal not a good candidate for engine build?

    • West Coast Engines

      Most vehicles early 90’s was the change to serpentine. Yes, 1 piece blocks are preferred.

  4. Deano

    I have an original Z28 302 crankshaft that I want to build an engine around. Obviously, it is a two-piece seal crank. Would you recommend an older 2 bolt main 327/350 block or an older 4 bolt block? Can this crank be adapted to a newer 1 piece seal block? Would that be a better option? Thanks for your inputs.

    • West Coast Engines

      Hey Deano! A 2 or 4 bolt on a 2-piece rear main block will do job just fine for everyday street performance 327 or 302 build. In that generation the block gurdles are real thick and the metals are strong.

  5. Shawn

    If new engine blocks are of low quality, why do engine last so much longer now. It is not uncommon for a LT or LS to go 300,000 miles with zero problems, as a matter of fact block issues other then the marriage of aluminum heads have been nonexistent .

    • West Coast Engines

      No that’s an entirely different topic. We are speaking of new aftermarket blocks specifically for small block 350 engines (classic hot rod carbureted engines) and those blocks being “new” but made cheaply. There are quality made new blocks out there, but most of the market is supplying very cheap blocks that are brittle… especially the target 350 blocks from GM.

  6. hugh kelly

    I have a 1970 350 4 bolt main engine. The block has never been touched but has camel hum heads. I wanted to have this engine rebuilt with a bottom end torque and slightly rough at idle cam using a quadra-jet carb and install it in my 1986 c10 p/u. Is it worth doing and could you build it. Thanks

  7. Dalton

    New and young buyer here. When buying an old Chevy 350 what’s the most tactful/easiest way for finding out all of this information from those old-timer sellers without seeming like a tire kicker? Is there a single serial number that can give me all the information I need after little research online?

    • West Coast Engines

      Yes there is. On the back of the block on the bellhousing lip there’s a casting number.


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