What Is A 350 Bored 40 Over? – Benefits and Downsides of Boring!

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what is a 350 bored 40 over

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The Chevy 350 is an all-around good engine that can comfortably put out up to 375 horsepower. This depends on the model of the car. And when bored to different bore sizes ranging from 30 to 40 and 60 over, it becomes even more powerful.

So then, what is a 350 bored 40 over? It refers to machining a 350 engine to increase the cylinder’s internal diameter by 40 thousand of an inch. The bore diameter is enlarged from 4.00 inches to 4.04 inches. As a result, this also expands the engine’s displacement from 350 to 357 cubic inches.

But is boring a 350-engine 40 over really necessary? If yes, why and what are the benefits? Keep reading to find out.

what is a 350 bored 40 over
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What Is a 350 Bored 40 Over?

This is a 350 cubic inch V8 engine that has been machined 40 thousand of an inch over. Typically, this is done as an aftermarket customization and involves modifying the engine stock bore diameter.

You see, the factory bore size of this engine is around 4.00 inches, while its stock stroke is 3.48 inches. So, the engine is bored 0.040 inches larger than the factory size.

Generally, the engine is machined to increase the internal diameter of each cylinder using specialized tools and equipment, such as:

  • Boring bar or machine
  • Honing machine
  • Bore gauges
  • Carbide drill bits

After boring the engine, this usually increases your vehicle’s engine displacement from 350 to 357 cubic inches. Below is the formula I used to arrive at this figure:

(4.04in/2) ^2 x pi (3.14159) x stroke x number of cylinders

Where 4.04 inches is 4.00 + 0.040, stroke is 3.48 inches, and number of cylinders is 8. So, to calculate the total displacement after the boring:

(4.04 inches/2) squared x 3.14159 x 3.48 x 8 = 356.812 or 357 cubic inches.

However, while boring a 350 40 over, you also must perform other engine modifications to ensure its proper function and durability. These include changing the pistons, rings, and other internal components to appropriate sizes.

In addition, before boring the engine 0.040 inches larger, it is advisable to get the cylinders tested for thickness. Because as it turns out, some blocks may become too thin from boring as little as 40 over. And this could lead to overheating problems in the future.

When Is Boring 350 Engine Block 40 Over Necessary?

Generally, boring a 350 40 over is done to clean up the cylinders from years of wear and tear when you rebuild it. This is because, over time, the cylinder walls can wear unevenly from high mileage and overheating, causing the engine to seize.

One common symptom of a seizing engine is the engine knocking the car shut off.

However, there are several other instances when it becomes necessary to bore the engine. These include:

  • If the engine pistons are seizing or damaged
  • You want to increase the engine displacement
  • The bore has an excessive bore diameter that is out of tolerance
  • The bore is out of round with taper top to bottom
  • You feel vertical scratches on the cylinder wall

What Are The Benefits of Boring a 350 Engine 40 Over?

Just like I discussed in the what is a 350 bored 60 over guide, boring an engine has some advantages. However, you can only realize most of these benefits when you make other modifications to your engine.

These benefits include the following.

1. Improved Power Output and Torque

Boring your 350 small-block 40 thousand of an inch increases its displacement volume by around 7 cubic inches. Consequently, this allows more air to be pushed into the cylinders, improving the combustion process. This ensures you get more horsepower and engine torque.

However, you should not expect significant power and torque increase since the bore size is not much.

2. Quicker Accelerations

With more power from your 350 small block engine, you will achieve faster acceleration. This is because it will be easier to attain higher top speeds when necessary.

3. Cost-Effective

If the engine block is damaged, boring it 40 over can restore it to usable condition and potentially extend its lifespan. I find the cost of doing this relatively cheaper than buying a new block.

Typically, a new 350 small-block crate engine costs over $2,000. On the other hand, boring costs a couple of hundred dollars, ranging from $200 to $400 or slightly more.

4. More Customization Options

During boring, you can change various engine components to suit your specific performance goals. For instance, you could customize the engine for racing or even street performance by upgrading the camshafts and valvetrain components.

Are There Any Downsides of Boring a 350 40 Over?

Boring your 350 also has potential challenges. The good news is that most of them can be avoided. These downsides include the following.

1. Reliability Problems

If you don’t upgrade the various engine parts appropriately, the engine can become less reliable. This is because boring the engine 40 over will increase the load on the old engine components. To avoid reliability issues, I would recommend installing new pistons and piston rings.

2. Thinned Walls

If the block has been re-bored before, the cylinder walls can become too thin from boring 0.040 inches larger. This could reduce the lifespan of the engine. However, this can be avoided by having the block’s thickness tested before boring it.

3. Reduced Fuel Efficiency

The 350 is a small block engine known for burning less fuel. However, increasing its bore size by 0.040 inches can potentially reduce its fuel efficiency. This occurs due to an increased air-to-fuel ratio.

People Also Ask

Below, I’m going to respond to the frequently asked questions about boring a 350 engine 40 over.

FAQ Gear Head New

Can I bore 350 40 over myself?

It is not recommended. This is because boring is a precision machining process that requires specific equipment, careful measurements, and knowledge of engine building. Therefore, the procedure should be conducted by experienced professionals.

How do you determine if your 350 block can be bored 40 over?

It’s only a reputable mechanic who can help you determine the maximum safe bore diameter for your 350 engine. They do this by measuring how thick the cylinder walls are and if they have porosity.

Do I use the stock or a new block when boring a 350 40 over?

In most cases, the mechanic will use the stock block for boring. This is unless the stock block is exceedingly worn out with thin walls due to having been rebored before. In such a case, you must buy a new block.

Conclusion

A 350 bored 40 over is a common aftermarket customization that entails increasing the cylinder’s diameter by 0.040 inches. This is mainly done to enhance the engine’s performance. However, boring your 350 engine 40 over has its own advantages and disadvantages. The good news is that the benefits outweigh the drawbacks.

That said, if you are planning to have your 350 engine bored soon 0.040 inches larger, I suggest you consult experienced engine builders. It does not matter whether you are boring the engine just to clean up the cylinders.

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