When you dive into the world of engines, especially classic small-block Chevy engines, you’ll often hear gearheads tossing around terms like “bored .030 over” or “bored .060 over.” These phrases may sound confusing to beginners, but they’re crucial for understanding how engine displacement and performance are modified.
One of the most common discussions centers around the legendary Chevrolet 350 small-block engine. So, what exactly does it mean when someone says they have a “350 bored 60 over”? Let’s break it down step by step.
Understanding Engine Boring
Engine boring is the process of enlarging the diameter of the cylinder bores inside the engine block. This is typically done during a rebuild to remove imperfections, scratches, or wear in the cylinders. It can also be done intentionally to increase engine displacement, which can add horsepower and torque.
For example:
- A stock Chevy 350 small-block engine comes with a 4.000-inch bore and a 3.48-inch stroke.
- If you bore it .030 inches over, each cylinder becomes 4.030 inches in diameter.
- If you bore it .060 inches over, each cylinder becomes 4.060 inches in diameter.
The increase might sound small, but across eight cylinders, that additional volume adds up.
What Does “350 Bored 60 Over” Mean?
When someone says they have a “350 bored 60 over,” they’re referring to a Chevrolet 350 cubic-inch small-block V8 engine that has had its cylinders enlarged by .060 inches beyond the stock size.
- Stock bore size: 4.000 inches
- Bored 0.060 over: 4.060 inches
This modification increases the engine’s displacement from 350 cubic inches to about 365 cubic inches.
So technically, a “350 bored 60 over” is no longer a true 350—it’s closer to a 365 small block.
How to Calculate the New Displacement
Let’s crunch the numbers to see how boring affects displacement. The formula for engine displacement is:
Displacement = (π ÷ 4) × Bore² × Stroke × Number of Cylinders
- Stock 350:
- Bore: 4.000 inches
- Stroke: 3.48 inches
- Cylinders: 8
Displacement = (0.7854) × (4.000²) × 3.48 × 8
= 349.85 cubic inches (rounded to 350)
- 350 bored .060 over:
- Bore: 4.060 inches
- Stroke: 3.48 inches
- Cylinders: 8
Displacement = (0.7854) × (4.060²) × 3.48 × 8
≈ 364.95 cubic inches
So, when bored .060 over, the engine becomes a 365 cubic-inch small block.
Why Bore an Engine 60 Over?
There are a few reasons why an engine builder might choose to bore a 350 out by .060 inches:
- Rebuilding Worn Cylinders
Over time, the cylinder walls wear down or get damaged. Machining them slightly larger ensures a smooth, even surface for new pistons and rings to operate properly. - Increasing Displacement
More displacement equals more air and fuel mixture inside the cylinders, which generally translates into more power. - Fitting Oversized Pistons
Oversized pistons are readily available in .010, .020, .030, .040, .050, and .060 increments. If the block has already been bored before, .060 might be the last viable option. - Performance Upgrades
Some hot rodders want every bit of cubic inch they can squeeze from a small-block, and boring to .060 over is one way to achieve that.
The Benefits of a 350 Bored 60 Over
- More Cubic Inches
Going from 350 to 365 cubic inches gives you a little more horsepower and torque potential without changing stroke length. - Fresh Cylinder Walls
Boring ensures your cylinders are perfectly round, which improves compression, sealing, and performance. - Better Ring Seating
New pistons and rings mate more effectively to freshly machined cylinder walls. - Compatibility with High-Performance Builds
Bored-out blocks pair well with aftermarket camshafts, heads, and intakes to create powerful street or race motors.
The Downsides of Boring to 60 Over
While boring out a 350 can give more power, there are trade-offs:
- Thinner Cylinder Walls
The more material you remove, the weaker the cylinder walls become. This can lead to overheating or even cracking under high stress. - Cooling Problems
Thinner walls transfer heat less effectively. A .060-over block may run hotter, especially in high-performance or towing applications. - Limited Future Rebuilds
Once an engine has been bored .060 over, there’s often no room left for further boring. If something goes wrong, the block might be junk. - Not All Blocks Can Handle .060 Over
Some 350 blocks are strong enough, but others (especially thin-wall castings from later years) may not safely handle being bored that much.
Alternatives to Boring 60 Over
If you’re rebuilding a 350 and want more displacement without risking thin walls, consider these options:
- Stroker Kit – By increasing stroke length with a new crankshaft, rods, and pistons, you can turn a 350 into a 383 stroker, which produces far more torque and horsepower than simply boring over.
- Small Overbore + Stroker – A combination of a .030 overbore and a stroker crank is often safer and more powerful than maxing out at .060.
- Bigger Engine Block – Sometimes, starting with a 400 small-block or an LS-based engine makes more sense for serious power builds.
Is a 350 Bored 60 Over Reliable?
Reliability depends on a few factors:
- Block Casting Year: Older 350 blocks from the late 1960s and early 1970s typically have thicker cylinder walls than later “lightweight” castings, making them better candidates for a .060 overbore.
- Cooling System Health: Proper radiators, fans, and coolant flow are crucial since .060 over engines tend to run hotter.
- Build Purpose: A daily driver or cruiser might be fine, but a drag car with nitrous or boost could overstress a .060 block.
In general, a well-built 350 bored .060 over can be reliable for street driving if cooling and tuning are handled correctly.
Common Myths About 350 Bored 60 Over
- “It’s unsafe to bore any block .060 over.”
Not entirely true. Some blocks can handle it, but others cannot. It depends on the casting. - “It adds massive horsepower.”
The extra 15 cubic inches won’t transform your engine alone. Most power gains come from heads, cams, intake, and exhaust upgrades. - “It’s always better than stroking.”
Actually, a stroker usually adds far more cubic inches and torque than boring alone.
Final Thoughts
So, what is a 350 bored 60 over? It’s a classic Chevy 350 small-block engine whose cylinders have been machined .060 inches larger than stock, making it a 365-cubic-inch V8.
This modification can provide more displacement and a fresh bore for oversized pistons, but it also comes with risks like thinner cylinder walls and potential overheating. Whether it’s worth doing depends on your block’s casting strength, intended use, and long-term plans for the engine.
