What is the bore diameter of a small block Chevy?

The bore diameter of a Chevrolet small-block V8 varies by engine variant and era. In general, the most common bore size for modern small-blocks is 4.000 inches, but early engines used smaller bores—3.750 inches on the 265 and 3.875 inches on the 283—while some later, larger-displacement small-blocks used 4.125 inches.

This article explains the typical bore sizes across notable small-block Chevrolet engines, how to identify the exact bore on a specific block, and what to consider when rebuilding or swapping parts.

Overview: common bore sizes for popular small-block variants

Below is a quick reference to common bore diameters associated with widely produced small-block Chevy engines, noting that actual bore can vary by year, revision, and factory oversizing during rebuilds.

• 265 cubic inch (1954–1955): bore 3.750 in (95.25 mm)


• 283 cubic inch (1957–1961): bore 3.875 in (98.43 mm)


• 302 cubic inch (late 1960s–1970s; widely used in various configurations): bore 4.000 in (101.60 mm)


• 327 cubic inch (1962–1969): bore 4.000 in (101.60 mm)


• 350 cubic inch (1967–present in many variants): bore 4.000 in (101.60 mm)


• 400 cubic inch (1970–1980): bore 4.125 in (104.78 mm)

Note: These figures reflect production standards for the main generations. Block casting and aftermarket rebuilds can alter the bore size, and many builders machine blocks to larger oversizes during resleeving or reconditioning.

How to confirm the bore on your specific block

To determine the exact bore on a particular engine block, you should measure the cylinders with a true bore gauge or have a machinist measure using precision tools. You can also reference the block’s casting number and verify it against Chevrolet’s specifications or a trusted engine database to determine the original bore and any oversizes that may have been applied during previous rebuilds.

Casting numbers and identification

Casting numbers etched into the engine block can help identify the exact model and bore size. Cross-reference these numbers with Chevrolet’s casting databases or reputable enthusiast resources to confirm the original bore and displacement. This is especially important for blocks that have been rebuilt or heavily modified over the years.

Why bore size matters for rebuilding or swapping

The bore size affects piston diameter, ring selection, head gasket geometry, and overall compression potential. Mismatched bore/piston dimensions or improper oversize choices can lead to poor sealing, reduced compression, or clearance issues. When planning a rebuild or swap, verify the bore, consider potential oversizes, and coordinate with a qualified machinist to ensure proper fitment and clearance.

Summary

Chevrolet’s small-block bore sizes evolved with the engine family: 3.750 inches for the early 265, 3.875 inches for the 283, and 4.000 inches for most later large-displacement small-blocks such as the 327, 350, and 302, with the 400 using 4.125 inches. Always confirm the exact bore of your block via precise measurement or casting-number reference, and plan any rebuilds with appropriate piston oversizes and machine work in mind. This ensures proper fit, sealing, and performance for your small-block Chevy project.

What is the bore size of the 327?

The Chevy 327 engine has a standard bore size of 4.00 inches (101.6101.6101.6 mm). This is combined with a 3.25-inch (82.5582.5582.55 mm) stroke, which is what distinguishes it from the 350 engine, which has a 4-inch bore but a longer 3.48-inch stroke. 

• Bore: 4.00 inches (101.6101.6101.6 mm) 
• Stroke: 3.25 inches (82.5582.5582.55 mm) 
• Distinguishing Feature: The 327's relatively short stroke is a key difference between it and the 350, a popular misconception is that they both have the same bore, but the 350 is 4 inches and the 327 is 3.48 inches. 

What is a 350 bored .030 over?

A "350 bored 30 over" refers to a Chevrolet 350 cubic inch engine where the cylinders have been machined to a diameter of 4.030 inches. This is a common practice during an engine rebuild to clean up the cylinder walls, and it requires the use of larger pistons and a new set of rings to fit the new bore size. This process results in a displacement of approximately 355 cubic inches.
 

• What it means: The engine block's cylinder bores have been enlarged by 0.0300.0300.030 of an inch to create a fresh, straight surface. 
• New bore size: The original 4.000-inch bore becomes 4.030 inches. 
• Engine parts needed: The rebuild requires pistons that are 0.0300.0300.030 inches larger than the standard size, along with new piston rings. 
• Displacement change: This increase in bore size adds a small amount to the engine's total displacement, bringing it to about 355 cubic inches. 

What is the bore and stroke of a 302 Chevy?

The Chevy 302 engine has a 4.00-inch bore and a 3.00-inch stroke. This combination, which creates a high-revving, oversquare engine, was achieved by using a 4-inch bore from the 327 and a 3-inch stroke from the 283 crankshaft. This configuration was designed specifically for high-performance racing, such as the SCCA Trans-Am series.
 
This video explains the bore and stroke of the Chevy 302 engine: 56sRatMaroYouTube · Jul 21, 2022

• Bore: 4.004.004.00 inches 
• Stroke: 3.003.003.00 inches 
• Design: The 302 was created to be a high-revving, oversquare engine for racing purposes. 
• Construction: It was built using a 4-inch bore, similar to the 327, and the 3-inch stroke crankshaft from the 283. 
• Application: This engine was most famously used in the 1967-1969 Camaro Z/28 to meet Trans-Am racing rules, which limited displacement to 305 cubic inches. 

This video explains how the Chevy 302 was designed to be a high-revving engine: 1mMILNERS SPEEDSHOPYouTube · Jun 27, 2021

What is the bore and stroke of a 400 small block Chevy?

A stock 400 small block Chevy has a bore of 4.125 inches and a stroke of 3.750 inches. This is different from a 350 small block Chevy, which has a 3.73-inch bore and a 3.48-inch stroke. The 400 was also known for its siamese-bore design and external engine balancing.
 
You can watch this video to learn about the differences between 350 and 400 small block Chevy engines: 48sMyvintageiron7512YouTube · Feb 12, 2023
Bore and stroke

• Bore: 4.1254.1254.125 inches
• Stroke: 3.7503.7503.750 inches 

Key characteristics

• Siamese-bore design: The 400 has no coolant passages between the cylinders, which allowed for a larger bore but required special steam holes in the cylinder heads for proper cooling. 
• External balancing: The 400's crankshaft and rotating assembly were externally balanced, meaning weights had to be added to the damper and flywheel to balance the engine. 
• Shorter connecting rod: To accommodate the longer stroke, the 400 used a shorter connecting rod (5.5655.5655.565 inches) than the 350 (5.75.75.7 inches). 
• Interchangeability: The 400 was less interchangeable with other small blocks due to its unique crankshaft, balancer, and flywheel. 

You can watch this video to see how a 400 small block Chevy engine is built: 59sZHP GarageYouTube · Jan 20, 2022