What size is the crank bolt on a 350 SBC?
The standard crank bolt size for a Chevrolet 350 small-block engine is a 7/16-20 UNC bolt, typically about 1.75 to 2 inches long to secure the harmonic balancer and pulley. Variations exist depending on year, aftermarket balancers, and spacers.
Understanding the exact bolt size matters for maintenance, restorations, and upgrades, since the wrong fastener can damage the crank nose or balancer. The following outlines the typical sizes you’re likely to encounter and how to verify the correct bolt for your specific setup.
Common bolt sizes and lengths you’ll encounter
Most 350 SBC installations use a 7/16-20 UNC crank bolt to fasten the harmonic balancer to the crank. Because pulleys and balancers vary in thickness and whether spacers are used, the bolt length can range from roughly 1.75 inches to about 2 inches. A smaller subset of aftermarket or vintage configurations may employ a 1/2-20 UNC bolt.
- 7/16-20 UNC: The standard thread size for the majority of factory and aftermarket SBC pulleys.
- 1/2-20 UNC: Used in some aftermarket balancer kits or unusual configurations; verify with your kit documentation.
- Typical bolt length: About 1.75" to 2.0" for standard setups; longer if spacers or thicker balancers are used.
Conclusion: These sizes cover the vast majority of Chevrolet 350 small-block engines. Always confirm the exact bolt size and length from your engine’s manual or the balancer kit manufacturer to avoid mismatches.
How to confirm the correct bolt for your engine
To verify the correct crank bolt for your 350 SBC, follow these steps:
- Check the vehicle’s service manual or GM official parts catalog for your exact year/model to confirm the standard bolt size.
- Inspect the bolt on the crank pulley or harmonic balancer. Measure the bolt diameter and thread pitch, and compare against 7/16-20 UNC and 1/2-20 UNC specifications.
- Cross-check with any aftermarket balancer kit instructions if you’re using a non-stock pulley or spacer combination.
If you’re still unsure, contact a GM dealership parts desk or a qualified mechanic to confirm the correct bolt for your specific engine variant.
Summary
The most common crank bolt size for a Chevrolet 350 small-block is 7/16-20 UNC, with a typical length around 1.75 to 2 inches. Some aftermarket or vintage configurations may use 1/2-20 UNC. Always verify with the exact parts documentation for your year, model, and balancer kit to ensure proper fit and safe operation.
