The most common SBC (Small-Block Chevy) motor mount bolts are 3/8-16 UNC, though some aftermarket or OEM-style mounts may use 7/16-14 UNC. Always verify the exact size from your mount kit or service manual.
Common bolt sizes for SBC motor mounts
Below are the sizes you’re most likely to encounter. The exact bolt size depends on the mount design and whether you’re using factory-style or aftermarket brackets. Use this as a quick reference, then confirm with your specific hardware.
- 3/8-16 UNC bolts: The standard size for many SBC engine-to-mount attachments. Typical engagement length is around 1-1/4" to 1-1/2" depending on bracket thickness.
- 7/16-14 UNC bolts: Used by some aftermarket mounts and certain OEM-style applications. Typical lengths range about 1-1/2" to 2" to accommodate different bracket stacks.
- Other sizes: In rare cases, certain heavy-duty or specialty mounts may employ larger or metric fasteners. Always check the manufacturer specifications for your exact kit.
When selecting bolts, ensure you’re using the correct diameter and thread pitch for your mount and engine block bosses, and choose the appropriate grade (such as Grade 5 or Grade 8) as recommended by the mount supplier. Improper fasteners can compromise alignment and safety.
How to determine the exact bolt size for your setup
Because SBC installations can vary by year, model, and aftermarket kit, there are reliable steps to identify the correct bolts for your particular setup.
- Inspect the current bolts and mounting ears with calipers or a thread gauge to determine diameter and thread pitch (for example, 3/8-16 or 7/16-14).
- Check the documentation or packaging that came with your motor mount kit for the specified bolt size and length.
- If documentation isn’t available, compare to factory service manuals or cross-reference with the mount manufacturer’s website or tech support.
- When using aftermarket mounts or adapters, verify the exact bolt size and length specified for that kit, and obtain the correct grade bolts accordingly.
Having the right bolts ensures proper thread engagement and load transfer between the engine, mounts, and frame, which is essential for safe operation and reliable engine installation.
Summary
For most Small-Block Chevy installations, 3/8-16 UNC bolts are the common choice, with 7/16-14 UNC appearing in some aftermarket or OEM-style setups. Always verify the precise size and length for your specific motor mount kit, and use the recommended bolt grade and torque as specified by the manufacturer.