What size thread are Chevy 350 spark plugs?
The short answer: Chevy 350 spark plugs use standard 14 mm threads (M14 x 1.25) with about 0.75 inches of reach in most stock configurations.
Overview
The Chevy 350, a cornerstone of General Motors’ small-block family, uses a widely standardized spark plug thread size across most of its production years. The typical specification you’ll encounter when shopping for plugs is 14 mm (M14) diameter with a 1.25 mm pitch, plus a common reach around 0.75 inches. While the thread size is consistent, details such as seat type and plug reach can vary by head design and aftermarket choices. Consult your engine’s manual or the plug manufacturer’s cross-reference to confirm exact requirements for your build.
Thread size and common specs
Before listing the core measurements, here are the essential specs you’ll encounter when selecting plugs for a Chevy 350.
- Thread diameter: 14 mm (approximately 0.551 inches)
- Thread pitch: 1.25 mm (M14 x 1.25)
- Common reach: about 0.75 inches (19 mm) for stock applications
- Seating style: typically tapered-seat on factory heads; aftermarket plugs may use gasketed seating
Concluding: For most Chevy 350 engines, you’ll be looking for spark plugs with 14 mm x 1.25 threads, about 0.75" reach, and a seating style that matches your cylinder head (tapered vs gasketed).
Seating and reach variations
Seat styles
Chevy 350 heads commonly use tapered-seat plugs, especially on older and many stock configurations. A number of aftermarket plug lines use gasketed seating. Matching the seat type is important to ensure proper seating, sealing, and heat transfer.
- Tapered-seat plugs are typical for many stock small-block Chevy heads
- Gasketed-seat plugs are common in some aftermarket designs and newer heads
Concluding: When purchasing plugs, verify the seating style to guarantee a proper seal and reliable performance on your particular head design.
Reach options
Reach, the distance the plug extends into the combustion chamber, commonly centers on 0.75 inches for stock Chevy 350 setups. Some aftermarket or high-performance applications may use different reach lengths, but the 0.75" reach remains the standard for most stock engines.
- Typical reach for stock 350 engines: 0.75 inches (19 mm)
- Longer or shorter reach plugs are available but are dictated by head design and application
Concluding: In practice, the standard reach you’ll encounter for the Chevy 350 is about 0.75 inches, with variations only appearing for specific heads or performance builds. Always verify reach for your exact engine configuration.
How to verify the exact size for your engine
Because Chevy 350 engines span several decades and feature different heads, confirming the precise plug size for your specific vehicle is essential before buying replacements.
Verification steps
- Consult the service manual or GM/ACDelco parts catalog for your year, model, and head design to confirm thread size, reach, and seating
- Inspect the existing spark plugs or use a thread gauge to measure diameter and pitch
- Match the replacement plug’s thread size, reach, and seating to the verified specifications of your engine
Concluding: The dominant specification for most Chevy 350 engines is 14 mm x 1.25 threads with about 0.75" reach, but always verify against your exact build to ensure proper fit, performance, and torque.
Summary
In short, Chevy 350 spark plugs generally use 14 mm (M14) threads with a 1.25 mm pitch, and a typical reach of about 0.75 inches. The seating is usually tapered on stock heads, though gasketed seats are common in aftermarket plugs. Always confirm the precise size for your year, model, and head design by consulting the service manual or a trusted parts catalog, and ensure you match reach and seat type when replacing plugs. This careful check helps maintain proper torque, sealing, heat range, and overall engine performance.
