What size is the distributor bolt on a 350 Chevy?

The distributor on a typical Chevy small-block 350 is usually held in place by a single 1/4-20 UNC bolt, roughly 1 inch long. Some aftermarket configurations or specialty setups may differ, so always verify with your exact engine or distributor manual.

To understand the issue in more detail: the bolt size affects compatibility, installation torque, and whether you’re dealing with a stock clamp or an aftermarket distributor. Knowing the standard size helps you source the correct hardware and avoid cross-threading or improper timing alignment.

Standard configuration and function

The standard hold-down method for most small-block Chevy 350 engines uses a single bolt that threads into a boss on the timing cover. The bolt passes through a small clamp or washer and holds the distributor body firmly in alignment with the cam gear. The hex head size is typically 7/16 inch, and the goal is to snug the distributor without binding rotation. Always consult the factory service manual for your exact year to confirm the proper hardware and torque.

Before listing common hardware variations, note that the exact choice can depend on whether you have a points-type distributor, an HEI, or an aftermarket unit.

Common hardware variations you might encounter on a Chevy 350:



• 1/4-20 UNC bolt, standard across most factory and aftermarket distributors; length is typically around 1 inch to 1-1/4 inch depending on the clamp.


• Some aftermarket clamps or older configurations may use 5/16-18 UNC bolts, though this is less common for stock 350 installations.


• Certain specialty or retrofits may use a two-bolt clamp or strap with smaller screws; verify the exact hardware on your engine.

Conclusion: when sourcing a replacement distributor hold-down bolt, match the diameter, thread pitch, and length to your engine’s clamp and timing cover to ensure proper fit and torque.

Variations by distributor type and year

Chevy small-blocks generally fall into a few hardware families. Most stock 350s with a conventional distributor use a single 1/4-20 bolt. HEI distributors, which became common in later models, may use a similar hold-down bolt but with different clamp geometry. If your setup uses a two-bolt clamp, the hardware is usually still in the 1/4-20 range or occasionally 5/16-18, depending on the manufacturer. Always refer to the specific distributor installation instructions for the exact hardware required.

Verification is essential: check the service manual or distributor documentation for your exact year and model to confirm the correct bolt size and torque specification.

How to verify on your engine

Here's how to confirm the correct bolt for your 350:

Before you replace anything, inspect the current hold-down hardware and the mounting boss:



• Measure the bolt diameter and count threads per inch to distinguish between 1/4" and 5/16" as well as UNC vs metric where applicable.


• Consult the service manual or a parts catalog for your year and distributor type (points vs HEI, stock vs aftermarket).


• If replacing, bring the old bolt or take a photo to ensure you match the diameter, thread pitch, and length.

Concluding note: using the correct bolt size and torque prevents distributor misalignment and potential engine timing issues.

Summary

The standard distributor hold-down bolt for a Chevy 350 small-block is typically a 1/4-20 UNC bolt, about 1 inch long, used with a single-bolt clamp. Always verify your engine’s year and distributor type, as variations exist with aftermarket parts or HEI configurations. When in doubt, consult the factory service manual or distributor installation guide to ensure proper fit and torque.

Bottom line: for most 350 Chevies, plan on a 1/4-20 bolt; variations may occur, so confirm with your exact setup.