Starter Shims: How To Shim A Starter Motor

Starter Shims: Why It’s Critical for Proper Engagement

If you’re installing a new or rebuilt starter, there’s one detail that too many technicians and DIYers overlook—starter shimming. It’s not optional in many cases. In fact, failing to install starter shims properly can lead to annoying noises, damaged flywheels, and repeated starter failure. In this guide, I’ll walk you through why it’s necessary, what symptoms indicate it’s needed, and exactly how to measure for and install them correctly.

Why Do Some Starters Need Shimming?

Many GM engines, especially small-block and big-block Chevy V8s, use starters that bolt directly to the engine block. The starter gear (called a pinion) must engage smoothly with the ring gear on the engine’s flex plate or flywheel. But manufacturing tolerances in the block, starter housing, and flex plate location can vary slightly from one engine to another—even within the same engine family.

This is where starter shimming comes into play. Shims allow you to fine-tune the position of the starter motor so that the pinion gear meshes perfectly with the flex plate ring gear. Without proper shimming, the starter might sit too close or too far from the flywheel, resulting in noisy operation or mechanical damage.

What Happens When You Don’t Shim a GM Starter Properly?

If you bolt in a GM starter without checking the gear clearance, you’re asking for trouble. Here’s what you might hear:

1. A High-Pitched Whine or Grinding Noise— This usually means the starter gear is too far away from the flywheel. When the teeth only partially engage, they grind past each other instead of locking in smoothly. Repeated starts like this will strip teeth from the ring gear or pinion.

2. A Loud Clunk or Bang During Cranking— This sound often happens when the starter is too close to the flex plate. The gears may bind, causing a hard metal-on-metal impact. This condition wears out the starter quickly and can even crack the starter nose cone.

3. Free-Spinning Starter Motor— If the pinion isn’t making contact with the flywheel at all due to excess clearance, you’ll hear the motor spin, but nothing else will happen. This is another classic symptom that starter shims are needed.

How to Check for Proper Starter Drive Clearance

Before installing any shims, it’s essential to verify whether you even need them—and if so, how thick they should be. GM provides a very specific method for checking the gear mesh clearance between the pinion gear and the ring gear.

Step-by-Step GM Starter Shimming Procedure:

Install the Starter with No Shims First
Bolt the starter in place snugly without using any shims. Then remove the starter solenoid wire and the solenoid cap.

Insert a Paper Clip or Drill Bit
Using a small section of bent paper clip or a 1/8-inch drill bit, measure the clearance between the top of the pinion tooth and the bottom of the flywheel tooth. Insert it into the gap between the two gears.

1/8″ drill bit placed between the starter drive and the ring gear tooth

Evaluate the Fit

Correct clearance: The 1/8” bit should slide snugly between the teeth without forcing it. Too tight: If the drill bit doesn’t fit, the starter is too close—you need to add a shim. Too loose: If there’s excessive space, the pinion is too far from the ring gear—use offset or angled starter shims to move the starter drive gear closer.

Shim Placement
Place flat starter shims between the starter mounting flange and the engine block. You can shim both mounting bolts equally to move the starter away from the ring gear, or shim only the outer bolt to tilt the starter slightly inward.

Retest the Clearance
After adding or adjusting the starter shims, reinstall the starter and check the clearance again. Repeat the process until the gear spacing is correct.

Types of Shims Available

There are several styles of shims designed to accommodate different needs:

• Flat – Move the starter directly outward from the engine block.
• Wedge or Angled – Tilt the starter to correct gear misalignment.
• Bolt-On Nose Cone – Installed on some newer GM starters to adjust depth without disturbing the mounting flange.

Flat and nose cone starter shims

Choose the shim type that best fits your alignment issue. Often, it takes a combination of shims to achieve perfect clearance.

What Happens If You Ignore GM Starter Shimming?

Over time, failure to properly install starter shims can result in:

Stripped teeth on the flex plate or starter gear
Broken starter nose cones due to gear bind
Premature starter motor wear
Intermittent or failed starting especially in high-heat conditions

All of these issues are avoidable by performing proper GM starter shimming during installation.

©, 2025 Rick Muscoplat