What Size Wrench Do You Need for a Side-Mount Battery?

Most side-mount (side-post) automotive battery terminals use an 8 mm wrench or socket; a 5/16-inch tool also fits in a pinch because it’s nearly the same size. This size applies to the bolt head on standard GM-style side terminals, though a few aftermarket adapters may differ. Below, we explain how to identify the right tool, avoid damage, and tighten to safe torque.

The Standard: 8 mm (or 5/16-inch) for Side-Post Bolts

On the vast majority of side-mount batteries—common on many General Motors vehicles—the terminal bolt is designed for an 8 mm hex head. Because 5/16-inch is very close in size, it typically works as well. Using the correct size helps prevent rounding the bolt and protects the battery’s embedded threads from damage.

Why the Size Matters

Side-post terminals thread into a lead insert inside the battery case. Over-tightening or using the wrong tool can strip the soft metal or crack the case, leading to poor electrical contact or battery failure. A snug fit with the right wrench size is the first line of defense against those problems.

Known Exceptions and Variations

While 8 mm is the norm, there are exceptions. Some aftermarket terminal extenders, accessory adapters, or replacement bolts may use different head sizes (occasionally 10 mm) or accept internal hex/Allen heads. Also note that battery hold-down hardware (which secures the battery to the tray) is a separate fastener—commonly 13 mm or 1/2-inch—and is not the terminal bolt.

Recommended Tools and Sizes

The following items help you remove, install, and tighten side-mount battery terminals safely and correctly. This list focuses on the wrench size you asked about and the essentials to avoid damage or shorts.

• 8 mm box-end wrench or 8 mm socket and ratchet (primary tool for side-post terminal bolts)
• 5/16-inch wrench/socket (acceptable substitute when metric isn’t available)
• Inch-pound torque wrench with 8 mm socket (to finish tightening accurately; optional but recommended)
• Insulated battery terminal wrench or socket sleeve (reduces risk of shorting against bodywork)
• Safety glasses and gloves (battery acid and sparks protection)
• Battery terminal brush or cleaner and dielectric grease (for corrosion prevention and good contact)

Equipping these tools will help you handle most side-post battery jobs smoothly and reduce the risk of damage or unsafe conditions.

Safe Removal and Tightening

Working around a battery requires a specific order of operations to avoid sparks, shorts, and damage to the vehicle’s electronics. Follow these general steps unless your vehicle’s service manual instructs otherwise.

1. Shut everything off and remove the key; if needed, maintain memory with an approved saver device.
2. Loosen the negative (black) side-post terminal first using an 8 mm wrench; remove and secure it away from metal.
3. Loosen the positive (red) terminal next with the same 8 mm tool; avoid contacting metal bodywork.
4. Clean terminal surfaces; apply a light film of dielectric grease to help prevent corrosion.
5. Reinstall the positive terminal first; then the negative terminal.
6. Tighten terminal bolts to a typical spec of about 8–12 N·m (70–105 in-lb). Many OEMs call for roughly 10 N·m (89 in-lb). Do not over-tighten.
7. Verify the connection is secure but not crushing the terminal; check for any movement or arcing.

Sticking to this sequence minimizes the chance of accidental short circuits and ensures a reliable, long-lasting connection.

Torque Guidance and Over-Tightening Risks

Because side-post terminals thread into a soft insert, the torque window is narrow. A good rule of thumb is about 8–12 N·m (70–105 in-lb), with many service procedures specifying near 10 N·m (89 in-lb). If you lack a torque wrench, aim for firm, snug engagement—then stop. If the bolt keeps turning easily, back off and inspect to avoid stripping the insert.

Bottom Line

For a side-mount battery terminal, use an 8 mm wrench or socket; a 5/16-inch tool is a workable substitute. Confirm any aftermarket parts before turning a wrench, and tighten to roughly 10 N·m (89 in-lb) to avoid damage.

Summary

Most side-post battery terminals use an 8 mm wrench (or 5/16-inch) on the terminal bolt. A torque of about 8–12 N·m—often near 10 N·m—is typical to secure the connection without damaging the battery’s lead insert. Remove negative first, reinstall it last, and avoid mixing terminal hardware with separate battery hold-down fasteners, which are commonly larger.

What size is a side terminal battery bolt?

3/8-16
Side Terminal Battery Bolt 3/8-16 Thread.

What size wrench for side post battery?

5/16-Inch
5/16-Inch Side Terminal Battery Wrench.

What size nut goes on a car battery terminal?

The most common car battery terminal nut and bolt sizes are 10mm for the bolt head (using a 10mm wrench or socket) and 5/16-inch (or 3/8-inch for side terminals) for the bolt diameter, with 10mm often fitting the bolt head on a 5/16″ bolt. However, exact sizes can vary by vehicle, so it’s best to check your specific battery or take the old bolt to an auto parts store for an accurate match.
 
Common Sizes and Considerations:

• Wrench/Socket Size: A 10mm wrench or socket is the most frequently used size for the hexagonal nuts found on most top-mount battery terminals. 
• Bolt Diameter (Thread Size):
  • 5/16 inch: (approximately 7.9mm) is a very common diameter for the bolt that goes through the battery cable’s ring connector and into the terminal post. 
  • 3/8 inch: (approximately 9.5mm) is a common size for the bolts on side terminal batteries. 
• Variations: Some vehicles may use a 13mm wrench, though this is less common than 10mm. You may also encounter other, smaller metric bolt sizes on the terminal itself. 

How to Find Your Exact Size:

1. Check for the size on the existing nut: Opens in new tabLook for the size (e.g., “10mm,” “5/16”) on the bolt head or the nut itself. 
2. Take the bolt to an auto parts store: Opens in new tabThis is the most reliable method. Take the bolt with you to an auto parts store, and they can help you find the correct replacement nut and bolt. 
3. Use an adjustable wrench: Opens in new tabIf you don’t have the right size socket, an adjustable wrench can work, but be careful not to strip the nut. 

Safety Tip: Always disconnect the negative (-) battery terminal first to prevent accidental short circuits and potential electrical hazards.

What size wrench do you need for a battery?

The most common wrench sizes for a car battery are 10mm for the top post terminals and 5/16-inch for side post terminals, with a 13mm wrench sometimes needed for the battery hold-down clamp. You will need a socket wrench, open-ended wrench, or a specialized battery terminal wrench to remove the nuts holding the battery terminals in place.
 
Common Sizes for Battery Terminal Nuts

• 10mm (0.4 inches): This is the most common size for the hexagonal nuts found on top-post battery terminals. 
• 5/16-inch: This size is commonly used for the bolts on side terminals, often found on GM and other domestic vehicles. 
• 13mm: You might also need a 13mm socket or wrench to loosen the hold-down clamp that secures the battery. 

Tools to Use

• Socket Wrench: A 3/8-inch drive socket wrench with 8mm, 10mm, and 13mm sockets is a versatile option. 
• Open-Ended Wrench: A standard open-ended wrench in one of the common sizes will also work. 
• Battery Terminal Ratchet Wrench: These specialized wrenches offer quick and easy tightening or loosening by flipping the reversible ratcheting end. 

Important Tips

• Identify the Terminal Type: Determine if your battery has top-post or side-post terminals, as this will dictate the size of the wrench you need. 
• Handle with Care: Use the correct size wrench to avoid stripping the nuts or damaging the terminals. 
• Check the Hold-Down Clamp: Don’t forget to loosen the clamp that holds the battery in its tray; it may require a different size wrench.