What size bolt is on the tensioner pulley?

There isn’t one universal bolt size for tensioner pulleys—the correct size depends on the engine and vehicle. Check the service manual or OEM parts diagram for your exact application.

Why bolt size varies by engine

Tensioner pulleys are used in the serpentine or timing belt system, and the mounting and pulley bolts can differ in diameter, thread pitch, and length from one engine to another. Some tensioners use a single mounting bolt through the bracket, while others use separate bolts for the bracket and the pulley itself. Using the wrong size or pitch can strip threads, misalign the belt, or reduce clamping force, potentially causing belt slip or tensioner failure.

How to determine the correct bolt size

Follow these steps to identify the correct bolt size for your tensioner pulley:

• Consult the vehicle’s service manual, owner's manual, or the OEM parts diagram for the specified bolt size, thread pitch, and torque for your engine.


• Inspect the existing bolt: measure its diameter and thread pitch with calipers or a thread gauge, and note its length, head type, and whether a washer is used.


• Check whether the tensioner uses a separate pulley bolt or a mounting bolt; both may have different sizes on the same engine.


• When replacing, use the exact OEM replacement bolt or an equivalent bolt that matches diameter, thread pitch, length, and grade (e.g., metric 8.8 or 10.9; SAE grade 5/8).


• Torque the bolt to the manufacturer’s specification to ensure proper clamping and belt tensioning.

By verifying these details before reassembly, you minimize the risk of damage and ensure reliable belt operation.

Common ranges you might encounter

While there is no universal bolt size, several size ranges appear across different makes and models. Use them as general guidance only and always verify with the OEM specs:

• Metric engines: mounting bolts typically range from M6 to M10 in diameter, with common thread pitches of 1.0, 1.25, or 1.5; pulley bolts are often M6 or M8.


• SAE/imperial engines: common sizes include 1/4-20, 5/16-18, or 3/8-16 for mounting hardware, depending on the engine block and tensioner design.

Sizes vary widely by manufacturer and model, so confirmation from official documentation is essential.

Summary

In short, there is no single bolt size for tensioner pulleys. The correct bolt is engine-specific, and you should rely on the vehicle’s service manual or OEM diagrams to identify the exact bolt diameter, thread pitch, length, and grade. When in doubt, consult a dealership or qualified automotive technician to avoid installing the wrong fastener and risking belt or tensioner damage.

What size bolt is on a crankshaft pulley?

The crankshaft pulley bolt size varies by vehicle, so you must check your specific car's make, model, and year for the correct size. Common bolt sizes range from SAE (such as 15/1615 / 1615/16 inch) to metric (like 191919 mm or 232323 mm). It's also crucial to know the correct thread pitch and whether the bolt has standard right-hand or reverse left-hand threads for your engine.
 
This video demonstrates how to remove a crankshaft pulley bolt, including some tips for different types of engines: 53sMerryfranksterYouTube · Aug 15, 2023
Common sizes

• SAE: 15/1615 / 1615/16 inch is a common size on some older vehicles, such as certain GM models. 
• Metric: Common metric sizes include 191919 mm and 232323 mm, though some may require a 222222 mm socket. 
• Specific examples: Some GM LS engines use a 161616 mm x 2.02.02.0 thread pitch bolt. 

Important considerations

• Thread type: Some bolts are reverse-threaded, so it's critical to verify the thread direction for your engine. 
• Thread pitch: In addition to the diameter, the thread pitch is vital. For instance, some GM LS engines use a 161616 mm x 2.02.02.0 thread pitch. 
• Bolt type: It's best practice to replace the crankshaft pulley bolt each time you remove it, as it is often a torque-to-yield fastener. 
• Torque specifications: Always tighten the new bolt to the manufacturer's specified torque value. Over-tightening can damage the crankshaft or pulley, while under-tightening can lead to slippage. 

This video explains why you should not reuse a crankshaft pulley bolt: 33sGilk The Great | HoodBilly Garage YouTube · May 22, 2025

How to remove tensioner pulley bolt?

System you first need to slightly loosen the tensioner pulley bolt bolt. Just put a wrench with the right size socket on there crack it loose. And give it a few turns. No need to remove it.

How do I tell what size bolt I need?

To measure a bolt, first determine its length by measuring from the underside of the head to the tip of the threads. Next, measure the diameter by measuring the widest part of the threaded shank. Finally, measure the thread pitch by counting the threads per inch (TPI) for standard bolts or by measuring the distance between threads for metric bolts. 
This video explains how to measure bolt length: 45sBoltsandNuts comYouTube · Oct 9, 2020
1. Measure length

• For most bolts: Place a ruler or calipers under the head and measure straight to the end of the threads. 
• For flat-head or countersunk bolts: Measure from the top of the head to the tip of the threads. 

2. Measure diameter

• Use calipers or a ruler to measure the shank, the unthreaded part just below the head. 
• Ensure you are measuring the widest part of the shank from one side to the other, perpendicular to the bolt. 
• For smaller bolts, you may need to use a bolt sizing gauge or look up the nominal size in a chart, as the measurement might not be a standard fraction or decimal. 

This video demonstrates how to measure the diameter of a bolt: 56sBoltsandNuts comYouTube · Feb 28, 2024
3. Measure thread pitch 

• For standard (inch) bolts: Lay the bolt flat and count the number of threads within a 1-inch section of the threads. 
• For metric bolts: Use a thread pitch gauge or place a ruler against the threads and measure the distance between a specific point on one thread (like the peak) and the same point on the next thread. 
• If you're unsure: You can also use a thread checker tool that has a variety of nuts and bolts to help you quickly find a match. 

What size wrench for tensioner pulley?

The size wrench for a tensioner pulley depends on the vehicle, but common sizes are 14 mm and 15 mm for the pulley's arm, while 12 mm or 13 mm sockets are often used for the pulley's mounting bolts. A universal or specialized serpentine belt tensioner tool that includes a range of sizes is also a good option for accessing the lug on the tensioner arm to relieve tension.
 
This video demonstrates how to use a serpentine belt tensioner tool: 53sCGFIXYouTube · Sep 16, 2024
Common wrench sizes

• For the tensioner arm: A 14 mm wrench is frequently needed to pivot the tensioner arm and relieve tension on the belt. Other common sizes include 13 mm and 15 mm. 
• 12 mm For the mounting bolts: or 13 mm sockets are often used to remove the bolts that hold the tensioner pulley assembly to the engine.

Specialized tools

• A dedicated serpentine belt tensioner tool is designed for this job and often includes multiple attachments.
• These tools are helpful for reaching into tight spaces and may include sockets in sizes like 13 mm, 14 mm, 15 mm, and 16 mm. 

Tips for the job

• Always know the correct belt routing before you remove the old belt, by taking a picture or finding a diagram for your specific vehicle. 
• If you don't have the right tool, you can sometimes use a breaker bar or a large socket with a breaker bar to rotate the tensioner arm, but be careful as some designs require a specific tool. 

This video shows how to remove the tensioner pulley without a special tool: 59sApache RanchYouTube · Jun 28, 2020