Motor mounts typically rely on high-strength fasteners to secure the engine to the chassis and absorb vibration. In most modern vehicles, engineers specify high-strength bolts such as SAE Grade 8 or metric grades 10.9 or 12.9, depending on the market and design.
Common bolt grades used for motor mounts
The bolt grade chosen for a motor mount depends on the required strength, vibration dampening, and torque. The following are the typical options you’ll encounter in maintenance or upgrades:
- SAE Grade 8 bolts (imperial): Widely used in North American and some import applications for engine-to-subframe and trans mounts due to their high strength and reliability. They are identifiable by a head marking indicating Grade 8.
- Metric Grade 10.9 bolts: Common in European and many Asian vehicles; these offer high tensile strength and are a frequent choice for engine mounts in modern cars.
- Metric Grade 12.9 bolts: Used on high-stress mounts or performance-oriented applications where extra stiffness and load capacity are required.
Key takeaway: Always verify the exact bolt grade specified by your vehicle’s manufacturer and match the diameter and thread pitch to the original fasteners. Using a lower grade can fail under torque and vibration, while higher grades may require different torque settings or hardware compatibility checks.
Practical considerations for replacement
When replacing motor mount bolts, follow these guidelines to ensure safety and proper function:
- Check the service manual or OEM parts fiche for the exact grade, diameter, and thread pitch the vehicle requires.
- Inspect bolt heads and threading for stretch, corrosion, or damage; replace any suspect bolts rather than reusing them.
- Use the correct torque specification and, if recommended, a thread-locking compound or lubricant per the manufacturer’s instructions.
- Replace mounting hardware in sets if multiple mounts are involved, ensuring consistent clamping force across mounts.
In practice, many mechanics and enthusiasts stick to OEM-grade replacements to preserve alignment, vibration control, and warranty considerations.
How to verify the correct grade on your vehicle
To ensure you’re using the right grade, check for markings on the bolt head, which indicate the grade in imperial (8, 5, 2) or metric (8.8, 10.9, 12.9) systems, and consult the vehicle manual or dealership resources. If in doubt, ask a qualified mechanic and adhere to the original hardware as the baseline.
Note: This guidance should not replace official service literature; always follow the torque values and grade requirements published by the vehicle manufacturer.
Summary
Motor mounts typically rely on high-strength fasteners: SAE Grade 8 bolts in the United States or metric Grade 10.9/12.9 bolts in many other markets. For replacements, adhere to OEM specifications, verify bolt markings, and torque to the manufacturer’s specifications to maintain engine alignment and vibration control.