What size is a motor mount bolt?
There is no universal size for motor mount bolts. Sizes vary by vehicle make, model, and engine. In modern passenger cars, you’ll typically encounter metric bolts in the M8–M12 range with thread pitches such as 1.25, 1.5, or 1.75, and high-strength grades like 8.8 or 10.9. Always consult the vehicle’s service manual or inspect the existing fasteners to confirm the exact size and grade before replacement.
Understanding motor mount bolts
Motor mounts secure the engine to the chassis and must withstand significant forces. Because mounting points, thread engagement, and the load path differ between designs, the bolt size is chosen to match the mount and block pattern. The most reliable way to know the correct size is to refer to OEM specifications or measure the existing bolt if you’re replacing a failed one.
Common sizes you may encounter
Below is a snapshot of sizes you might see on common automotive applications. The exact bolt for your vehicle will depend on the engine and mounting arrangement.
Metric sizes
- M8 x 1.25
- M8 x 1.0 (less common, varies by design)
- M10 x 1.5
- M12 x 1.75
These metric sizes cover the majority of front- and mid-engine mounts in small to mid-size cars. Always verify the exact size and length for your specific vehicle.
Imperial sizes
- 5/16-18
- 3/8-16
Some older models and certain heavy-duty applications may still use imperial bolts. In modern models, metric fasteners are typically standard.
How to determine the correct size for your vehicle
If you are replacing a bolt or mounting a new engine, use a systematic approach to identify the correct size and grade. The steps below help you avoid mismatched hardware.
- Inspect the existing bolt: measure diameter with calipers, count the threads per inch or use a thread pitch gauge, and measure the threaded length that engages the mount.
- Check the mounting points on the engine and subframe or bracket to confirm compatibility with known bolt patterns (M8, M10, M12, etc.).
- Refer to the vehicle service manual, OEM parts catalog, or a reputable bolt kit for the exact specification and torque values.
- If in doubt, bring the bolt or a sample to a parts counter or hardware store to match the size with a replacement bolt.
In practice, the most important rule is to use hardware that matches the OEM specification in size, thread pitch, length, and strength. Reusing damaged bolts or using incorrect grades can compromise engine mounting integrity.
Safety and maintenance considerations
Always replace fasteners with the same grade and property class recommended by the manufacturer and tighten them to the specified torque. Engine mounts transmit dynamic loads during acceleration, braking, and rough driving, so proper torque and bolt condition are essential for long-term reliability.
Note on choosing the right bolt
When replacing motor mount bolts, it’s usually best to source a complete torque-rated bolt kit or OEM bolts designed for your exact make, model, and year. This reduces the risk of mismatched threads, incorrect length, or insufficient strength.
Summary
Motor mount bolt size is not universal. It depends on the vehicle and engine design, with common metric sizes ranging from M8 to M12 and possible imperial sizes on older models. To ensure a correct fit and safe operation, consult your service manual or OEM hardware, measure the existing bolts, and verify thread pitch, length, and strength rating before replacement.
