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Why do you need to match the durometer (hardness) of BMW bushings and mounts?

You must take into consideration durometer (or hardness) when replacing mounts and bushings on your BMW. This is particularly important when it comes to BMW motor & trans mounts and BMW subframe & diff bushings. These parts work together as systems to hold components in place (the drivetrain and rear cradle, respectively). In each situation, both parts must be of a similar hardness, or excessive stress will be placed onto the *softer* material part and/or its mounting points. Harder material parts are virtually fixed in place so all of the movement in the system is transferred to the softer part, causing excessive stress on it, which may lead to a failure.


BMW Motor and Train Mounts
When it comes to BMW motor and trans mounts, both parts must equally contribute to holding the engine and trans assembly in place. For example, if you loosened the stock trans mounts and drove the car around, a failure would occur either with the mounts themselves or at the mounting points. This is due to the added stress put onto them from the excess transmission movement. Motor and trans mounts are designed to work together as a system and that requires that they both contribute equally in holding power.

The same situation happens when you replace one part with a hard material while the other part is of a softer incompatible material. For example: combining 75D motor mounts with stock trans mounts. The stock trans mounts will move considerably more, putting excess stress onto them, which could cause a failure. Matching hard (75D) trans mounts with hard (75D) motor mounts, allows the mounts to work together equally to provide the correct amount of holding power along the drivetrain without promoting undo stress onto either part.

What BMW motor and trans mount combinations are compatible?

- Stock (OE) Motor Mounts and Trans Mounts
- Poly 75D Motor Mounts and Poly 75D Trans Mounts
- Poly 75D Motor Mounts and AKG Performance Trans Mounts
- Performance Motor Mounts and Performance Trans Mounts


BMW Subframe & Diff Bushings
When it comes to BMW subframe and diff bushings, both parts must equally contribute to holding the subframe and diff assembly in place. For example, if you loosened the stock diff bushings and drove the car around, a failure would occur either with the diff bushings themselves or at the mounting points (i.e. broken bolts). This is due to the added stress put onto them from the excess differential movement. Subframe and diff bushings are designed to work together as a system and that requires that they both contribute equally in holding power.

The same situation happens when you replace one part with a hard material while the other part is of a softer incompatible material. For example: combining 75D subframe bushings with stock diff bushings. The stock diff bushings will move considerably more, putting excess stress onto them, which could cause a failure. Matching hard (75D) subframe bushings with hard (75D) diff bushings, allows the bushings to work together equally to provide the correct amount of holding power along without promoting undo stress onto either part.

What BMW subframe and diff bushing combinations are compatible?

- Stock (OEM) Subframe Bushings and (OEM) Diff Bushings
- Poly 75D Subframe Bushings and Poly 75D Diff Bushings
- Aluminum Subframe Bushings and Poly 75D Diff Bushings (or vice versa)
- Poly 95A/85A Subframe Bushings and Poly 95A/85A Diff Bushings (or vice versa)
- Poly 95A/85A Subframe Bushings and (OEM) Diff Bushings (or vice versa)