Don't neglect your motor mounts! Most stock BMW motor mounts are fluid filled and the stiffness of the mount depends on this fluid. But over time the fluid leaks out and the strength of the mount is greatly reduced. A worn motor mount allows the engine to twist and shift around. The harder you're driving the more shift will occur. As the engine is moving around it can contact other parts in the engine bay - usually cooling related. The mechanical fan can hit the radiator; the A/C compressor can hit a radiator hose, the exhaust will flex and may break. Twisting of the engine can also lead to shifting problems as the transmission twists with it. There are countless issues related to worn or compressed motor mounts so don't overlook these as a regular replacement item (every 50,000 miles).
Instead of completely replacing the mount, simply make a billet aluminum insert for them. This leaves the feel of the factory mount in place but stiffens them up quite a bit removing unwanted engine movement without rattling your teeth out or emptying your wallet.
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