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Old 01-07-2019, 08:14 PM
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andrewwynn andrewwynn is offline
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Raising the engine to get at exhaust bolts

I will have a very helpful case study soon. When I replaced my stiffening plate a while back I used less than grade 8 bolts and definitely not torqued to spec. I just used a good during strong yank on a breaker bar.

The only problem that could be expected if the bolts aren't doing their job is that the plate will slide sideways during a body twist situation.

As I'm well known for tight parallel parking shortcut of driving straight in over the curb about a foot and back onto the road my car gets a very significant "ditch witch" twist test regularly. I need to remove the plate soon to track down an oil leak and will take close looks at all the six mounting points and let everybody know if there is any sign of movement of the stiffening plate.

If there is or isn't the question can soundly be answered.

The primary job of the bolts during normal operations (not a wheel off the ground off-road operations which I'm also no stranger to), is to hold the anti sway bar down.

The sway bar doesn't need half the strength of the bolts used for this job: implication is that the only reason they are overkill bolts is to prevent side slip and that's pretty clear by the name of the part or holds: stiffening plate. It stiffens by preventing the subframe from twisting out of square. (Think of it like a giant "X" under the engine to keep the front left and back right of the engine bay the same distance as the front right and back left. Rather than a couple 1" thick steel bars BMW choose to use a 5mm thick aluminum plate that coincidentally blocks road hazards from the engine, noise and catches drips from messing up people's garage floors (until the foam is saturated and that's usually a second owner problem).

The math I'm sure worked out to "holy shit that's a lot of sideways force" so when they added on the typical 200% to 300% safety factor it landed where it did.

As far as a safety concern by the time you get to where one of these bolts fail there will be 8 things far more risky to your well being and more than likely the bolt failing would improve your chance of survival by decreasing the forces required to crumple the engine bay and reduce the g-forces transferred to the cabin.
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Last edited by andrewwynn; 01-07-2019 at 08:20 PM.
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