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I never hold wheels when using an impact driver on them. I've never had to apply the brake or otherwise stop the wheel from turning. The same is true of a FWD hub nut that I broke free last weekend; nothing was holding the hub from turning. The transmission was in neutral and the brake was not engaged, but the nut came right off. That nut is on with a whole lot more torque than your steering wheel.
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Agreed - but there's a very big difference between the mass of a wheel/tire/hub assembly and the steering wheel/steering shaft. As for the FWD hub nut - you've got the hub, the shaft and all the gearing attached to the spindle - lots of rotational mass.
There has to be sufficient mass to oppose the force of the impact.
On my 300SE (140 chassis), there are 5 factors contributing to removal difficulty
1) Bolt cross section is probably 12mm - it's huge
2) Bolt is fine threaded - more threads - more friction
3) Bolt is allen-headed/flat head and has a tapered shoulder with a wide bearing surface which mates to a similarly tapered area on the steering wheel. Lots of contact area - lots of friction.
4) A ton of blue lock-tite on the threads.
5) From rim edge to top of fastener is 6+ inches - the longer the extension, the more difficult it is to effectively transfer the torque of the wrench to the bolt.
Looking at the math of it all - if I'm right in my dimensions and calculations, the bolt head is about 1.125" in diameter. If the diameter of the threads is .5, that leaves 5/16" (.3125) of shoulder (per side). Now, taper the shoulder at a 45 degree angle and 5/16" grows to .44194. Take that dimension and multiply it by the circumference of a 1.125 diameter (3.53") and you get 1.5600 square inches of contact. (I don't think this is completely accurate because the area decreases as you move from the rim of the bolt head to the edge of the threads - but it serves well enough to illustrate my point.)
Apply the same math to a hex nut of similar dimensions (1.125 diameter with a 1/2" bore) and you get 5/16" shoulder X 3.53 circumference = 1.103 square inches of contact - maximum, because there's actually less contact due to the hexagonal shape "cutting" into the diameter.
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..........so, he used the steering lock to provide countertorque..........I'm not confident that he's correct regarding "no damage".
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Which is why I added my disclaimer. This is a reputable shop - in business for 10+ years. They work on MB and BMW, but mostly MB. They've yet to offer advise or provide service that I didn't find completely accurate/worthy.
I shouldn't have been so absolute in my original post. In my experience, muscle was the solution to the problem.
I would NEVER recommend using an impact wrench on an allen-headed screw. For a given size, allen head screws have far less fastner/tool contact surface than a similar-sized hex-headed bolt. I came way too close to stripping the socket out on my steering wheel when I attempted to use an impact wrench.
Maybe the major reason why the fasteners are typically so tight is that they're the only thing that holds the steering wheel onto the shaft. On many of the American-made cars I've worked on, there's a hex-nut on the end of the shaft, and usually some sort of c-clip in front of that as a belt & suspenders prevention. The nut comes off easily, but then you must use a puller to get the steering wheel free from the shaft. In my 92' W140, and also on a number of other german autos I've wrenched on over the years, you remove the bolt/nut and the steering wheel comes off easily in your hands.
Again - the solution to the original poster's problem is getting full transfer of the torque from the wrench to the fastener. It would be helpful to get an assistant to hold the wheel, and don't yank - just provide constantly increasing torque until the fastener releases.