Hello,
As long as the bolts are tightened to specification, replaced if needed, and free of oil, there is no reason to be concerned.
Most EU vehicles use aluminum fasteners for low torque applications, even including camshaft retaining hardware (of all the various German types).
The good part is, if the valve cover gasket is going to leak because of the fasteners stretching, it should happen after the first few heat cycles of the engine.
Really, I would expect some stretch out of the bolts, except they were replaced, meaning they are what is known as 'torque-to-yield' bolts.
What that means is that as soon as the bolts are tightened to spec, the bolts are already stretched, and the pressure exerted on the bolts during normal operation is insufficient to cause further stress, because of the clamping force of the combination of several bolts.
Great question, I know exactly why you are asking, and the issue is not the material, it is that people think they have it torqued properly, or don't even look up the specs. Also, if the mating surfaces are not cleaned thoroughly, there is a great chance of leakage. Finally, nobody believes they should replace bolts, it is just now catching on with the DIY community to change TTY fasteners when removed...
Proven quality! > *VICTOR REINZ* Valve Cover Bolt Kit (Aluminum) 11120409288 .. Without getting into a metallurgy course here, due to close tolerances, 'corrosion' prevention as well as torque is main concern which usually occurs between steel and aluminum materials over time... I must say that 'stainless steel' bolts properly torqued would work equally as well as the aluminum OEM, but cost would be about the same .. Just trying to give the installer a break!
After I question him about the risk of causing damage to my engine when the they get heated, he stating that he will go ahead and change out to the aluminum bolts. . When I looked up the parts for the valve cover gasket, and the bolts for my car. It give me set of aluminum bolts, not steel. The mechanic (Non-BMW) stating that steel is better and the BMW mechanic is saying the aluminum bolts are required for this car. Confused. I just want my car fix right and Mechanics to be honest and fair. Thanks.
I guess I read something wrong, sorry, I thought you were saying aluminum bolts were used. Yes, this should have aluminum bolts. BMW does not sell steel bolts for these, and aftermarket manufacturers, if not reputable, should not be trusted for dead-on accuracy of material blends, tensile strength, and length. Make SURE you ask him for the plastic packaging from the BMW bolts that he purchases to properly complete the job. The package had better say genuine OEM, or you are not using the right bolts. This is a simple thing to get right, and though there may never be an issue, right is right. If you end up spending a few extra dollars to purchase OEM parts over and above the quote, that should be expected. But a few dollars at max, since you are only paying for the DIFFERENCE in cost of (23?) 90 cent bolts. You should not pay for the labor to replace TTY bolts with new bolts, as that is included in the job.