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Old 04-19-2012, 08:35 AM   #98
Dan330Ci~
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Join Date: May 2011
Location: UK
Posts: 216
My Ride: 330Ci Clubsport
Lol I agree with dmast, as a fellow stupid engineer.

A jointing torque plus angle does not make a torque-to-yield fastener. It is clear to an engineer with experience in fasteners that 60 NM plus 90 deg. is not torque-to-yield for an M10 fastener, of the specified grade 10.9.

I would consider that these fasteners are specified to be replaced each time they are removed due to the thread locking compound being pre-applied, plus as dmast states they are plated - ZNS3 (hexavalent chromium-free). With a plated fastener you may choose to specify replacement each time due to the natural degradation of the plating, which can range from say 0.1 um per year for a dry indoor (or storage) environment to 8 um per year for an exterior coastal environment (or let's say salty road conditions). The plating thickness of BMW's GS 90010 ZNS3 specification is 8 um minimum, therefore in their location underneath the car you may expect a few years of corrosion protection, depending on the country, then the substrate will corrode which in this case is the fastener. BMW design does not know when these fasteners will be removed in-service, it may be every 1 month or 15 years, therefore they have specified replacement upon removal.

However, this does not preclude replacement of the fasteners each time as specified, as these are just my considerations of the available information.

It is for the above reasons I replace most plated fasteners (and most unplated) in exposed locations on the car when I remove them, as I am aware that the plating service life may be nearly expired with my car being 7 years old. The main benefit of this is ease of dissasembly next time I have to disassemble them - they won't have corroded in the meantime.

Another example of one-time-use fasteners on the E46 is the driveshaft to diff output flange bolts, which are one-time-use because the underside of the head is ribbed which becomes distorted upon tightening, providing an anti-vibration feature.

There are further examples of one-time-use such as the steering column torx fasteners, again plated with locking compound pre-applied.

If either of the above examples used a jointing torque plus angle I would expect people to believe they were torque-to-yield however neither would be.

dmast you stated that the bolts are 'extruded' into the aluminium surface, I would use the term embedding. However, your observation is correct.

Hopefully this provides some food for thought.
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Last edited by Dan330Ci~; 04-22-2012 at 04:24 AM.
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