[Tpeterson]

I have some Dinan springs to install on my 228i.The Bentley manual says to replace most of the fasteners that are removed to do the install,28 in all.These must be BMW instructions.Is it because the nuts and bolts are aluminum? Is it because some of them are torqued then turned 90* farther? Is it OCD German engineers? Has anyone who has done this mod replaced the fasteners? Is it necessary?

[aerobod]

Most of the fasteners are torque-to-yield, so they need to be replaced each time they are removed to be able to meet the required clamping force on assembly without stretching the bolt further than the designed yield point.

[jmwilson125]

I ordered new ones for the lca swap. Turned out they sent me the wrong length bolts so I just re used oem.

As mentioned above these are strech bolts and should be replaced.

Personally I am not terribly concerned about not replacing them. My guess is that the stretch in the bolt has more to do wih ensuring torque accuracy than it has to do with the stength of the bolt.

[Tpeterson]

There's $60 worth of nuts and bolts involved,but I probably will use new ones to be on the safe side.

[NISFAN]

I believe the requirement to replace them is more down to them being 'safety critical' than for engineering reasons.
For example, the 10 top hat bolts on front dampers are tightened to not a particularly high torque for the thread size/bolt rating. I can't imagine there is significant stretch going on. Even the pinch bolt isn't torqued very high.

I reused all of mine, in the knowledge that more upgrades were coming and would replace at last fix.

[Sail Boat]

Great question.

"Stretch Bolts" or torque-to-yield fasteners have advantages over traditional fasteners because when they are torqued to their plastic range, any reduction of the fastener's overall length caused by suspension parts settling or relaxing results in a comparably small reduction in clamping force.

I think this works very well with engine components, such as mating the head to the block, where gaskets settle over time. This settling/relaxing can significantly reduce clamping force if the head studs are in their elastic region.

For suspension components, I'm convinced manufacturers use them to keep customers from complaining about small clunks and rattles during the warranty period.

Don't be worried about the fasteners not being effective (they are) or falling apart (they wont). For what it's worth, I've never replaced a suspension related stretch bolt.

[Tpeterson]

Originally Posted by NISFAN

I believe the requirement to replace them is more down to them being 'safety critical' than for engineering reasons.
For example, the 10 top hat bolts on front dampers are tightened to not a particularly high torque for the thread size/bolt rating. I can't imagine there is significant stretch going on. Even the pinch bolt isn't torqued very high.

I reused all of mine, in the knowledge that more upgrades were coming and would replace at last fix.

BMW recommends upgrading the M8 top hat bolts to M10. As a matter of fact,in my parts search at GetBMWParts the M8's are listed as not for our struts and the M10's are. If I replace those 10 bolts I might as well replace the rest.

Originally Posted by Sail Boat

Great question.

"Stretch Bolts" or torque-to-yield fasteners have advantages over traditional fasteners because when they are torqued to their plastic range, any reduction of the fastener's overall length caused by suspension parts settling or relaxing results in a comparably small reduction in clamping force.

I think this works very well with engine components, such as mating the head to the block, where gaskets settle over time. This settling/relaxing can significantly reduce clamping force if the head studs are in their elastic region.

For suspension components, I'm convinced manufacturers use them to keep customers from complaining about small clunks and rattles during the warranty period.

Don't be worried about the fasteners not being effective (they are) or falling apart (they wont). For what it's worth, I've never replaced a suspension related stretch bolt.

Thanks for the explanation.

[zipphreak]

You can't just put an M10 bolt in an M8 hole. First your strut tower has to have the same bolt holes, and even if you had them you need to replace the strut mounts to that match... You don't need to change them.

BMW has M10x3 for pre-2015 production, M8x5 for post 2015 production F22, and now M8x6 strut mounts for F22 LCI... Different years, different models, different combinations. You can't really mix and match

I have a MY2015 with 2014 production date, that has M10x3 bolts.

[Tpeterson]

Originally Posted by zipphreak

You can't just put an M10 bolt in an M8 hole. First your strut tower has to have the same bolt holes, and even if you had them you need to replace the strut mounts to that match... You don't need to change them.

BMW has M10x3 for pre-2015 production, M8x5 for post 2015 production F22, and now M8x6 strut mounts for F22 LCI... Different years, different models, different combinations. You can't really mix and match

I have a MY2015 with 2014 production date, that has M10x3 bolts.

Thanks. I'd like to know why GetBMWparts says the M10 bolts fit my car and the M8 don't.

[zipphreak]

Don't trust those parts sites, look at your strut tower. It's not complicated But 10mm is 10mm and 8mm is 8mm. You cannot fit one in the other, right
Any time you order parts from them your supposed to provide the VIN, so they can check the production month/year.

[Tpeterson]

Originally Posted by zipphreak

Don't trust those parts sites, look at your strut tower. It's not complicated But 10mm is 10mm and 8mm is 8mm. You cannot fit one in the other, right
Any time you order parts from them your supposed to provide the VIN, so they can check the production month/year.

It's obvious when you point it out. I wasn't thinking the numbers meant mm.I guess I'm still confused by metric parts measurements.