Rear Suspension Banging

[stuarthardy]

I know this is late and off topic but I cannot let the comments about stainless steel fasteners go unanswered.

Alec is correct in that most grades of stainless steel bolts are of low strength compared to the average engineering type bolts, having grade 8.8 (I'm using the metric system here, for simplicity). There are some stainless steel fasteners that are of a reasonable engineering quality, the best of which are produced by a Swedish company (surprise, surprise) called Bumax. In these you can get strengths very similar to the 8,8 grade upto M6 and similar to grade 10.9 in larger fastener sizes. When we specify stainless we always use more fasteners than we necessarily would with high tensile steel bolts (usually grade 12.9) and obviously use a lower fastening torque than we would for high tensile steel bolts.

If you do use stainless steel fasteners then be aware that they are prone to stress corrosion and can fail spectacularly and without warning as the cracks are very difficult to spot in advance. Grade A2 stainless (also known as 304 stainless), which is what the majority of Ebay supplied fasteners are made from, is particularly susceptible to stress corrosion (also known as crevice corrosion). Grade A4 (aka 316 stainless) is much better in this respect but use grade 316L, if you can get it. Stress corrosion occurs in the presence of an electrolyte (even humid air) and does not even require the presence of dissimilar metals to set up the corrosion path.

Then there's the problem of galvanic corrosion, which will usually be exacerbated using stainless steel, even over un-plated carbon steel fasteners. Here, I am talking about corrosion of the surrounding materials, not the fasteners themselves. Zinc is your best friend when it comes to this situation so good quality fasteners that have been zinc plated and passivated to a good standard are to be preferred.

Galling is indeed a major problem and I have seen plenty of stainless steel nuts and bolts just lock together, even when simply running nuts down the bolts. Once this happens you will never get them apart without damage and replacement.

Finally, you do NOT want to use ductile fasteners, this is a ridiculous suggestion. Nuts and bolts are used to make a joint. Their purpose is to provide the clamping force to keep the joint together under all operating conditions. This is why fastening torques are specified. They ensure that each bolt is providing an equal and known clamping force, the sum of which is the figure required to maintain the joint under all conditions. The only way a bolt can bend or shear is if the joint has already failed (possibly due to incorrect assembly torques or inferior quality fasteners). Please do not think that you are doing your car a favour by fitting low tensile strength fasteners, quite the opposite is more likely the truth. Mild steel is great for body panels and clamps and brackets but, please, not fasteners.

Buy good quality fasteners that have been manufactured and plated/finished to correctly quoted (British) Standards and you won't go far wrong.

[Goss]

Couldn't agree with you more, it's usually accident investigation engineers that find the misuse of fasteners and then its too late for the owner, occupants and other victims. I always use fasteners from a reputable source and to the manufacturers specification because that insurance question "has the vehicle been modified" would have to be a yes!

Still not found the banging so plan on removing the rear part of the exhaust for inspection as its joint has a small leak.

Regards

Goss