Join Date: Oct 2013
Location: Chilliwack, BC, Canada
Too much tension will break fasteners. Under tightening where there are reciprocating loads (pulling and releasing) eventually causes fatigue and subsequent breakage.
If the fastener breaks despite being correctly tightened, a option is to replace with a higher strength one. Specialized, special configuration fasteners are difficult to assess in terms of the strength grade category in which they belong however one may be able to choose another fastener which will substitute with the addition of a washer, or change in other drive type (i.e. Allen versus hex head).
What typically happens is that someone grabs a substitute without consideration for the strength of the replacement. A frequent issue is the use of stainless fasteners which have acquired the mystique of superiority despite that they are never higher strength grade. Most are in the junk category in terms of strength.
If the fastener is breaking rather than the threads being pulled from the mounting, there is obviously room for a higher strength fastener.
A frequent issue in addition to the use of insufficient strength fasteners and under tightening, is the use of low grade washers. This is almost completely unappreciated in the repair field but if one considers subsituting a nylon washer under a connecting rod nut, one will immediately see that the nylon will sqeeze out under the clamping/tightening. What is generally unappreciated is that the squeezing out is a process. The washer will continue to yield over time such that the fastener will become loosened even if was sufficiently tightened initially.
Increasing the fastener diameter is another often ignored option. As 19willys51 mentioned, a small increase from 6 mm to 5/16" provides an exponential increase in fastener strength as well as increasing the anchor strength of the mounting threads.
I cannot recall the sizing of the rack holes sufficiently to know whether 8 mm might also fit but seem to recall that there may be a sizing issue so that 5/16" is the best choice? Perhaps someone whose memory works can comment?
If one has not done so, compare tightening torque for 5/16" Grade 5 inch capscrew with 6 mm Grade 8.8 ISO metric. Also compare 5/16" and 8 mm. An even better excercise is to calculate the cross sectional area of the fasteners. Simply comparing the torques will show clearly why 19willys51's selection of 5/16" is so much stronger than the 6 mm.
A caution: it is generally best practice to avoid mixing fastener thread types such as installing inch sized fasterners onto a metric application or vice versa as subsequent service may not recognize the substitute. I've seen many problems which arose from the use of some inch fasteners into a metric engine. The person doing the work grabbed the wrong fastener and cracked the case. I happens....
General Motors vehicle of years past were really "interesting" because they used both inch and metric. As an added bonus, instead of using standard metric they created their own metric by adapting sort of standard size inch fasteners by threading to a sort of metric thread. Really "interesting".
The rack is not likely to be so affected by mixing up of fasteners but as a general principle, professionals avoid the practice. If one does decide to resize to another thread type, it is best to do them all. Nothing worse than fasteners of similar size with a slightly different thread.
Rats are not the only creature so affected, IMO, 19willys51. LMAO There is a huge depth of wisdom in that signature line.