Before someone injures themselves, there are a few things you should know.
1. The OEM crossmember bolt are not standard with respect to the length of
the threads. The threads are longer than the current standards, so you
would have to use washers or shorter bolts to avoid damaging the threads in
the frame tube nuts.
2. Graded bolts are heat treated to develop strength. They do not take
kindly to the addition of threads and the disruption of metal will weaken
the bolts actual strength and decrease it's fatigue limits.
3. If you must change bolts, your far better off turning the diameter of
the bolt to reduce it just below the minor diameter of the threads so that
it won't jam in the tube nuts. The best method to do this diameter
reduction is by grinding to maximize surface smoothness and minimize tool
marks which cause stress risers.
4. I've never heard of anyone failing a crossmember bolt other than by
physical damage due to rust or mechanical abuse. The weak point in the
system are the tube nuts buried in the frame. They most commonly fail due
to de-lamination of the frame at the lower spot welds. They can typically
be pulled back into their correct position and reinforced to better
distribute the applied loads. The de-lamination must also be addressed to
complete this repair.
5. If you haven't done the physical examination of your frame elements in
the last year, and haven't already done the proper reinforcement, you are
asking for future problems. The unibody structure is 40 years
old. Structural fatigue is taking it's toll and it's only a matter of time
before it finds your Tiger.
6. Rob and I are hard at work on a web site and we will be putting up many
of the Tech Tips I have written over the years. I expect this will be on
line sometime this summer.
Tom Hall
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