[Mgs] 80 MGB - Cross member to Body bolt

[Max Heim]

Crazy, maybe...

The major load on those bolts is not compression, or stretch, as you infer
-- it is shear, and it occurs anytime the car changes direction, or brakes.
One way to look at it is that the tightness of the nuts is protecting the
bolts (or studs) from the shear forces (transferring them to the frame of
the vehicle). The nuts, as usual, are not directly subjected to the major
force, but their integrity is critical to the integrity of the assembly
(also as usual).

A parallel situation might be lug nuts (the studs being loaded chiefly in
shear). Of course, you don't think of them as being "graded" quality (are
they?), but I suppose they are to some extent redundant (multiple per
wheel).


--

Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires


on 7/19/07 3:26 PM, Matt Trebelhorn at matt.lists at trebelhorn.com wrote:

> While I'm not going to argue that weaker fasteners are necessarily
> better than a nice Grade 5, the load on these isn't ever going to be
> all that high.  I'll explain why I think that, and you can tell me if
> it sounds crazy.
> 
> The way I see it, those nuts carry a vertical load -- they hold the
> crossmember on when the car is jacked up, or if you pick the car up
> and hold it upside down by the crossmember, they hold the car up.
> Now, that second scenario is unlikely, so we're looking at other ways
> in which a vertical load is carried by the nuts and washers.
> 
> That boils down to loads put on the suspension by an anti-roll bar,
> or momentary loads that involve similar issues -- a pothole, say, or
> braking loads that would pull the crossmember rearwards -- some of
> this turns into a vertical load, pulling the crossmember down.
> 
> In both of these scenarios, though, the load is leveraged against the
> nuts, and the weak point in the system isn't the hardware, but the
> rubber mounting pads.  If you have an anti-roll bar that puts enough
> vertical load on the crossmember to break even a grade 2 fastener of
> that size, you would long ago have crushed the rubber pad, and
> probably broken some other suspension bits, too.  If you hit a
> pothole hard enough to pop those fasteners, you would also destroy
> the rubber pads first; at any rate, your suspension would be in bad
> shape.
> 
> I was in an accident a few years ago that pushed the front wheel back
> a tiny bit -- and the only suspension bit destroyed was the rubber
> pad on the hit side; when it came back from the bodyshop, the car
> drove like hell until that got replaced.
> 
> Yes, a really weak fastener in that application might have broken.
> But it would have been the difference between a broken suspension in
> a badly broken car and a badly broken suspension in a badly broken car.
> 
> So that's my story -- that they're not loaded in such a way that you
> would ever put enough of a load on them to break the nuts without
> breaking lots of other things first.
> 
> Crazy or no?
> Matt
> 
> 
>> 
>> on 7/17/07 8:03 PM, WJHS1960 at WJHS1960 at comcast.net wrote:
>> 
>>> <<BTW,  if you get stainless locknuts and washers from
>>> the hardware store, they polish up nicely.....>>
>>> 
>>> You must have a Death Wish, Bob.  Just what I want on any
>>> automobile is Grade
>>> 2 fasteners.
>>> 
>>> N    O    T    !!!
>>> 
>>> Suicide comes to mind!!
>>> 
>>> ONLY Grade 5 fasteners are used on any/all LBCs!!!
>>> _______________________________________________