What Would Cause This?

[Machspeed 219]

Gutting out the interior of my car today and noted cracks in the area pictured below. The car has been in no major accidents and I just can't account for why this would be like this. What would cause this?  BTW, the access for both rear shocks it cracked like this. 

 

 

[craigm 25]

Looks like fatigue cracks but they are in a strange place.

[RPM 1,190]

Strange location indeed. Usually they crack in the upper shock hole when past owners installed air shocks.

[Machspeed 219]

Had air shocks, Bob. They are still on it. Surely they are not the cause for this?   

[SWPruett 83]

Machspeed,

 

Not that unusual with air shocks actually.  The spring rate and substantially increased damping rate transfers A LOT more energy into the chassis at the upper shock mounting points.  Just a guess, but has she seen her fair share of hard launches in her time?

 

Also, the cracks in the vertical section of the rear transition pans are also not entirely rare.  Most times, this is often seen with car that have had exhaust work done that is not very compliant with temperature.  As the exhaust grows with heat, it oil-cans this surface and cracks around the mount and the rear seatbelt mount over time.  I have seen some very late 70's that had doubling plates on this surface to help prevent this.

 

Its a fair bit of work to properly end-drill, relieve and weld these up, but obviously something that has to be done.  While you're at it, look closely at the front shock towers too.  That's another very common place for fatigue cracks.

 

Best,

[Mike65 475]

I am on my 5th Mustang & all of them have had the cracks by the rear shock bolt access hole like the 1st pic.

[fvike 173]

My car had those cracks around the rear shocks too. Welded it. My friend's '70 Mach 1 also has just the slightest beginning of a crack on one of the holes, but not like mine was. That '70 car actually has a factory rear swaybar. Perhaps that have saved the body a bit ?

 

[1969vert 55]

The lower crack...Did someone attach a muffler hanger to the rear brake hose support?

[Machspeed 219]

Machspeed,

 

Not that unusual with air shocks actually.  The spring rate and substantially increased damping rate transfers A LOT more energy into the chassis at the upper shock mounting points.  Just a guess, but has she seen her fair share of hard launches in her time?

 

Also, the cracks in the vertical section of the rear transition pans are also not entirely rare.  Most times, this is often seen with car that have had exhaust work done that is not very compliant with temperature.  As the exhaust grows with heat, it oil-cans this surface and cracks around the mount and the rear seatbelt mount over time.  I have seen some very late 70's that had doubling plates on this surface to help prevent this.

 

Its a fair bit of work to properly end-drill, relieve and weld these up, but obviously something that has to be done.  While you're at it, look closely at the front shock towers too.  That's another very common place for fatigue cracks.

 

Best,

SW, thank you for the input. Regarding your comment on the doubling plates for reinforcement, you wouldn't have pics would you? And, was that a factory thing?  

 

Have looked over my shock towers and they are in very good condition.....No cracks there!

 

John  

[1969_Mach1 333]

Deleted my original post.

 

Simply, these are probably high stress areas when the body twists and flexes and the air shock further increased those stresses.  Thus, fatigue failures occurred sooner than would normally have or not at all.

 

Drill small holes at the ends of all the cracks, then fully weld them.  Hopefully, the metal in those areas has not become brittle and cracks won't start elswhere.  I also see cracks around the seat belt hole in the lower picture.  After the repairs, consider adding sub frame connectors to help the chassis.

[Ridge Runner 1,113]

Gutting out the interior of my car today and noted cracks in the area pictured below. The car has been in no major accidents and I just can't account for why this would be like this. What would cause this?  BTW, the access for both rear shocks it cracked like this. 

 

 

This looks like the brake line bracket .If the air shocks were aired up to high the brake line may have been pulling tight and caused this damage .Air shocks and coil over shocks will cause these cracks because they remove the weight that is normal on the springs and transfers it to the shock bracket causing the bracket to flex .I have seen air shocks punch through the shock bracket .

[SWPruett 83]

SW, thank you for the input. Regarding your comment on the doubling plates for reinforcement, you wouldn't have pics would you? And, was that a factory thing?  

 

Have looked over my shock towers and they are in very good condition.....No cracks there!

 

John  

John,

 

I wish I had taken pictures back then, but I never thought much of it.  It was probably 30 years ago when I saw them.  What I DO remember of them is that they were on a '70 Shelby GT500 and looked pretty rough.  Normally, I would doubt they were factory, but the car was a very nice original with a factory exhaust, so I kinda left with the thought they might be the real deal.  Never seen another car like it and I've probably looked under hundreds (literally) of 69-70 cars since.

 

Great news on the shock towers!  Keep us posted as you go after it!

 

Regards,

[stangnet33 88]

One of mine,has air shocks but didn't put very many miles on after they were put on.

 

[ray1970 88]

Air shocks do that... mine went right threw the floor......