Machspeed 219 Report post Posted January 22, 2017 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. 12 ncdruroSor, Brianguple, lapedVem and 9 others reacted to this Quote Share this post Link to post Share on other sites
craigm 25 Report post Posted January 22, 2017 Looks like fatigue cracks but they are in a strange place. Quote Share this post Link to post Share on other sites
RPM 1,190 Report post Posted January 22, 2017 Strange location indeed. Usually they crack in the upper shock hole when past owners installed air shocks. Quote Share this post Link to post Share on other sites
Machspeed 219 Report post Posted January 22, 2017 Had air shocks, Bob. They are still on it. Surely they are not the cause for this? Quote Share this post Link to post Share on other sites
SWPruett 83 Report post Posted January 22, 2017 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, Quote Share this post Link to post Share on other sites
Mike65 476 Report post Posted January 22, 2017 I am on my 5th Mustang & all of them have had the cracks by the rear shock bolt access hole like the 1st pic. Quote Share this post Link to post Share on other sites
fvike 173 Report post Posted January 22, 2017 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 ? Quote Share this post Link to post Share on other sites
1969vert 55 Report post Posted January 22, 2017 The lower crack...Did someone attach a muffler hanger to the rear brake hose support? Quote Share this post Link to post Share on other sites
Machspeed 219 Report post Posted January 24, 2017 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 1 SWPruett reacted to this Quote Share this post Link to post Share on other sites
1969_Mach1 333 Report post Posted January 24, 2017 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. Quote Share this post Link to post Share on other sites
Ridge Runner 1,113 Report post Posted January 24, 2017 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 . 1 SWPruett reacted to this Quote Share this post Link to post Share on other sites
SWPruett 83 Report post Posted January 25, 2017 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, Quote Share this post Link to post Share on other sites
stangnet33 88 Report post Posted January 25, 2017 One of mine,has air shocks but didn't put very many miles on after they were put on. Quote Share this post Link to post Share on other sites
ray1970 88 Report post Posted January 29, 2017 Air shocks do that... mine went right threw the floor...... Quote Share this post Link to post Share on other sites