71-74 B-body door cracks

71Runner · Post by **71Runner** » Wed Jul 23, 2008 9:12 am

My wifes '71 RR has devoloped the cracks in the doors that are somewhat common. These cracks begin at the top of the door, in front of the side view mirror, and run vertically for usually under an inch. After making the repair to the metal, how is this condition best prevented? Should I replace the door hinges, and if so, who has them? Thanks.

moparmodeler · Post by **moparmodeler** » Wed Jul 23, 2008 9:32 am

while repairing the crack drill small holes at both ends of the crack to stop the crack from traveling. then do the body work. This trick is done on aircraft all the time.

72RoadRunnerGTX · Post by **72RoadRunnerGTX** » Wed Jul 23, 2008 12:21 pm

In my humble opinion on this matter, the cracks form from movement created by the weight of the mirrors and the fact that the outer skin is not secured to the shell aft of where the cracks form. Grab the mirror and move it around you will see how the crack forms. Of the six doors I have, all but one have/had the cracks, and that pass side door didn't have a mirror. Some of the doors showed signs of a past attempt; either by the factory or some time thereafter, to tack weld the skin to shell at the upper most point but didn't take, skin cracked. Completely welding the skin to the shell at the time the cracks are repaired should solve the problem.

rr6pak · Post by **rr6pak** » Wed Jul 23, 2008 12:39 pm

moparmodeler wrote:while repairing the crack drill small holes at both ends of the crack to stop the crack from traveling. then do the body work. This trick is done on aircraft all the time.

Do it ALL the time

Smellslike1974 · Post by **Smellslike1974** » Wed Jul 23, 2008 12:41 pm

*goes and checks his door*

71Runner · Post by **71Runner** » Wed Jul 23, 2008 1:25 pm

Guys, thanks for the info on drilling the crack to prevent its spread.

72RoadRunnerGTX, thanks for taking the time to post the photo. Your explaination makes much more sence. I never thought the mirror exerted that kind of pressure on the door skin, but the way you explained it makes it much more likely than a door hinge. Since you have welded the area indicated, I guess the problem has the not resurfaced, is that right? Thanks again.

72RoadRunnerGTX · Post by **72RoadRunnerGTX** » Wed Jul 23, 2008 2:13 pm

The 73-74' doors are slightly deferent in this area, not sure if this applies to them. Do they have cracking problems there as well? Drilling holes at the end of cracks is performed to stop a crack from expanding past the drilling point. Does little if anything to address the stress loads that cause the cracking in the first place.

Not so much how much what the mirror weights as it is the number of times the doors are opened and closed in what 37+years now. Any stress no matter how small over time will find the weakest point and crack.

I'm afraid I can't provide much feedback on the fix as of yet, I only have about 150 miles on the car now. It will be some time before I have racked up some door open/close cycles on it. I am confident my problem is resolved.

Update; about 4 years and almost 6k miles on the car now with no sign of cracks.

71Runner · Post by **71Runner** » Wed Jul 23, 2008 2:45 pm

We'll you've got me convinced. I made an appt w/my body guy for the week of Aug 4th. He will address it then with the aid of your photo. I would have expected the d/s door to go 1st as it's opened alot more than the p/s. I'll have him do the welds on the d/s to prevent the crack from appearing on that door also. Thanks again.

73sunroofSSP · Post by **73sunroofSSP** » Wed Jul 23, 2008 2:58 pm

thanks guys you just helped me diagnose what cuased the crack on my pass door.

72RoadRunnerGTX · Post by **72RoadRunnerGTX** » Wed Jul 23, 2008 3:09 pm

One or two of my doors looked as though there may have been an attempt by the factory to tack weld higher up on the skin but the welds were of poor quality and let go. You could find your driver's door may have a weld there that may be holding up.

AZ-Nick · Post by **AZ-Nick** » Wed Jul 23, 2008 9:22 pm

wow, I have never seen this before or never bothered to look.
Thank you for the time to explain what and why and how to fix it.....

mopar71 · Post by **mopar71** » Thu Jul 24, 2008 12:17 pm

Thanks guys! good info,

Post by **Eric** » Thu Jul 24, 2008 8:08 pm

Every 71B I've owned has had this problem! I think it's more than just the mirrors though. My B7 car has only the drivers non rallye mirror and none on the pass. side.....and the crack is worse on the pass side than the drivers!

72RoadRunnerGTX · Post by **72RoadRunnerGTX** » Thu Jul 24, 2008 11:49 pm

Any movement at the top edge of the door skin would create stress at the point of the cracks. That would include the windows up/down cycles, over all door/body flex, and door open/close cycles. Having a mirror mounted so close to the stress point would only amplify this movement. A racing type mirror, I would assume, has more mass than say a standard mirror and would accelerate a problem like this. The appearance of these cracks, in my mind, is mainly caused by the lack of proper welds securing the door skin completely. The presence of mirrors would be a primary contributing factor. I'm sure there are plenty of no mirror doors with cracks, but all things being equal, I'll bet the ones with mirrors got them sooner.

Many years ago I had an old girlfriend that figured out the easiest way to really piss me off was to slam the door of my 70' RR as hard as she could. All I could think about at the time was all that unnecessary stress on my sheet metal. I had fewer things to worry about back then.

A closer look at one of my untouched later production 72' doors shows the tab that would normally be tack welded but isn't. This door did have a racing mirror; however the crack has just started at the weak point. The nearest secure point is at the front edge of the door. Movement at top of the door skin is not focused as much at the weak point with the tab allowed to move, I believe that is why this door isn't cracked an inch down the outside of this door skin. Maybe they figured out if they omitted all the welds here the doors would last longer without cracks.