Oil-canning door sheet metal
I'm restoring a '70 Datsun 240Z. The passenger's door has previous damage that was repaired, but the metal is evidently stretched to the limit for holding its shape. When I push on the metal in the center of the door, it oil-cans in and stays. I can pop it back out easily, but the skin is really bouncy. Is this fixable by using a shrinking hammer and working the metal to tighten the skin? It is a rust-free door, so I hate to discard it if it can be repaired. Thanks.
Terry that door needs to be heat shrunk, either by torch, hammer and dollie,or a stud gun heat shrink att. .if you try to use just the shrinking hammer to try to gather the metal you will still be left with the excess metal in the form of that high area that keeps popping in and out when you push on it.
Shrinking can be done is one of several ways but first you should try to repair the problem without "shrinking". Many times the face of a panel can loose it's stiffness because it gets dented and the dent pulls IN or pushes out the surrounding metal. On a door you may have pushed the top of the door up or the bottom of the door down causing the center to pop in and out easily. If you can push the metal out then drive the bottom up or the top down it may make the sheet metal rigid enough to keep it's shape. I usually do this type of "persuasion" using a block or board and a heavy hammer but I'm careful not to do further damage by restraining myself from hitting the board too hard.
If the above hammering doesn't work then you'll probably need to shrink the metal. A "shrinking hammer" doesn't really shrink anything, it has a textured face so that when you hit the metal it has less tendency to "pinch" it and cause it to thin and stretch. The old method of shrinking was done by heating the metal until you had a blister which was tapped down onto a dolly which would drive the softer hot metal together then it was quenched with water to stabilize the shrinkage. On newer "high strength steel" this type of shrinkage quickly causes the metal to become brittle so it's not usually the best fix for the problem.
There is also the "shrink disk" which is an inexpensive method but still takes water to stabilize the metal. I've never used a shrink disk so I can't comment on how well it works but there are a few here that like that method.
I'm a big fan of using a shrink tip on a stud welder. On a piece of clean metal you can shrink it quite easily and quickly by holding the shrink tip against the metal and zapping to heat a small spot, hold the gun in place for a couple seconds then move an inch or two away and zap it again. I do this several times in about a minute or two and it usually takes care of any shrinking I need done.
If you're straightening metal you'll find that a stud welder is one of the best tools you can have at your disposal. This tool can pull out most dents quickly and easily without damaging the metal as well as do your shrinking quickly without the need of a torch or water.
If you find that you're interested in one of these tools you're in luck because they JUST went down in price AND they have been further reduced and placed in the Specials Page on this site. This special will run through the end of this month or until they are no longer available.
Last edited by Len; 12-05-2006 at 06:34 PM.
Len posted good information...here is a slightly different take on it... something I wrote a while back (edited) :
What helped me work with oil canned panels is to address the perimeter of the panel first, and work your way into the middle. Think about the tension in the plane of the metal, and the smoothness of the surface working together. You must make the panel smooth to read it. If this means making it have a little too much crown in it, so be it. You can easily shrink it back down. The crown a panel has puts just the right amount of tension in it to hold it in place without being so floppy as to pop in and out. Try pushing in on various spots around the perimeter of the area with your thumb while popping the oil can in and out. Once you find a spot that helps stop the oil can popping, check this small area for smoothness. You may need to hammer and dolly to get it back to a smooth but slightly high condition, then shrink to get the tension just right. There may be several areas that need this attention. It is easy to overlook a spot that is affecting the panel and spend too much time playing with the oil can itself. You can also try pushing out instead of in at various spots on the panel to see the effect on the oil can. If you are going to use a torch to shrink, try heating the metal to the point right before it turns blue instead of heating to red. It will shrink with very little or no hammering, and stay a bit more workable than metal that is heated to red. A shrinking disc is the best way (my opinion) to shrink an over-stretched panel because it heats just the high spots without over-heating them. You can stretch with the hammer and dolly and shrink with the disc many times without damage to your panel until you get it right. This allows you to free yourself from worrying about over use of the hammer and dolly which can keep you from getting the job done.
Oil cans can be very tricky. I recently spent about 7-8 hours fixing one on a quarter panel that I had flared. All the stretching of the fender had resulted in a different pull and tension in the panel. I ended up shrinking metal that had not been previously stretched to remove a large bow that inhibited the reverse I wanted, stretching various areas around the oil can, and pounding from inside the quarter panel against the inside of the door opening flange just a little for tension in the plane of the metal. The bow actually shrunk down 3/16" measured in the middle of a 20" verticle template of the curve before I started. I learned a few things on this panel that made it well worth the effort. Hope this makes some sense!
Hey, thanks for the good info, guys. This will give me some options to try. I'll get back to you if I run into any problems or need further advice.
Len, I shrink the same way, but wasnt aware of a seperate shrinking tip. I use the same tip to weld on the studs. Can you get just the shrinking tip, and how much more effective is it than just the tip for the studs. I do agree with John as well, the surrounding high spots are what I address first, and sometimes eliminates the need to shrink, or at least lessens the shrinking needed.
Originally Posted by Len