MP&C
09-10-2015, 10:42 AM
Get some PM's on occasion asking about fixing oil cans on sheet metal, so I thought I'd share some ideas....
I've seen on far too many automotive forums that any request for help dealing with an oil can invariably generates a response to pick up the shrinking disc, despite no detailed information given besides "I have an oil can"
Before I suggest to someone how to fix an oil can, it is best to find out what kind of oil can you are dealing with, first and foremost. I consider there are two kinds, a tight oil can and a loose oil can, each will require a different method of repair.
Tight oil can:
This is almost exclusively caused by body damage, whether a dent, glancing crease, or media blast damage, and is especially noted by displaced metal that will oil can when considerable pressure is applied, and may or may not forcibly spring back. When the body damage occurs, it stretches the panel throughout the dent or crease. A typical dent, whether straight in or a glancing blow, will have direct and indirect damage. The direct damage goes inward, stretching the panel as it goes. The indirect damage, is a much lesser amount of springback, compounded by the internal stretch pushing outward circumferentially, and you will see an outward bulge around the perimeter of the dent/damage. Although the initial inclination may be to shrink this outward bulge, for the most part this adjacent area is relatively damage free, it is mainly being spread outward by the stretch forcing outward. Shrinking the center damage will start the process of relieving the stresses pushing outward, relaxing some of the bulge surrounding the dent. After a bit of shrinking, using the shot bag against the outside of the crease/dent and some light taps with a flat body hammer or slapper from the inside will help to start manipulating the crease/dent back into it's original place. I would add that too much shrinking all at once may give you the loose oil can, so profile templates are highly recommended as they work well to let you see how the panel is reacting so you don't go too far too quick.
Tight oil can, part two:
Where some tight oil cans from dents may be challenging to determine where to start your shrinking (if it doesn't have an obvious sharp crease to show where to work from) the following process will normally find the area that needs shrinking.... Cycle the oil can in and out a couple times in order to find the outer perimeter. If it helps to mark it with some painters tape, a sharpie, so be it, use whatever works. Now using your thumb from one hand apply slight pressure on a point on this perimeter. Use the other hand to cycle the oil can again, using the same pressure as before. Keep moving your pressure point around the perimeter and cycle the oil can for each spot until you get to a point on the perimeter where the pressure will keep the oil can from cycling, it locks it from moving. This should identify your "sweet spot" that needs shrinking, and there may be more than one sweet spot
Loose oil can:
This is typically caused by welding, over-eager torch shrinking, or shrinking something when you should have stretched, (or fatigue over the many years that has caused a larger panel/hood to settle). Any panel will shrink from heat, causing the crown to draw in from the surrounding area. This is especially noted by a loose, easily flopped back and forth oil can. This is fixed by stretching, typically in the area of the weld and HAZ.
Loose oil can part two:
In some cases we'll see that a dent (or tight oil can) has actually caused a loose oil can in the outer reaches in the adjacent area. The direct force (dent) may have caused displacement of the inherent stresses of the panel (crown) such that it pulled at the adjacent metal elsewhere, resulting in a loose oil can outside the area of the dent. Here the loose oil can should be left alone and focus on removing the stretched area (dent) that moved the panel. Once the dent is removed, this action alone should correct the loose oil can in the adjacent area.
I've seen on far too many automotive forums that any request for help dealing with an oil can invariably generates a response to pick up the shrinking disc, despite no detailed information given besides "I have an oil can"
Before I suggest to someone how to fix an oil can, it is best to find out what kind of oil can you are dealing with, first and foremost. I consider there are two kinds, a tight oil can and a loose oil can, each will require a different method of repair.
Tight oil can:
This is almost exclusively caused by body damage, whether a dent, glancing crease, or media blast damage, and is especially noted by displaced metal that will oil can when considerable pressure is applied, and may or may not forcibly spring back. When the body damage occurs, it stretches the panel throughout the dent or crease. A typical dent, whether straight in or a glancing blow, will have direct and indirect damage. The direct damage goes inward, stretching the panel as it goes. The indirect damage, is a much lesser amount of springback, compounded by the internal stretch pushing outward circumferentially, and you will see an outward bulge around the perimeter of the dent/damage. Although the initial inclination may be to shrink this outward bulge, for the most part this adjacent area is relatively damage free, it is mainly being spread outward by the stretch forcing outward. Shrinking the center damage will start the process of relieving the stresses pushing outward, relaxing some of the bulge surrounding the dent. After a bit of shrinking, using the shot bag against the outside of the crease/dent and some light taps with a flat body hammer or slapper from the inside will help to start manipulating the crease/dent back into it's original place. I would add that too much shrinking all at once may give you the loose oil can, so profile templates are highly recommended as they work well to let you see how the panel is reacting so you don't go too far too quick.
Tight oil can, part two:
Where some tight oil cans from dents may be challenging to determine where to start your shrinking (if it doesn't have an obvious sharp crease to show where to work from) the following process will normally find the area that needs shrinking.... Cycle the oil can in and out a couple times in order to find the outer perimeter. If it helps to mark it with some painters tape, a sharpie, so be it, use whatever works. Now using your thumb from one hand apply slight pressure on a point on this perimeter. Use the other hand to cycle the oil can again, using the same pressure as before. Keep moving your pressure point around the perimeter and cycle the oil can for each spot until you get to a point on the perimeter where the pressure will keep the oil can from cycling, it locks it from moving. This should identify your "sweet spot" that needs shrinking, and there may be more than one sweet spot
Loose oil can:
This is typically caused by welding, over-eager torch shrinking, or shrinking something when you should have stretched, (or fatigue over the many years that has caused a larger panel/hood to settle). Any panel will shrink from heat, causing the crown to draw in from the surrounding area. This is especially noted by a loose, easily flopped back and forth oil can. This is fixed by stretching, typically in the area of the weld and HAZ.
Loose oil can part two:
In some cases we'll see that a dent (or tight oil can) has actually caused a loose oil can in the outer reaches in the adjacent area. The direct force (dent) may have caused displacement of the inherent stresses of the panel (crown) such that it pulled at the adjacent metal elsewhere, resulting in a loose oil can outside the area of the dent. Here the loose oil can should be left alone and focus on removing the stretched area (dent) that moved the panel. Once the dent is removed, this action alone should correct the loose oil can in the adjacent area.