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Thread: fixing an oil canned hood

  1. #1
    Registered Member WagonCrazy's Avatar
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    fixing an oil canned hood

    Here's the hood i'm starting with (for my 57 Nomad). It's been thru a carb fire at one point, and has oil canning in the center.


    Been watching youtube videos on how to fix oil canning, by basically heating with a torch, then rapid cooling. Seems to bring back stiffness.

    Any clues from you guys before I start this process?
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

  2. #2
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    A couple of comments while we wait for the master, MP&C, to step in.

    1. You don't need rapid cooling to shrink with a torch, the heating only will shrink a stretched area. The only effect that rapid cooling will have is it will let you see your results faster.

    2. That said, I don't think I'd be doing this one with a torch. It heats too big a spot at a time, and it's imprecise. I think I'd attack it with a shrink attachment on a stud gun. I'd draw a grid of where I'd apply the shrink attachment, maybe on a 2" grid.

    That said, I'll stand aside for some thoughts and comments and hopefully Robert will chime in soon.

  3. #3
    Registered Member Custer55's Avatar
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    If the oil canning was caused by the carb fire that spot on the hood may have shrinkage already. Maybe just needs some hammer and dolly work.
    Robert would be the one who should know what to do with it.

  4. #4
    Registered Member MP&C's Avatar
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    First, let's add the reference material here so you can have it in one place...


    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 spring-back, 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 be aware that there may be more than one sweet spot needing attention.

    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.


    Now, with that done, the lack of paint suggests where the damage occurred, the heat from a fire would have shrunk the metal in the area of the surface rust. PAY NO ATTENTION TO THE OIL CAN IN THE FRONT!!!! That is secondary to a shrink at the rear pulling low and pulling that down as well. When you get the back fixed, it should fix the front.

    IN your video, the initial push you did showed an oil can affected the surface rust area and nothing else. Try to focus on that, see if you can cycle that from the center of the damage where you should be able to see the outer perimeter of the oil can... the part of the sheet metal that cycles inward and then outward. Although what I'm going to suggest is normally used on stretched oil cans, try it to see if it points to an area to address first. This means it may or may not work. If it doesn't report back.

    Look at the instruction under "tight oil can part 2" above. In it you find the outer perimeter of the oil can and apply thumb pressure on a spot (any spot) on that outer perimeter. (perimeter does not mean it has to be round) With pressure applied, cycle the oil can again. Move pressure point, cycle again. Repeat until you have gone all the way around the perimeter. You are looking for any and ALL areas where thumb pressure either locks the oil can from cycling or decreases it's amplitude. If you find any such locks, mark them and continue around the entire perimeter. IF this method works and does show any such spots, I would start by planishing the areas that completely locked the cycling first, and any that decreased amplitude next. I would suggest that the locks would need more planishing than the "limits". Also, don't think that the first round of ANYTHING in addressing oil cans is an immediate fix. You may go through many evolutions of this to get things back where they should be.

    Disclaimer: Don't get overzealous and beat the shit out of the hood, even though it may look bad now, it doesn't take that much planishing to overcome the effects of shrinking. too much in one place and you'll have a nice pucker sticking up... Also, some profile templates (side to side, front to back) from a good hood would help to read what you have, and let you see sooner that perhaps you're using too much force.. The video is your friend, take more as you go...
    Last edited by MP&C; 04-01-2020 at 01:57 PM.
    Robert



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  5. #5
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    Excellent point on the fact that the original damage shrunk the metal, didn't stretch it.

  6. #6
    Registered Member WagonCrazy's Avatar
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    This afternoon I removed the paint from the front.
    IMG_5861.jpg

    And based on the amount of rust on the back, I'm considering cutting out all the underbracing (removing spot welds) so that I can media blast everything and put a coat of epoxy paint on it.
    IMG_5862.jpg

    But before that, I'll do another video of the oil can spot so you can confirm that I have a loose oil can situation here. Thanks Robert.
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

  7. #7
    Registered Member MP&C's Avatar
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    That bottom view shows where most of your heat took place, and where most of the shrinking occurred. I would almost work the oil can before media blasting so you don't lose track of exactly where the damage occurred. Removing the brace will definitely give you more room to work.
    Robert



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  8. #8
    Registered Member WagonCrazy's Avatar
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    Today I managed to get the inner brace removed (spot welds drilled) and started to media blast the brace. It's slow going with a little pot blaster outside. Have to keep scooping up the media, sifting it, and reloading it. Its just so friggin' s_l_o_w....

    IMG_5863.jpg


    Here is the backside of the brace (sits against the inner hood). Think it needs a little clean up and epoxy primer?

    IMG_5864.jpg

    Tomorrow I hope to finish the media blasting and then go after that hood oil-can with some heat...

    Robert, should I heat the bottom side? (right where the previous carb fire occured) OR heat from the top side?

    The hood only flexes when I push down on it (from the top), and oil cans right back into position when I release the pressure. There are no obvious dents or creases in the hood. It's just weak right there in the middle.
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

  9. #9
    Registered Member MP&C's Avatar
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    Heat is what caused the problem to begin with. Not sure what you're reading that tells you to heat it more, but believe me, you have quite enough shrinking that has taken place, you don't need more...
    Robert



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  10. #10
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    WagonCrazy, you need to read Robert's initial reply very carefully again. No heat yet.

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