View Full Version : choppped top issue
10-26-2004, 05:48 AM
Just chopped the top of my son's 55 F100. After tack welding in the two filler strips to lengthen and widen the roof I noticed the center has now lost it's crown, in fact it is now lower in the middle. Can someone tell me whether I have to shrink or stretch it to bring the center up. Any tips on the best way to re-crown the roof would be appreciated!
10-26-2004, 06:09 AM
You will need to stretch the metal where it is low, but first bump up the low areas to get them a little closer to where they belong if possible. To stretch metal that is in place, I use a ballpeen hammer and a shot dolly filled with sand. The sand absorbs the stretch from the hammer. For broad low crown areas that have to move a lot, you can use the hammer on top, and the shot dolly held firmly on the underside to stretch a little more easily than hammering from below, then bump the stretched area up with a dolly, and smooth with a hammer and dolly.
A faster way is to get a palm nailer for $100, a carriage bolt to fit in the palm nailer as a hammer head, and use that instead of a ballpeen hammer...very fast and noisy. You must spend a lot of time with a hammer and dolly smoothing out the lumps between stretching sessions to see where you are at. And finally, a shrinking disc to tune up the panel, and help you find minor low spots. A very important step before any of this is to lay a flat flexible strip of material like a ruler against the roof skin so you can see the low spots, and how the roof flows. I've done a lot of stretching using this method. It is time consuming and physically demanding, but rewarding. Do you have any pictures of the truck?
10-26-2004, 09:52 AM
Does this panel require stretching, as you stated, because the heat of welding has shrunk the metal and drawn it lower? I did hammer and dolly the spot welds as I went along. I assume this only reduced the amount shrinkage and the amount the metal was sucked down. Could I have done anything different to further reduce this lack of crown?
10-26-2004, 11:07 AM
Did your patch panel have correct crown etc prior to installation or was it a flat sheet?
10-26-2004, 07:17 PM
After tack welding in the two filler strips to lengthen and widen the roof I noticed the center has now lost it's crown, in fact it is now lower in the middle.
Why the two filler strips and where? :( Is this your first chop?
Check to see if it's "oil canning" or the weld-shrink pulled it down. If it oil cans, you need to shrink near the weld areas. If it doesn't, you need to stretch near the weld areas. If at all possible, DO NOT touch the roof crown itself. You'll open a whole new can of worms and make more work.
A pic would come in handy. I'm really confused about why and where you put the strips.
10-26-2004, 08:32 PM
Here's where I usually make my cuts on tops. You can use any combination of "a" and "b". If you keep as much of the original top complete, it's much easier work. It's easier to work or form the pillars than the top. You might consider laying the front of the hood back too. It will help blend in the layed back windshield.
After--The top stays uncut.
10-26-2004, 09:32 PM
Sorry I don't have a way to get you pictures. Let me try and describe what we did. The top was chopped 4". The roof section is narrower than body section by 2 1/2" front to back and 1 1/2" side to side. My son didn't want to to tilt the A pillar back so we had to cut the roof in quarters. We then tacked each quarter to its appropriate pillar with the center temporarily supported with the correct crown. We then cut and formed sheet metal (strips) to fill the front to back and side to side voids. These were tacked welded in working slowly and hammered / dollyed lightly when we noticed metal movement. Once it was all tacked in place the temporary support was removed and the center dropped below level by maybe 1/4".
SNOTZLOT - the filler strips were formed to fit the contours of each void and a slight gap was maintained so as not to induce any stresses into the roof.
CCWKEN - how do you check for oil-canning at the welds or did you mean does the overall roof panel oil -can? If I understand you correctly oil-canning of a panel is caused by stretched metal and requires shrinking to bring it back to shape. If the roof does not oil-can then you are saying that the welding has shrunk the metal and requires the welds to be stretched.
I really appreciate all your expertise as this is only my second chop job, but the first with this much expanse of metal.
10-27-2004, 05:49 AM
Remember don't try to shrink the oil can, shrink the areas causing the oil can.
10-27-2004, 07:29 AM
It sounds like you need to stretch for three reasons. The welds shrink, the inserts you put in need shape, and the edges of the roof sections that you quartered probably need to be stretched a little to flow into the insert pieces you made. If you put a flow strip across the panels before welding, I'm guessing you would have seen valleys between the pieces. For a flow strip I use a plastic strip with a magnet imbedded in it (part of a storm window kit), but you can use anything that is flat, stiff, and flexible like a machinists ruler. When you lay it flat on the surface and push down on the ends you can see the curve in the roof, and where it needs to be raised. Another somewhat unusual way of fixing this might be to reduce some of the shape in the areas adjacenct to the welds by shrinking the crowned areas down with a shrinking disc until your flow is good between the sections. This method would depend on how flat or crowned you want the roof to look. I've done this type of cheating before with good results.
10-27-2004, 05:58 PM
If I understand you correctly oil-canning of a panel is caused by stretched metal and requires shrinking to bring it back to shape. If the roof does not oil-can then you are saying that the welding has shrunk the metal and requires the welds to be stretched.
Yep, that's about it. Oil-canning can also be caused by displaced metal in the surrounding area, as in a dent. That's why it's important to NOT work the low spot or the canning area. You should work the area around it.
If you wanted to enlarge the top, I would have removed the top at the lead seams, set the pillars and door frame then made a new one-piece top to fit the hole. Tops are inherently under an edge compression. That is when the top is assembled, it's weight is suspended during welding/leading to the outer fixed edges. That's what helps keep the dome and give it strenth. One way to tell: If it has drip rails, it's probably a section roof and can be removed.
When you welded in the strips, the top should have been OVER supported. You're going to have to stretch your weld areas and/or new panel areas to get the dome to stay. Since the top was quartered, plan on some extra work. Start at the outer areas (all the way arround) and work in toward the center of the top. After you get the dome back in, you may have to shrink areas to fine tune the dome.
By the way, I never liked enlarging a chop-top. It will make the body look boxie. With the rounded front-end, stuff just won't look right. :(
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