Technical TECH WEEK: Chopping a Pre-War GM Coupe

A while back I took my ’41 Buick Super to @Cody Walls shop, Traditional Metalcraft, and we spent the week chopping the top and taking a bunch of photos. We ran a short, one-page story on it in issue #82 of The Rodder’s Journal, but I’d always intended to post a step-by-step here on the HAMB, so here it is. I think a lot of this can be applied to many of the turret-top GM coupes from about 1939 to just after the war (pre-’39 GMs have a similar roofline, but the tulip/catwalk area doesn’t have the defined bodyline wrapping below the back window that the ’39-and-up cars have, so laying down that backlight and dealing with the sail panels would be a little different).

Some of these notes are very basic “Customizing 101” types of things, but I’m putting them in here anyway. Twenty years ago when I first found the HAMB I learned an awful lot digging through threads like Cleatus’ “A Fool Chops at Home” (still one of my favorites). I always liked that you could find everything from the most basic info to the most advanced right here, so hopefully this adds to that archive.

PREPARATION, PLANNING & MEASURING

I removed all the trim, glass, window tracks, vent windows, etc. in my shop before taking the car to Cody’s. That stuff always takes longer than you think—especially when you’re wrestling with 80-year-old window gaskets and trying not to break the back glass. With that out of the way, we stripped the primer off the areas around where we’d be cutting.

I studied a bunch of different chops on early-’40s cars while figuring out how I was going to chop mine. I really liked the proportions of Jake Christmas’ Buick from Texas. He was kind enough to send me some measurements off of his car—which had 3” taken out of the pillars—so we knew we’d be cutting at least that much.




Cut lines were marked using a 3” wide cardboard template that we could wrap around the A and B-pillars. Cody uses a ballpoint pen to make his marks (in these photos I went over them digitally in red to make them easier to see). I suppose this could be done with Dykem and a scribe, but pen is quick and easy and gives you an accurate line. Just don’t use tape because it will burn back as you cut and then all that measuring you did is out the window.

The “vertical” lines on the pillars provide a centerline that will help with alignment when everything’s coming back together. It goes without saying that those vertical lines should extend above and below your horizontal cut lines so they’re visible once the pillars are chopped.


Here’s one last look at the un-chopped profile. I didn't take pictures of the bracing, but the car was braced with 1” square steel tube welded inside the cabin in an X shape below the window line—driver’s side A-pillar to passenger’s side B-pillar and vice versa. That keeps the body from getting out of shape when the structure normally provided by the roof is removed. Once that’s done, it’s time to cut your perfectly good car to pieces.

THE CHOP

The first cuts should be at the top cut line. That way you can chop the pillars while they’re still attached to the car (it’s difficult to cut the part of the pillar that’s attached to the roof after it’s separated from the body because the roof becomes so flimsy). A 4-1/2” metal cut-off wheel was used to make most of the cuts. When you're cutting through those pillars, the inner structure can grab the wheel, which can make a standard wheel explode. To avoid shrapnel, spring for a few of the diamond blade metal wheels. They’re about $12 at Home Depot and will last a whole lot longer than the standard discs.


Here Cody’s cutting out the inner quarter window area, which will be chopped and replaced later on. These inner supports help stiffen things up and they also provide things like mounting points for the headliner bows.


The proportions of the quarter windows kind of drive the rest of the profile, and since I didn’t want to alter the angle of the B-pillar or shrink the length of the windows too much we cut out the top halves and left the bottoms intact. That way we could chop the quarter windows first, then do the rest of the roof around them.


The center windshield support has mounting points for the stainless trim that separates the glass, so we were careful to leave those intact.


Last cut, then the roof was free.

 

With the roof removed you can see the bracing inside the car.


We set the roof back in place and I really liked the 3” chop. I thought the windshield height and the profile looked good, so we didn’t cut any more. Sometimes you have to make relief cuts in the roof just inboard of the A-pillars so that you can stretch the tops outward to meet the bottoms of the pillars, but at 3” that was not necessary here.

 

QUARTER WINDOWS

Quarter windows can make or break a chop, so you’ve got to get them right. Corner #1 was going to stay the same because I didn’t want to change the B-pillar angle, and corner #2 stayed the same because I wanted to maintain the smooth visual arc from the top of the side windows rearward through the quarter windows. With the quarter window frame clamped in place, you can see that corner #3 will need tweaking in order to get everything lined up.

To bring everything in line, the upper radius of the window opening (the section between corners #2 and #3) had to be tightened up. That window frame is made up of two pieces of 18-gauge steel pinch-welded together. Cody’s got this killer ’40s-era shrinker and stretcher set that made quick work of the double-wall 18-gauge. If you don’t have access to equipment like this, you could probably achieve the same result by either cutting the inner and outer structure apart and shrinking them separately on a smaller home shrinker, or by making a series of very small relief/pie-cuts around the radius and welding them up.

This is after the first couple of passes through the shrinker. With the tighter upper radius it comes close to aligning with corner #3, but it’s not there yet. We needed to shrink a little more, which we realized would then require that we shorten the length of the window a little bit to get everything to line up.


Here we’ve done two things…first, the upper radius is tighter. Second, we cut out corner #3, removed a small horizontal section from the bottom of the window frame, and slid the corner forward. In these photos, it looks like there is a kink right at the top of the corner, but there isn’t…there are just a bunch of weld seams and grinder marks that make it look a little funky.


Two kooks and a half-chopped top. With the stock driver’s window frame, you can see how much we removed from the height of the car. It’s not a super heavy chop, but I like the proportions.

 

#dreamteam

Looking good!

 

THE LOWERED LID

The cut edges of the pillars were cleaned up to make the seams as tight as possible.

You can see how Cody likes to cut out corners with a radius rather than a right angle. This prevents the corners from getting sucked in from the heat when you’re welding things back together.


Here’s the roof clamped back in place. If you squint you can see things taking shape. We left the back window surround attached throughout the chop to maintain alignment.


With everything looking good, the A and B-pillars were welded up. This is where there centerlines on the pillars are helpful.

PATCHES AND PANELS
Next we chopped the tops of the window frames off (again at the top cut line) and set them aside for later.


When the roof came down, it also slid forward, leaving a gap in the drip rail and the roof itself.


The patches needed for the gap in the roof could be fabricated, but it’s a lot easier if you’ve got spare parts from the scrapyard. @FLAT-TOP BOB at Owens’ Salvage in Texas had a ’41 Buick fordor in his yard and was able to sell me 8” sections cut out of the roof on both sides, and corresponding sections from the tops of the window frames and garnish moldings. I didn’t take photos before we cut them apart, but here’s what was left after we cut the patches we needed.

 

Here’s one of the patches clamped in place. The cool thing about using donor pieces is that you get the roof, drip rail and inner doorjamb section all in one piece.


Although we left the sail panels in place when we cut the roof off, they will need to be replaced to fit the new contours of the roof. Heavy construction paper makes a good, flexible template. The other thing you can see in the first photo above is that we’ve cut along the bodyline below the backlight, leaving only the center couple of inches intact. This allowed the window to lean forward and show us where the rear profile of the roof needed to be. In the second photo you can see that the crown of the roof was still a little high here, but that was adjusted before the sail panels were finished.


Sail panels are tricky because they’re one big compound curve, and everyone’s got their own way of doing them. Cody uses a combination of a Pullmax, an English wheel and a planishing hammer. Here he’s using the Pullmax with a shrinking die to form the basic contour in the 18-gauge steel.


Next, several passes through the English wheel help to further shape and smooth the panel.


After a dozen trips back and forth between the English wheel and the car, the sail panel was ready. Clecos held it in place so that the shape could be transferred to the original sail panels as cut lines.

Then we cut a big ass hole in the roof.


After tacking the sail panel in place, we looked at it from a bunch of different angles to make sure it was right before it was fully welded.

 

Next we cut a long, thin 18-gauge piece to bridge the gap from the drip rail to the rest of the roof. One trick I learned from Cody was to use 18-gauge for patches that will be metalworked because once you’re done beating and grinding on them, they thin themselves out to 19-gauge, matching the factory steel.

A similar patch was made to connect the window surround with the roof.

Here it is all welded up. I wish I could take credit for any of the welding, but that’s all Cody. My welds keep abrasive wheel manufacturers in business.

The angle of the A and B-pillars didn’t change, so replacing the window frames was a matter of splitting them in half and filling the gap in the horizontal section at the top.

This is where those spare window frame tops come into play. The fact that the donor car was a fordor didn’t matter because all we needed were a few inches of the straight section from the top.


Here’s the window frame with the patch welded in place.

A little hammer and dolly work got everything in shape. We’ll still need to add steel to perfect the gaps around the doors and window frames, but that will come later.

 

Cody begins smoothing out the highs and lows in the new metalwork with a pneumatic planishing hammer…

…then follows up with hammers and dollies for fine tuning.

This just shows how the internal structural steel was chopped and reshaped to match the new quarter window. This stuff will be covered by the garnish moldings, so there’s no need to spend a bunch of time dialing in the corner radius like we did on the outside.

We had to cut out the package tray support in order to access the underside of the catwalk area for metalworking (hammer and dolly, etc.) A new patch panel with a bead rolled down the center provided the replacement structure.

The center windshield support looks pretty gnarly, but it was sound enough, and it will be covered up anyway. While we were welding that back together, we also filled the hole for the aerial.



Here are a few different angles of the finished chop. Of course everything will get a skim coat of filler and tons of sanding before it’s painted. We also haven’t chopped the garnish moldings, vent windows, or stainless trim yet. But hopefully this gives an overview of one way to chop one of these pre-war GM coupes.

 

Beautiful, superb, details. Phenomenally done.

 

Sweet Tech!! I love that is not a crazy low window chop, feels right!

I combined your upper Before and After at the bottom for those who don't want to scroll!!!

Looks like Magic!

 

I have a horrible eye for that, both present and future. Awesome talent. Congrats. I envy your capabilities. Hats off. And cool ass equipment to boot..

 

Amazing how a talented person like Cody makes it look easy! But we know it isn't!

 

Great pics and explanations. The basic process can be applied to many chopped tops.

Sent from my SM-A102U using The H.A.M.B. mobile app

 

Why do I get the feeling that Cody’s boss (his wife) was looking at you two and wondering what mischief you were getting into.


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Very cool and great documentation!

 

Nice work, thanks for all the photos. Bob

 

Fantastic tech! Chop looks just right! Love the lines of the car.
Cody has some mad skills and shop equipment.

 

Great process, Thanks for showing All your Work ( Just like Math class)!

 

Great Tech showcasing the tools of the trade and a Wonderful trip in the Custom Shop...great play by play in imagery and explanation...great outcome by all involved...looks just like the Artist Rendered Factory Print Advertising...

Thanks for sharing @Curt Six...

 

Amazing chop! That rear 3/4 view blows me away

 

Nailed it!

 

Great Tech Week post -- And quite impressive work on the chop. Well done, guys!

 

Awesome!

 

I think she was just sick of us begging her to run to town to pick up Po Boys for lunch every day.

Thanks for the kind words on the chop, fellas. I've made a good bit of progress since it was at Cody's, so I'll have to post some updates soon.

 

What a great chop and documentation! You said in the opening post it was at Cody's for a week. Any idea of the man-hours involved? I would guess that between your preliminary prep work and what's left to finish up, you could probably double that number. Then multiply that by 2 or 3 for regular humans.

 

Great job, and great looking car!

 

@GearheadsQCE we had about 60 hours at Cody’s shop...I was there for five full days, then the following week he put another 15 or so hours in it on his own (he also nosed the hood while it was there). Your estimate of doubling that time before it’s done is probably accurate, once you figure chopping and bodyworking garnish moldings, chopping the stainless trim, vent windows, etc. then reinstalling everything (gaskets and glass, etc.)

 

That chop looks fantastic. You guys nailed the proportions.

 

Thanks for showing so much detail regarding the process and fitting


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Thanks a lot fellas. And I don't know about "dream team" @CTaulbert unless your dream team is one guy who's a kickass metalshaper and another guy who stares at stuff way too long trying to decide if it looks "right." Thanks man.