How To Notch GM G-Body Frame Rails

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Wayne Scraba investigates how to notch the frame of a GM G-Body to accomodate a big, sticky tire.

GM’s G-Bodies and more specifically, ’78 and later Malibu’s and Monte Carlo’s are popular cars, particularly if you want to burn rubber, either on main street or down the straight and narrow. There’s good reason for that. G-Bodies are the last of the mid size rear wheel drive platforms from General Motors, they have a full (albeit a wee bit flimsy) frame, they have relatively spacious engine compartments that swallow any number of different GM engines, there’s a ton of aftermarket support for them and they’re easy to work on.

Bliss. Right? Maybe not. Unfortunately there’s limited room out back for big sticky tires. Typically, the most street tire these things can accommodate is in the range of a P275-60R15. And if you run slicks, a 9.00X28.00-inch example is the biggest you can get away with (and that might be a stretch for many cars). Even with those two examples, you’re almost always forced to jack the back end up to gain clearance. The reason is, the leading and trailing edges of the frame rail (closest to the back of the door jamb and closest to the bumper) are tight to the tire. The gentle production line frame “notch” simply limits the amount of tire you can fit, even though there’s plenty of room in the OEM wheel well. Then there’s the catch with jacking up the back of the car. If you raise it, you mess with the geometry of the back suspension. The factory 4-link instant center location is changed and typically, the car will be a handful to hook.

The solution is simple: Notch the frame. Sounds simple enough, but if it isn’t done correctly, it can turn ugly, simply because, as pointed out earlier, the factory frame isn’t the strongest piece in the world. The truth is, the frame has to be carefully notched, then reassembled (plated) with fresh material and reinforced (with a crossmember) so that it doesn’t droop or bend following the surgery. Once that’s complete, your G-Body can swallow tires right up to P325-50R15’s or 11.5X28.00-inch slicks, using the stock inner wheel well (tub). Best of all, it can be accomplished at stock (or close to stock) ride height which means the suspension geometry isn’t screwed up. Sound interesting? Read on. In the captioned photos (and the subsequent article) that follow, we’ll show you how the frame was notched in our old G-Body. FYI, what works on Buford will work equally well on your Malibu, Monte Carlo or Cutlass. And for this segment, we’ll show you how the frame was cut. Next issue, we’ll show you how it was completed.
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Here’s a snake’s eye view of the back end of the car. At the time, the car had a set of P235-60R15 tires out back, wrapped around stock wheels (the rolling stock measured barely 26-inches tall). While it looks like plenty of room in the wheel well, it’s not, particularly when the tire diameter is increased. In the second photo, you get a better idea of the restriction. See where the frame swoops around the tire? That’s one spot where tall, wide tires get fouled by the frame.

First things first: The lowest 1/2-inch to 1.00-inch of the inner wheel well (tub) where it meets the frame is actually a sheet metal lip. At various spots, that lip tends to flare out to meet the outside of the frame rail. Clearance is made with a big hammer, whacking it several times to get it into shape. It really doesn’t require a huge amount of hammer clearancing.

This is how the frame was cut. A common 4.5-inch angle grinder with a cut off disc was used for this part of the surgery. The frame rail was cut back to just past the factory seam. Check out the next two photos for more detail.

Once the side and bottom cuts were done on the frame rail, attention is turned to the top. Here, a torch was used to cut it out. In the second photo, you can see the (rough) finished cut.

With the rough cuts complete, the notch was detailed with a grinder. In the following photos you can see a close up of the notch, prior to finally deburring, filling and boxing it.