Automotive History: General Motors’ 1950 Body Interchange Program

Last year, CC’s JP Cavanaugh examined the mystery of the 1949-50 GM B-body. His excellent two-part article looked at the post-war changes occurring at General Motors that seemingly put its body sharing program in turmoil. Coincidentally, when JP originally published this article I had just finished reading Thomas Bonsall’s book The Cadillac Story, which discusses some of those changes in finer detail. There are few historians that have delved deeply into this time period of General Motors. Consequently, I started doing my own digging and I ended up going down a bit of a rabbit hole; the result is this article on GM’s body interchangeability program.

Lockheed P-38 Lightning

In the late 1930’s Harley Earl received special permission from the US Government to have sneak peek at the newly developed Lockheed P-38 Lightning. Earl brought along his top designers including Bill Mitchell and Frank Hershey. The viewing had significant impact on the designers, and was the inspiration for Hershey’s tail fins on the 1948 Cadillac. Bill Mitchell later said in an interview with James Howell “You have to understand the value of what we saw in that plane’s design. We saw that you could take one line and continue it from the cowl all the way back to the tip of the tail – that you could have one unbroken flowing line.”

Both Mitchell and Hershey ended up serving during the war. Hershey returned and was assigned to start working on the new 1948 Cadillac. Mitchell had no significant involvement, as he was still involved in the service during that time. By the time Mitchell returned, the design for the 1948 Cadillac had been locked in place.

Above are some early styling sketches for the 1948 Cadillac.  Note the themes from the P-38 being present, including tail fins.

Hershey initially worked several design ideas which were influenced by the P-38 and eventually evolved into the Interceptor. The concept of the continuous line from the front to rear was used, as he’d seen on the P-38. Hershey first experimented with tail fins on some of these early concepts. The Interceptor was eventually developed into two running prototypes. Ultimately, Harley Earl deemed the Interceptor to be too radical and too advanced for the public.  Earl told Hershey to take Cadillac in a new direction.

These styling themes developed into the Interceptor.  Although this styling theme was abandoned for the 1948 Cadillac,  Cadillac continued to  develop the car and created a running prototype of the Interceptor (see video here).  Several of the styling themes would be revisited in the 1950 Cadillac.

During the winter of 1945-46, there was a UAW strike which caused some havoc for the design team who were also supposed to be participating in the strike. Instead of crossing the picket lines, the dedicated Cadillac design team relocated to work out of the basement of a farm house owned by Frank Hershey. The team toiled away and within four or five months the 1948 Cadillac design was completed.

All this hasty change in the design process created other problems. According to Richard Stout, who worked at GM styling from 1947 to 1950, although the 1948 Cadillac was a C-Body, it actually used the body shell that was originally destined to be the new B-body shell. It was essential that Cadillac, GM’s flagship, be one of GM’s first divisions with a new post-war design and the new C-Body shell wouldn’t be ready for 1948. Although no source specifically spells out the reason, I suspect the abandonment of the Interceptor design and a hurried redesign of the Cadillac had much to do with this. Therefore, the B-body shell was promoted to be the new C-body putting Cadillac at the forefront of GM’s new post-war car designs. This smaller C-body shell is the reason why the 1948 C-bodies had shorter wheelbases than the previous C-bodies, being roughly the same as the 1947 B-body. This change had a ripple effect that would upset GM’s nicely organized A-B-C body program.

1948 Oldsmobile 98 clay and production model

Despite the design team toiling away to complete 1948 C-body designs, the cars were not ready until well into the 1948 model year. Oldsmobile and Cadillac began production in February of 1948, and the cars went on sale in March of 1948, resulting in a shortened model year for both makes. The 1948 C-body shell was to be used by Cadillac, Buick and Oldsmobile, unfortunately, things didn’t play out that way. A new Buick was also supposed to be released for 1948. The design for the 1948 Buick had been locked in place and was ready for production. Even the catalog illustrations had been commissioned. Nevertheless, at the eleventh hour Harlow Curtice had second thoughts about the design. He was not happy with the front end styling on the new Buicks, and according to Richard Stout, Curtice had a nightmare where the new Buick was mistaken to be a Chrysler Airflow. Whether or not Harlow actually had the nightmare, history clearly shows that he had serious concerns about the styling. He believed the front styling as too “soft” and ordered it revised immediately. The last minute alterations meant that there was no possibility that Buick would have a new car ready for the 1948 model year.

Twins? Harlow Curtice’s nightmare

Ned Nickles, who had worked on the 1948 Cadillac, was newly assigned to Buick. His first duty was to revise the Buick front-end design. His changes were small but had a significant impact. The hood was reworked to have a more upright leading edge for a bold look and the grille was changed to a soon to be traditional toothy Buick grille. It was during these revisions that Nickles added the ventiports to the Buick. This idea was first used on his own personal 1948 Buick, although on his car each port lit up with an amber light as a corresponding spark plug fired. Nickles’ revised design satisfied Curtice and Buick finally got the new C-body for 1949.

This was the 1948 Buick that never was.  Note the hood’s leading edge is much more curved than the production 1949 Buicks, giving it a softer look.

1949 Oldsmobile 88

Meanwhile, the 1949 Cadillac and Oldsmobile C-bodies continued with few changes. The new A-body used by Chevrolet, Pontiac and Olds 76/88 lines was introduced for the 1949 model year. It was styled much in the same fashion as the 1948 C-Body. With no new B-body for 1949, all Pontiacs were reverted to the smaller A-Body shell while Oldsmobile also demoted all of its models below the 98 to the to the A-body shell. I suspect this may have been a compromise for Oldsmobile, but it was the best it could do considering there was no new B-body. Buick on the other hand, was likely unwilling to downgrade its base model Special to the A-body, so it carried on with the 1941 B-body for the 1949 model year.

A 1949 Buick Special, the only GM B-body produced for the 1949 model year, still using the 1941 body shell

1949 Buick Roadmaster

Despite all the work that went into the new C-bodies, some in General Motors had second thoughts about the designs. Originally, the 1948 C-body shell was intended to be developed further. There were plans to introduce B-body and D-body variations and even a facelifted 1950 Cadillac design was proposed. Nonetheless, a last minute decision was made to restyle the C-body for 1950. The fact that there were plans for a 1949 Cadillac Series 75 and a 1950 facelifted Cadillac, and no true B-body variation of the 1948-49 C-body was ever developed as planned, supports the 1950 redesign was likely a crash program.

1949 Cadillac Series 75 styling proposal

This 1950 Cadillac proposal is a mildly facelifted 1949 Cadillac.

While on the surface the 1950 C-bodies appeared to be all new, they were actually heavily reworked 1948-49 C-body shells. After GM’s large investment into these new bodies, it would make little financial sense to completely abandon and develop a new body in such a short order. According to historian Thomas Bonsall, the 1950 body shells carried over the same cowl, the most complex of the body’s stampings, while the exterior panels were heavily reworked. Earl directed his designers to drop the fender line lower, more so on the Buick, to create a distinctive look from the competition.

Some early 1950 Cadillac styling proposals.

The question arises as to why General Motors undergo such a heavy restyling in such a short order? This was a time when Alfred Sloan was being particularly conscious of spending. Sloan remembered that shortly after World War One, once supply caught up with demand, there was a recession that hurt General Motors significantly. Accordingly, he took steps to safeguarding GM to avoid this from happening again.

Evolution of the 1950 Cadillac to the production models.  The fastbacks wouldn’t see production.

This 1950 Cadillac Series 75 appears to be nearly ready for production.

However, GM had to remain competitive, and the 1948-49 C-body was not breaking new ground. It was really just a continuation of the same styling concepts GM had used on the previous 1941 bodies.  At this time there was a  shift in design language in the industry with cars such as Hudson, Lincoln, Packard, Mercury, Kaiser and Frazer all of which used new bathtub-like designs.  GM was not following this trend.  This new design language was showing quick success in the market place and it shared some elements that Harley Earl had deemed too radical when he rejected the Interceptor. Undoubtedly, the perceived shift in design by the industry was unsettling for GM, and immediate changes needed were required.

Historian Thomas Bonsall believed the above, and states the resulting 1950 restyle for the Oldsmobile, Buick, Cadillac lines was a crash program to bring the cars more in line with the styling trends of the day. On the other hand Tim Howley of Collectible Automobile Magazine, claimed the opposite. He claimed the “P-38 inspired” 1949 Buick was too radical, and that Harlow Curtice was lukewarm to the design. Certainly, Harlow Curtice had his reservations about the initial ’48 Buick design and it’s also possible that even after the revisions Curtice was never truly satisfied.  Nevertheless, I find it difficult to believe that anyone thought the conservative and evolutionary 1948-49 C-bodies came off as too radical. I believe the evidence shows that Thomas Bonsall’s theory is correct, which is supported by the fact the 1950 GM designs have elements closer to that of its competitors.

These Buick styling proposals show a high fender line and use the old 1949 style tail lights.  Production 1950 Buicks had a much lower fender line.  The top proposal is a B-Body while the lower is a C-body.  

A pre-production Oldsmobile 98 being compared to some of its competitors.

Furthermore, by this time Bill Mitchell had returned to the studio. He was not particularly fond of the 1948-49 Cadillacs. He described them as too tall, and not wide enough. No doubt, part of his distaste was due to the fact he had no involvement in this design. Undoubtedly his ego came into play, and he likely wanted a car with his design. As such, the 1950 design was led by Mitchell and he chose a lower, wider and more substantial look, despite little actual growth.

This B-Body shell as the basis for the 1950 GM Body interchange program. All the B, C, C-special and D-bodies shared this same basic structure.

The new 1950 C-bodies were rather revolutionary for General Motors. As stated previously, Alfred Sloan was looking for ways to save costs. Body sharing wasn’t a new concept, however for 1950 GM revised its method of manufacturing body shells to maximize part interchangeability and minimize costs. Previously the A-B-C body shells were each separate and unique structures, sharing little between. For 1950 the B, C, C-special and D-bodies all used the same basic engineering and body structure with the major differentiator being length of the body. As of 1950, the GM body shells, with the exception of the A-body, were based on one basic body shell that was simply stretched or shrunk. The designation, no longer discerned unique structures, rather, only referenced the specific variation of the one basic body shell. For the 1950 model year, all Cadillac, Buick and Oldsmobiles used this new body shell in its four variations.

Released August 8th 1949, the 1950 Buick Special was the first of GM’s cars to wear the new 1950 B-body.

The first car to market with the new body shell was the Buick with the early introduction of the 1950 Buick Special on August 8th, 1949. Likely the reason Buick was allowed to debut the new body first was as a consolation for having no base model for numerous months for 1949. The 1949 Buick Special was stuck with old 1941 B-body, however, it was a very short production year, ending in December 1948. It is probable that using the old 1941 B-body was a last minute solution for Buick, much like Cadillac had to reused the pre-war body on the 1949 Series 75. After the 1950 Special’s hit the market, the new redesigned 1950 Cadillacs, and the remaining Buicks and Oldsmobiles followed later in the year.

Note the same roofline on all B-body sedans.  Also note the relationship of the rear wheel on the Cadillac when comparing B-body to C-Body.

So, if these cars all used the same basic body shell, where exactly was the size difference? One of the easiest identifier between the cars bodies was the rooflines, notably the B-body sedans having a vertical C-pillar. This was a clue to the difference between the body shells, as different sized bodies required different roof structures. It may be tempting to examine wheelbases and body lengths to determine body shells, but this is not an accurate way to measure a body shell size. The true differentiator was the dash to rear axle distance. Fisher Body defines the “dash” as the front of the body or the “zero –inch” body line. This dash to rear axle distance grew larger with each body shell and the extra space was added to the rear the passenger compartment. So a C-body had more interior length than a B-Body, a C-Special was longer than a C-body and so forth.

The Cadillac Series 62, Sixty Special and Series 75.  Not the longer trunk on the Sixty Special.

Cadillac is the best car to examine as it had models with each of the four variations, the B, C, C-special and the D-body. Cadillac had a dash to rear axle distance of 98” on the Series 61 (B-body), the Series 62 (C-body) was 4” longer at 102”, and the 60 Special (C-special) was 4” longer at 106.” The size between the body shells grew in a logical progression of 4 inches. The result was a noticeable size increase, while minimizing the cost to the General Motors. The Series 75 D-Bodies were the only shells to have more than a 4” increase with a 123.75” dash to rear axle length. However, unlike previous Series 75 Cadillacs, it shared much of its structure with the smaller cars.

Buick B-Body four-window sedans (top) and C-body six-window sedans (bottom).  This diagram shows the dash to front axle distance and the dash to rear axle distance.

Buick primarily used B-bodies, with the only exceptions being the C-body Model 52 Super sedan and Model 72 Roadmaster sedan. The extra body length is noted when one compares the same make with different body shells. For example, when the Model 50 Buick Super sedan (B-body) is compared to the Model 52 Super sedan (C-Body), there is precisely a 4” increase in length, and the difference is entirely in the longer rear passenger compartment. Oldsmobile 98 had the simplest line-up, with all models using the B-body.

This diagram shows the difference in the Buick hood lengths and the difference between the B-Body and C-Body.

Each division further individualized its styling by changing the dash to front axle length. In front of the cowl, there isn’t much costly sheet metal, so to vary the nose length on a vehicle really does not add significant cost. Oldsmobile and Cadillac only had one nose length respectively, both of which were unique for each make. On the other hand, Buick had two variations of the straight-8. Accordingly, it had two corresponding nose lengths, keeping with the tradition of long hoods on its premium cars. These changes significantly altered the styling theme, allowing Buick to have a more sporting appearance with its long nose and Cadillac to have a more understated conservative design.

On the left is the Cadillac B-body coupe compared to the C-body coupe.  Note the longer trunk on the C-body Cadillac.   The Buick B-body coupe has a longer hood but shorter rear deck than the C-body Cadillac.

Cadillac did not have the varying nose length of Buick, but it did have two rear deck lengths. The entry level Series 61s and the Series 62 sedans got a short stubby trunk, each with 53.18” of rear overhang. Moving up to the Series 62 coupe got you 5 extra inches of overhang, which increased the overall length a corresponding 5 inches. The 60-Special also got this long deck variation, to ensure its size was imposing, while the already long Series 75 used the shorter deck.

GM’s 1950 B-body sedans

Ultimately, this 1950 B-C body interchangeability program’s objective was to lower cost, which it did by sharing the major engineering and body structure over multiple bodies. Nevertheless, when changes were required between the different bodies, the styling department attempted to maximize the unique styling features for each body. Since the shorter B-body 4-door sedan needed a new roof, stylist designed a unique C-pillar to help differentiate the cars. While the six window roof of the C-body showed it was the larger more premium car. To help minimize cost further, both the Cadillac and Oldsmobile B-bodies shared the exterior front door skin. Moreover, the D-Body Cadillac Series 75 used a reworked B-body Cadillac coupe quarter panel, rather than requiring its own custom quarter panel.

GM’s 1950 C-body sedans.  Note the six windows.

Below is a chart for the 1951 B-C body Oldsmobile, Buick and Cadillacs. For each car it shows the body designation, the dash to front axle distance, the dash to rear axle distance, wheelbase, front overhang, rear overhang and overall length. Looking at these dimensions, it is very easy to spot the B, C, C-special and D body shells. It should be noted that the dash to rear axle distances vary slightly between makes. This is simply due to difference in rear suspension geometry between the makes. Although all cars shared the body structures, the chassis were unique to each division. That said, the 4” graduation between the body designations remains constant regardless of the make.

This body interchangeability program was quite sophisticated for the times, and was one of the most ingenious product rationalization programs of the industry. When Richard Stout left General Motors to work for Ford, he enlightened Lincoln-Mercury execs with a presentation on the body interchange program. Known within the Lincoln-Mercury division as the “paper doll show,” Stout used cut-outs of basic body shells along with cut-outs of different body components from the Cadillac, Olds and Buick, each with magnetic backings. Using his presentation board he could literally build different variations of the cars using the various cut-outs for fenders, doors, and quarter panels. This level of body interchangeability was completely unheard of at Ford, and the Lincoln-Merc execs realized how much money this program would save their company. So then, why when 1951 rolled around did GM mess with such a good thing?

This is a photo of the presentation Richard Stout made to Lincoln Mercury Execs in the early 1950s.

For 1951, GM introduced another new body, called the OB-body. This new body was the hiccup that altered GM’s well played plan. The OB body was used on the new 1951 Olds Super 88, which eventually replaced the 88 with its conservative A-body shell, and the Buick Special, demoting it to this smaller body shell. According to Thomas Bonsall, Chevrolet and Pontiac were scheduled to have the OB body shell for 1951, but it ended up being deferred. Historical sources suggest this may have been due to the Korean conflict, or because the new more advanced OB-body shell was too costly to be profitable for Chevrolet or Pontiac, due to the thinner margins per car. When Chevrolet and Pontiac passed on this new body, it left Oldsmobile as the only division using the OB body. This gave Buick the opportunity to jump on board by downsizing the Special. Undoubtedly it reduced production costs for the Special while it helped amortize the costs for the OB shell. The new body downsized the Special, but really only in width, as the OB body was closer to the A-body in width. It still had the same approximate dimensions as the 1950 model in other areas.

The 1951 Oldsmobiles. Top is a 1950 vs 1951 Oldsmobile 98.  Bottom in the new OB-body Oldsmobile Super 88 and an A-body Oldsmobile 88.

A C-body proposal for the Oldsmobile 98 didn’t make production.

The OB-body was sized between the A and C body shells, in effect taking the place of traditional B-body. However, it couldn’t be called a B-body since the B-body that was created in 1950 (the shortened C-body), still existed for 1951. So what exactly does the OB designation mean? It’s rather simple – Oldsmobile-Buick. As the story goes, once it was determined this body would not be the new A-body and that Buick and Oldsmobile were to use it exclusively, it was designated “BO” for Buick-Oldsmobile. The obvious negative connotation resulted in the change to OB. All this may be a little confusing, but we must remember that these body designations were for internal use only. It is noteworthy that the OB-body structure was the most modern design of all of GM’s body shells used at this time. The B, C, C-special and D-bodies all used bolt on rear quarter panels, while the OB body had welded on quarter panels. The remainder of Buick, Oldsmobile and Cadillac’s line-up was unchanged for 1951, using the same body shells as in 1950, as previously outlined.


Early Buick sales literature showed a Series 44 “Custom Special” which was essentially a facelifted 1950 Special using the old B-body shell.  These cars were never produced, and only the OB-body Series 40 Special went into production (see below).


The Series 40 Special with the new more modern OB-body.  Note the narrower width and the higher fender line from the B-C-body Buicks.

More adjustments occurred in 1952 when the Oldsmobile 98 was revised to use a longer variation of the OB-body. This change resulted in all Oldsmobiles using the smaller OB-body shell. Cadillac dropped its Series 61 B-body, meaning all Cadillacs used a C-Body or larger body shell. Buick was unchanged from 1951, using the same B-C body shells, and the 40 series Specials using the OB body. It should be noted that the B-body shell continued for 1952-53. However, Buick was the only car line to use the B-body shell during those years, albeit, solely as a 2-door. Buick dropped the B-body Model 51 Super Tourback sedan after 1951. The only remaining 4-door sedans were the Model 52 Buick Super and Model 72 Roadmaster 4-door sedans, both of which used the C-body.

A styling proposal for the 1952 Oldsmobile 98 using the B-body that didn’t see production.  For 1952 the Oldsmobile would switch to the OB body. 

An OB-body Oldsmobile 98 styling proposal, with an extended rear deck.

This OB-body 1952 Oldsmobile 98 proposal was close to the production model.

Many historical sources revise the nomenclature for the body shells for the 1952 model year. The OB body shell sometimes gets re-dubbed B-body and the former B- C-body are grouped together as the C-body. This makes some sense, as the OB-body was sized between the A and C-body, was a unique body structure, and was in fact filling the spot of the traditional B-Body.  However, the B-body with its short dash to rear axle length was still used by all 1952-53 Buick 2-doors. This shorter body should have a separate designation which is why I believe it’s less confusing to simply continue to use the OB designation clearly differentiate the two.

1949 compared to 1952 Chevrolet and Pontiacs

While the medium and high priced GM cars were shuffling through all these body shells, the A-body Chevrolet and Pontiacs soldiered on from 1949 – 52 with no major changes. By 1953 they were long overdue for revision. Oldsmobile and Buick had commandeered the OB-body shell, so Chevrolet and Pontiac had to make do with the old A-body shell for two more years. As a result, the 1953 Chevrolet and Pontiac bodies were heavily revised but were still based on the 1949 A-body shell. Examining the interior dimensions and cowl area reveals its true roots.

When 1954 came along, things went back to the old pre-1948 system for General Motors body shells. By this time, the worries of a recession had vanished while medium priced cars were selling like hotcakes, so cost savings were less of a concern. New separate B and C body shells were introduced for 1954 and despite the similar appearance, the B-body was a significantly smaller body structure than the C-bodies. No longer were the B and C-body only differentiated by length. The OB-body was abandoned. A new A-body was released for 1955 which finally brought Chevrolet and Pontiac back into the game with a modern body.

Fast forward to 1958, and the A, B and C bodies have again all been revised and things appeared to have gone mostly back to GM’s old body system. That was until the 1957 Chrysler products caused GM’s 1959 crash program. The story of GM’s radical ‘59s is a story that is well documented. While the 1959 designs were new and cutting edge, GM did go back in time for one aspect of these cars – the body interchangeability. Much like GM had done in 1950, the 1959 B,C and D bodies reverted to using the same basic body shell, that was simply varied in size. The other major change was that Chevrolet and Pontiac were added into the mix, both being promoted to a B-body with the demise of the A-body structure. What had been laid out in 1950, finally came into fruition in 1959.

1952 Chevrolet and 1954 Chevrolet.  Note that the firewall and cowl areas are nearly identical.

Looking back, it seems GM was a mess of activity during the late 1940s and early 1950s. It is interesting to me that while the late 1950s are so well documented with GM’s radical crash redesigns and major changes to body programs, but there is so little detailed history on the early part of the decade. Even contemporary road tests of the day, hardly mentioned the fact that cars such as the Oldsmobile 98 had major body shell changes from 1951 to 1952. There is a lot of detail in this story, so I will summarize my findings in a simpler summation.

GM initially planned on releasing new post war designs for the A-B-C-bodies. Due to changes in the Cadillac program where the Interceptor design theme was abandoned, the new quickly redesigned 1948 Cadillac ended up using the B-Body shell, which was re-designated the C-body to ensure it could be one of the first new cars GM released post-war. Oldsmobile and Buick followed Cadillac’s lead, until Harlow Curtice forced a last minute redesign of the Buick. Design teams worked on new B-body and D-body variations of this 1948 C-body. The revised Buick was released for 1949, but by this time, it was decided a crash redesign was required for the 1950 B-C cars to move the cars more in-line with the styling trends. This resulted in no new B or D-body for 1949. The 1948 C-body shell was heavily reworked for 1950 with four separate variations, each with their own body designations. They were only differentiated by dash to rear axle length. The C-body was still the same size as the 1949 C-body, but a shorter variant, now called the B-body was also introduced along with the longer C-Special and D-body.

The A-body line was to be updated for 1951, with a newly designed OB-body shell. Chevrolet and Pontiac couldn’t afford the body shell, which left Olds using it alone. This allowed Buick to jump on board and use this same body shell. As a result, GM now inadvertently had three unique body shell structures, A-body, OB-Body and the B-C Body (including C-Special, D-Body). In 1952 Oldsmobile moved the 98 down to a longer variation of the OB body, and Cadillac abandoned the B-body. This left Buick using B-C bodies exclusively, while Cadillac used the C, C-special and D-body for 1952-53.

Even in the 1980’s GM’s BOF cars still used the same formula created in 1950.  This D-body Cadillac is just a longer version of the B-body used by the more pedestrian Chevrolet Caprice.

The 1950 body interchange program resulted in GM reducing costs and simplifying its product line. While successful in doing so, there were a few bumps along the way that made things work-out less than ideal. However, the end result was the 1950 B-C program ultimately became the template for GM’s body on frame body interchangeability until their demise in 1996. Taking it further, this basic concept of body interchangeability eventually evolved in to the modern concept of platform sharing. Undoubtedly, this time period in General Motors history should be remembered as being historically significant.


Further Reading:

Automotive History: General Motors 1949-50 – The Mystery Of The Missing B Body (Part 1) 

Automotive History: General Motors 1949-50 – The Mystery Of The Missing B Body (Part 2)

A special thanks to JP Cavanaugh for his consultation during the writing of this article.