Solid Axle RPO Cars


Ninth Edition

by Dr. Gerald C. Wagner



From the mid 1957 through 1962 model years, 935 corvettes were shipped with a factory heavy-duty brake package and are called 'RPO cars' by collectors and restorers. RPO is an acronym for Regular Production Option. They all included finned brake drums, vented backing plates with eccentric centering pins, a fast steering adaptor, and four backing plate air scoops. Other components were added and deleted over time and many of the pieces underwent design changes.

Most of these cars were raced and modified. The RPO pieces, in fact whole conversion kits, were available separately from Chevrolet. As a result there are many different opinions as to the original factory configuration of these cars. This article has been reviewed by many owners of RPO cars and its goal is to arrive at a consensus view for restorers and judges.

A listing of the factory options and list prices during the six years they were offered is a good starting point:



Year Price Count
684 Heavy Duty Brakes and Suspension 1957 $780.10 51
684 Heavy Duty Brakes and Suspension 1958 $780.10 144
684 Heavy Duty Brakes and Suspension 1959 $425.05 142
687 Heavy Duty Brakes and Special Steering 1960 $333.60 119
687 Heavy Duty Brakes and Special Steering 1961 $333.60 233
687 Heavy Duty Brakes and Special Steering 1962 $333.60 246

The price decreases and option name changes reflect items that were deleted from the package over time. Most of these became over-the-counter ("OTC") service parts and were subsequently added by racers to the confusion of later restorers. The success and popularity of Corvette racing contributed to the increased sales.

1957-59 RPO 684

Research and testing supervised at Sebring in 1956 by John Fitch developed many brake and suspension modifications which were used on 'factory' racers in 1956 and then offered as a RPO in the 1957 model year. The RPO brake shoe linings were Ceremetalix which only worked well when hot; conventional shoes had to be substituted for street use. Ceremetalix linings were used on aircraft brakes and are copper in color. The RPO 684 front shoes are 2 1/2" wide and the rears are 2". Early ceremetalix linings came in pairs with two pads on the primary shoe and four pads on the secondary shoe; in 1959 this was changed to three and five pads respectively. Ceremetalix linings were also available OTC to fit regular, base drums. These linings were so tough that you used up and replaced drums, not brake shoes.

Besides the vastly improved brakes, a heavier front sway bar, heavy-duty shocks, firmer springs, fast steering adaptor, and longer rebound straps were included. Initially, in 1957, there was to be a heavy suspension package separate from the brake package; however, it appears that this tentative RPO 581 was never offered. RPO 684 became available in early April, 1957, and 51 cars were sold with this option during the remainder of the model year. As we shall see, RPO 276, the wider wheels, was a necessary option because of the shape of the early finned brake drums. Probably not coincidentally, in the 1957 model year, 51 cars were shipped with wide wheels. This implies that no '57s should have wide wheels and not also have RPO 684. Most likely these 51 cars included the 43 that were equipped with RPO 579E, the special fuel injection airbox assembly.

Heavy Duty Springs

The RPO rear springs had five leaves without anti-squeak liners compared to the four leaves on base springs which had impregnated cardboard liners. The four long leaves of the RPO springs have lengthwise grooves like the base spring but their short fifth leaf was flat. These rear heavy-duty springs generally have six heavy riveted retaining straps. A few springs have been reported with thin bands like base springs. The ratings in pounds per inch of spring travel were: 

Front Rear
Base springs 300# 115#
1957-58 RPO 684 340# 125#
1959 RPO 684 and OTC 550# 145#

Early literature shows the 340# and 125# rates, but later '57 literature shows them as 550# and 155#. The part number on heavy duty springs never changed so we welcome more information on early spring differences. The consensus opinion is that all RPO springs, from inception, had the 1959 ratings of 550# and 145#.

Air Ducts

The 1957-58 RPO cars included a complex systems of ducts to lead cold air from the front of the car to the rear brake scoops. The 1958 rear ducting was more refined than that of 1957 in that it included 'elephant tusk' fiberglass ducts up high inside the front fender wells. These ducts were attached to the metal reinforcing straps that early '58s had in their front fenders. The base car fender reinforcements were dropped half way through the model year, yet, necessarily, '58 RPO cars retained them until the end of the model year.

Rear ducting was dropped in 1959 and the RPO cost was reduced by $355 as a result. Despite the Assembly Manual (AIM) not changing until October 17, 1958, the price dropped at the start of the model year and no '59s with rear ducting have not been reported. AIM dates are very rough approximations of when running changes were actually made. The rear duct outlets would have had clearance problems with the radius rods (traction bars) and their towers which were added in 1959. The metal rear air scoops were redesigned for 1959 and are flatter to clear the just introduced radius rods.

Late 1958 through 1962 RPO cars have two short ducts mounted on the fiberglass at either side of the radiator core support. These were to direct air to the front scoops mounted on the vented brake backing plates. 1956 and 1957 factory racers had triangular holes cut in their inner front fiberglass to accomplish the same purpose. The AIM date for adding these fiberglass front ducts was February 25, 1958.

Scoops and Dust Covers

The front and rear scoops were in a bag in the trunk when the RPO cars left the factory. The cars were shipped with metal dust covers blocking the screened vents in the backing plates for street driving. These covers had narrow paper gaskets around their edges and were mounted with indented hex head bolts with integral star locking washers. The bolts are the same type as are used to mount the brake cylinders on the backing plates. In '60-'62 cars they commonly have B&H headmarks and in earlier cars the headmark is usually an E. The two small circles around the headmarks indicate that they are of a harder grade than common bolts. These bolts are recycled to attach the scoops when the dust covers were removed. These covers are among the harder RPO pieces to find today as most racers discarded them. There are two per wheel but only six different part numbers as the rears were interchangeable.

An instruction sheet showing how to remove the dust covers and mount the scoops was included in the trunk at the start of the 1959 model year. Though not mentioned in the instructions, the front scoops could be switched from side to side. This changes how low they ride - the tradeoff is how much airflow versus debris you pick up. Mounting the scoops makes the cars more exotic to show, but I got caught in a rainstorm on the freeway with the scoops installed and learned to appreciate the dust covers.

Shock Absorber Mounting

All RPO 684 cars, except early '57s, had extra parts to accommodate their heavy-duty shocks. The RPO shocks were thicker than base ones and to clear the front springs the lower clamshell retainer for the front shocks had an offset hole. The base retainer has its shock rod hole in its center. The clamshell retainers seem to often be broken and many RPO 684 cars are found with the base retainers which were serviced by GM until recently. The rear shocks required a larger, heavier top support on the frame crossmember. At the factory when the suspensions were being assembled onto the inverted frames, the base retainers were knocked off and the heavier RPO Vee shaped pieces were welded on. This welding is crude ("bubble gum" tacks rather than neat seams). See photo below.

Brake Pivot Pins

On all 1957-62 RPO 684 and 687 cars, eccentric pivot pins were used at the top of the backing plates to allow up-and-down centering of the shoes. The back of the pin itself has a hex profile. The large locking nut is loosened and the pin is turned with a wrench to the point of minimum drag. All brakes are self-centering front-to-rear, and base and RPO 686 pivot pins are not eccentric. The front base pivot has a squared shaft to lock into the base front backing plate; base rear pivots are round and can be substituted for the eccentric RPO pivots if necessary.

Transition in 1959

In 1959 thicker, stronger front wheel hubs appeared on all Corvettes and gas-filled shocks replaced the earlier ones - both on base and RPO cars. The stronger hubs dropped the extra holes necessary to rivet the base front brake drums to them. The 1959 base brake drums also changed to a heavier, bell shaped design. The 1959 advance parts catalog shows that the new radius rods were not intended to be put on 1959 RPO cars, yet all seem to have them. Again this was probably due to interference with the rear ducting and when it was decided to drop the ducts, the go-ahead to install radius rods on 1959 RPO cars was given.

1960-62 RPO 687

Even though the AIM dropped RPO 684 on July 10, 1959, and added RPO 687 in early August, the 684-687 transition took place with the '59 to '60 model year change.

Metallic Linings and Fans: In 1960 the package changed substantially and was now designated RPO 687 rather than RPO 684. Riveted metallic linings replaced the cerametalix ones and centrifugal cooling fans were added inside the finned brake drums. The RPO 687 rear linings were 1 3/4" rather than 2" as used in RPO 684; the fronts remained at 2 1/2". As a consequence, the welded washer was omitted from the rear backing plates as noted below.

Anti-Roll Bars: The heavier 13/16" front sway bar became standard on all Corvettes, though with a different part number and shape, and 1960 Corvettes introduced a rear sway bar, not previously used, which was 5/8" in diameter. With the improved anti-roll handling provided by the sway bars, the heavy duty shocks and springs were dropped and base ones were used. These deletions and the other changes allowed the RPO cost to fall another $91.

These revised RPO 687 cars could be driven comfortably and safely on the street. The bone shuddering 1959 springs had been dropped, and the metallic linings worked without having to be at racing temperatures. The 1960-61 RPO cars rode no harder than base cars, but they stopped better and had quicker steering. Despite factory publicity that stated the sway bars added more road holding ability than was lost by dropping the heavy duty springs and shocks, many racers bought the heavy shocks and springs over-the-counter and added them to their RPO 687 cars.

Last Year was 1962

In 1962 the RPO 687 name changed to reflect heavy duty shocks which were put back into the factory package. Also for 1962, the metallic linings were bonded rather than riveted to the brake shoes. The factory brake shoes are distinguishable from after-market metallic linings since factory pads are made up of paired segments rather than one-piece pads. What appear as 5 segments are really 10 in pairs. The 1962 RPO shocks had new part numbers and the rear ones did not require the upper Vee supports welded onto the crossmember (as in late '57-'59), nor did the front shocks need the '57-'59 offset hole lower clamshell retainers. 

Over the counter items such as an auxiliary front sway bar and dual belt water pump pulleys were available. The idler and pulleys for this dual fan belt setup was available as far back as 1960 and was used on the LeMans racers. The 1962 Assembly Manual shows that the dual belt system was to be installed on the two solid-lifter camshaft engines that year, but in actuality they received the standard single belt system.

Racers found that the cars handled best with the addition of the heavy duty springs and the second front sway bar with the stock rear sway bar removed. In the middle of the '62 model year, over-the-counter kits were designed which automatically adjusted the brakes while driving forward! These adjustors became standard on the Z06 option in '63 cars.

Evolution of the RPO components

There are early and late versions of many of the RPO parts and the point of change of many is between 1958 and 1959 rather than the 1959 to 1960 RPO 684/687 juncture.

Brake Drums

All drums had the same casting number but there are four types, i.e., early and late, front and rear. The early drums have fins which wrap around the rim of the drum onto its outside face. The later ones began appearing in 1959 and have more squared-off fins which stop on the outer circumference of the rim.

The wraparound fins on early RPO drums protrude 1/8" from the drum face. The fins interfere with base 5" wheels where the flat attaching pads of the wheel center spider are welded to the rim. The optional 5.5" wide wheels fit fine. NOS early drums have been found with their front fins machined off by GM; this base wheel interference is most likely the reason the drums were redesigned. Another type of early drums had fins that protruded 1/2" on their faces and were designed for use with the Halibrand alloy wheels used mainly by factory racers.

The backside of all front drums have inner core braking surfaces which protrude 1/8" beyond the fins whereas the rear drums are machined flat, i.e., when set on a flat surface, the fins of rear drums touch the surface. The front drums have a more bell-shaped stamped metal face; the rears are flatter. An early '59 RPO car has been found with mixed drums, i.e., early ones on the rear and late on the front. Many RPO cars were raced and wore out their drums, so many '57 and '58 RPO cars also are found with mixed drums although originally they came with four of the early style drums. This is why early drums command a much higher price with collectors than do later drums.

Backing Plates

The 1957-58 backing plates have fine screen vents while the 1959-62 ones are coarser. The fine screens have 11-12 holes per inch while the coarse ones have 8 holes per inch. This change was made because the fine screens tended to clog up with dirt. Late 1958 and early 1959 RPO cars have been found with mixed screens.

RPO 684 rear backing plates have a washer welded onto them under the top brake shoe pivot pin to extend it. When the option was first announced in 1957, the specifications called for 1 3/4" rear shoes. Apparently when it was decided that the rear cerametalix shoes would be 2", the tooling for the backing plates had already been made. Consequently 2" rear shoes will not fit on backing plates that don't have a spacer under their eccentric pivot pin. Thus all 1957-59 RPO cars should have the welded spacing washers.

My early 1959 has fine screen front backing plates and coarse screen rears with the spacing washers welded onto them. This should put to rest the view that the screens changed with the 684-687 transition. RPO 687 rear backing plates have no washers. Very early 'prototype' vented backing plates (circa 1956) do not have rolled flanges, i.e., they are flatter around their circumferences. All front RPO backing plates are attached to the steering knuckle with tubular bolt spacers and bolts that are 1/8" longer than the base ones.

Shock Absorbers

As noted there are three distinct sets of shock absorbers: 1957-58, 1959, and 1962. The redesigned 1962 heavy-duty shocks did not need the RPO 684 modified upper rear shock supports or offset front shock lower clamshell retainers. Although the '62 RPO AIM called for rebound bumpers to be welded to the radius rod frame towers to control spring wind-up, they were not actually installed.

Scoops and Ducts 

Another evolutionary RPO part was the rubberized canvas of the front 'elephant ear' air scoops. The front scoops are rubber because they can rub the suspension when driving hard. The first 1957 ones were Green and proved too stiff; they were replaced by a lighter-weight Black canvas. These were too thin and wore out so 1958 through early 1960 scoops were a thicker Black canvas. Finally, from late 1960 through 1962 the canvas was white except some early '61 RPO cars have been found with a softer, Red/white canvas. Jan Quast has made an extensive study of these front scoops, and an employee of the supplier told him that quality control was always a problem.

There were many running changes over time. The short front fiberglass ducts were added to late '58 RPO cars, and up through most of the 1960 model year they were made with a fiberglass cloth having a coarse weave. In 1961-62 they were made with a finer weave cloth. The rear brake scoops were made of formed metal and were redesigned in 1959 to clear the new torsion bars. These later rear scoops are flatter than the early ones.

Brake Springs

All RPO 684-687 cars used pullback springs which are Dark Green and although the same diameter as base springs, they are stronger and much more heat resistant. RPO 686 pullback springs have a larger diameter and were never used on 684-687 cars.

Until RPO 686 was offered in 1959, the RPO 684 cars used base springs for the hold-down pins, adjustors, and parking brake struts. The RPO 686 metallic brake hold-down springs were also used with RPO 687; they are Avocado Green and their inside retaining washers are larger and flatter than the outer cups which appear like base ones. The correct hold-down pins are set forth below. The number is stamped on their head and their length is shown below:

Front Front Rear Rear
Base Cars #4 2" #8 1.875"
RPO 684 #6 2.25" #4 2"
RPO 687 #6 2.25" #8 1.875"

Mike Ernst (see Restorer; Volume 17, #4; Spring 1991; pages 28-32) has noted that the adjuster springs should be Black on the front and Baby Blue on the rear of 1960-62 RPO 687 cars. Base adjustor springs are Dark Blue and were used on most RPO 684 cars. The nuts (female part) and sockets of the RPO brake adjusters carry a different part number than base ones; they appear about 1/16" longer.

Brake Cylinders

The base front wheel cylinders were 1-1/8" in 1957-59 and 1-3/16" in 1960-62. RPO cars kept the 1-1/8" front cylinders all the way through 1962. The RPO rear cylinders are 7/8" as opposed to the 1" base ones. The part numbers for RPO 684 rear cylinders are 5456398-399 and show casting number 5456400. Cylinders cast 5456400 look as large as 1" cylinders but are actually bored only to 7/8". RPO 687 rear cylinders show a casting number of 5450996 and were assigned part numbers of 5459986-987. They are cast as true 7/8" cylinders and smaller on the outside than the earlier RPO 684 cylinders.

RPO 686

Metallic brake linings were introduced at the beginning of the 1959 model year in a much simpler and fairly common option which used the base brake backing plates and drums. The base drums were honed to a finer finish for the metallic linings, hence a different part number although they appear identical to base drums. The shoes were the base size - 2" in front and 1 3/4" in the rear. Heat resistant pullback springs were used which are Whitish Pink and Yellow and much larger in diameter than base springs.

"Special Equipment for Sports Car Meets"

Chevrolet offered several other 'racing and handling' options which were priced separately. RPO 684 and 687 were limited to Corvettes with the solid-lifter dual four-barrel or fuel injection engines and a Positraction rear axle was mandatory. In fact I know an original owner of a 1961 RPO 687 car with the standard wheels and full hubcaps. He is proud of, and played for me, the 45 rpm record narrated by Zora Duntov and called the 'Sound of the Corvette' which came with his car, however, he recalls the dealer doing something with the tires because his wife didn't like the way the car looked!

Wide Wheels

Original wide 15"x5.5" wheels are hard to find today but interestingly twice as many Corvettes had them as were sold as RPO 684 and 687 cars. The RPO 276 wide wheels cannot hold the normal Corvette wheelcovers and cars with them were shipped with simple passenger car small hubcaps. The 1957 and 1958 early drums interfere with base wheels so we believe that the wide wheels were necessary on early RPO 684 cars. From 1957 through 1962 there were 1,813 Corvettes shipped with the wide wheels. They were also an option available on full-size Chevrolets such as taxicabs and police cars. These 'police wheels' can still be found in junkyards and are identical to Corvette wide wheels.

With the small hubcaps, a large portion of the wide wheels are visible. Despite different opinions among RPO buffs, it appears that the painting of the wide wheels followed the model year base wheel painting policy except for '62. The whole wheel was painted a semi-gloss Black when supplied by the vendor and then the outside face was repainted at the St. Louis assembly plant. '57 wheels are body color except they were Black on White cars; all '58 wheels are Silver; and all '59 are Black.

All '60-'62 wheels (both base and wide ones) are in body color, except '62s with whitewalls and base 5" wheels had Black wheels. In thinking about it, the designers probably made this change for a better appearance with the new '62 narrow whitewalls which were Black between the wheel and the whitewall. Until a '62 turns up with original whitewalls mounted on Black 5.5" wide wheels on a non-Black car, we should agree that all wide wheels in '62 were painted body color. Dale Pearman (see Restorer; Volume 18, #1; Summer 1991; pages 36-37), Ray Masciarella and John Amgwert (see Restorer; Volume 18, #2; Fall 1991; pages 30-31) have addressed this color issue in detail.

Original RPO 276 wide wheels are of riveted construction rather than welded, do not have tire bead safety depressions on their rims, and have large valvestem holes. The manufacturer markings inside were stamped in before the rim was formed. One is a circular 'Kelsey Hayes -Made in USA' logo; another looks like a 'cattle brand' - a round logo with the number 4 inside it. Also inside, near the logos, some original wheels have 15X5 1/2K and no date; others have a smaller 15X5 1/2 - K (note the dash) and date, i.e., 60 for the year and one a small number stamped crookedly near it denoting the month. The only outside marking is one embossed 'GM' on the front near the lug holes.

Replacement '8080' wide wheels have their rims welded to their centers rather than riveted and have two nubs near their smaller valvestem holes. Another noticeable distinction are bead safety grooves which originals do not have. These wheels will not fit over 1959-62 front hubs unless their center holes are ground out slightly. Original base 1963-64 wheels are also 5.5" wide and don't have the tire bead safety retaining depression on their back rim, just on the front, and are similar to '8080' wheels.

Nylon Tires

LPO 1408 called for nylon 4-ply blackwall tires and was offered at the beginning of the 1959 model year. When they were mounted on the optional wide wheels, the spare tire board was raised enough to interfere with the trunk latch. As a result an AIM change on February 24, 1959, called for additional notching of the board and mat to clear the trunk lid latch.

Large Fuel Tanks

LPO 1625, a 24 gallon tank became available in mid 1959. It was made of fiberglass unlike the early '56-'57 factory racers which had large metal tanks. In 1961 LPO 1625 cost $161.40. In 1962 the large tank became RPO 488 and its cost fell to $118.40; 65 cars were shipped with RPO 488. These 'tankers' had to be hardtop only cars since the large tank used up the space where the folding top was stowed. In 1962 the previously optional heater became standard equipment so a new RPO 610 was added which was 'heater delete'.

Why Confusion?

The majority of RPO cars were purchased by enthusiasts who planned to race them. Jack Podell (of fuel injection fame) owned a 1959 RPO car which still had its original window sticker and dealer paperwork. Bob Bondurant borrowed it from its owner and won the Palm Springs race that year. This car left the St. Louis factory with a Solid White exterior, Red interior, five Black RPO 276 wide wheels, standard blackwall tires, and was fully optioned with two tops, heater, radio, courtesy lights, visors, and a windshield washer, as well as, FI, a 4 speed, 4.11 gears and RPO 684.

With the original sale, Harry Mann Chevrolet in Los Angeles added three more wide wheels ($39.21), painted all eight wheels ($28.00) and the body coves Red ($16.15), and then mounted four standard whitewalls ($31.55) and four Firestone 170 Super Sports racing tires and tubes ($261.20). Harry Mann made a little extra on the deal, they kept the five blackwalls and charged the full upgrade-to-whitewall price yet only supplied four whitewalls.

This historic car is a good example of how Corvettes were modified before they even left the dealer. Don't we wish we could still buy wide wheels for $13 apiece? 

Any comments? Click
Jerry Wagner

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