Introduction
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:
RPO |
Name |
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? |