CRG Research Report - 1967 Camaro Radius Rods

C R G	CRG Reports	Exterior	Engine	1967 Model ID
Numbers Decode	General Info	Interior	Transmission	1968 Model ID
Drivetrain Decode	Options	Underhood	Chassis	1969 Model ID

A CRG Research Report

1967 Camaro Radius Rods

Reviewed by the CRG
Last Edit: 21-May-2004
Previous Edits: 08-Aug-2000, 18-Aug-1999, 23-Dec-1998
Original Release: 24-Sep-1998

Introduction

At mid-year, high-performance 1967 Camaros received a passenger-side traction bar, referred to by Chevrolet as a "radius rod." There were two factory designs - the first design used a round rod while the rod in the second design was rectangular with increased stiffness. Additionally, later in the year, a rectangular rod service retrofit kit was made available to dealers to address wheel-hop complaints; the service kit could be applied either early vehicles with no rod at all or to intermediate vehicles with the round rod.

**First Design - Round Radius Rod**

**Second Design - Rectangular Radius Rod**


Two views of 1967 12-bolt axles showing the "Mono-Plate" single-leaf, width-tapered, rear spring, with the later rectangular radius rod (traction bar).

The question of which rod was applied to what model, and when, has often been a subject of much confusion. This bulletin defines, as clearly as possible given the known data, the applications of each rod, and also discusses specific areas where data and information is lacking or weak. We encourage all 1967 Camaro owners with installed radius rods or rod brackets to please contact the CRG to report information about their vehicle, that we might continue to improve this report.

Summary

The overly-simplified summary of radius rod application is that starting mid-December 1966, cars with 12-bolt axles and automatic transmissions (L48/M35 and L35/M40) received a round rod, while all other cars with 12-bolt axles received a square rod. This is overly simple because there are three caveats, as well as a couple of grey areas that CRG is continuing to research. The caveats:

Since the round rod was released for production well before the rectangular rod, obviously all rods on 12-bolt cars built after mid-December. but prior to release of the rectangular rod, are round, including models that would later receive the rectangular rod, namely: Z28, L48/M20, L48/M15, L48/M13, and (eventually) L30/M20.
The square rod was not introduced to all manual transmission models at the same time, therefore some manual transmission cars continued to receive the round rod after other models had changed to the second design. The start date of the rectangular rod for each model is still being refined.
And lastly, the L48/M35 car, while fitted with round rod brackets, did not always have the rod itself actually installed at the factory. Data to date indicates that only about one-fourth of this model have installed rods. No specific patterns of installation have emerged as of the August 2000 update.

Table 1 illustrates, in a different format, these findings to date, model by model.

A significant complication is the factory authorized rectangular radius rod service retrofit kit. This makes it a bit more difficult for us to be sure if a rectangular bar was factory installed or dealer installed. However, by comparison of the service replacement instructions with the original factory assembly instructions, it appears that there are three clues that should distinguish a service replacement rectangular bar from its factory brother:

When the service kit was applied to a chassis with no previous bar, the chassis bracket was welded to the chassis, rather than bolted-on as was done at the factory.
Regardless of application, the service kit required the bumper stop bracket to be welded to the axle bracket, rather than bolted-on as was done at the factory.
When the service kit was applied to a chassis with a pre-existing round bar, the round bar axle brackets were not replaced with the new brackets but remained on the axle and were used as-is. The two bracket types have distincly different shapes.

**Table 1: 1967 Camaro Radius Rod Conclusions To-Date**
Application	Early Use/Type mid-Dec to Feb-Apr	Late Use/Type Feb-Apr to End-of-Model
L30 4-speed^{1, 2}	(late Feb start) proven/round	proven/rectangular
L48 3-speed	probable/round	assumed/rectangular
L48 4-speed²	proven/round	proven/rectangular
L48 Powerglide³	sporadic/round	sporadic/round
L35 3-speed	assumed/rectangular	assumed/rectangular
L35 4-speed	proven/rectangular	proven/rectangular
L35 THM400	proven/round	proven/round
L78 4-speed	N/A	proven/rectangular
Z28 4-speed⁴	proven/round	proven/rectangular
Introduction of the radius rod into the L30/M20 lagged behind the L48 by about two months, not appearing until circa late-February at the time of the introduction of the 12-bolt axle to the L30/M20 (see the axle section of the L30/M20 CRG Research Report). The L30/M20 and L48/M20 transition to the rectangular rod is still being studied due to a lack of quality data from the mid-March to mid-April period. The latest known with round rods are dated 04C. The earliest known with rectangular rods are also 04C, but there isn't enough data to be certain that the transition occurred cleanly within that one single week. About one-fourth of L48/M35 cars have been noted with the round radius rod installed, while the remainder have the rod brackets but are absent the rod. No definite installation pattern has emerged. Z28s starting late March have the rectangular bar. The change may have occurred with early March Z28s, but this hasn't been determined. The rarity of Z28s (much less original Z28s from this restricted time period), along with existence of the retrofit kit discussed in the main article, all complicate this determination. Terms: "proven" means that the preponderance of documentation evidence, supported by vehicle data, indicates this conclusion. "probable" means that the document evidence is conflicting but that there are confirming vehicles available. "assumed" means that the documentation indicate this but that the CRG has no confirming vehicles in its database. "unproven" means that there is some documentation that indicates this - but most documents don't include this version, and there is no confirming vehicle evidence. "doubtful" means that there is no evidence to conclude that this application continued - but also none that absolutely says that it ended.

Problem

High-performance 1967 Camaros under heavy initial acceleration experienced a rear suspension problem called "wheel-hop" (or "wind-up"). This problem was accentuated by a combination of insufficient suspension stiffness about the axle centerline and insufficient suspension damping in the monoleaf spring/vertical shock system. High engine torque applied to the differential rotated the entire differential assembly about the axle transverse centerline toward the front of the vehicle and twisted the monoleaf springs into an "S" shape, until the torque broke the tires loose. With the tires off the ground the engine torque no longer applied a load to the deformed springs - with the result that the springs violently rebounded and forced the tires back on the ground, starting the cycle over again. This shuddering, traction-losing, U-joint-breaking, behavior repeated at high frequency until the driver backed-off the accelerator.

The temporary fix implemented by Chevrolet engineering during the middle of the 1967 model year (until 1968 model year suspension changes were implemented) was a form of traction bar, labeled by Chevrolet as a "radius rod." The radius rod was a single, end-hinged, rigid-link (traction bar) between the passenger side of the rear axle and the chassis. In contrast, the related Pontiac Firebird included both left and right traction bars of a different cross-section design on their highest performing models. See Firebird rod - view 1 and Firebird rod - view 2 for illustrations of the Pontiac-designed radius rod.

The theory was that such a rod would allow an acceptable amount of vertical translation in the rear suspension, but resist rotation of the suspension about the axle centerline such as occurs when a large torque is applied to the driveshaft during a hard start. In practice the Chevrolet factory rod reduced, but did not eliminate, wheel hop, and therefore neither of the two Camaro versions of the radius rod were fully successful.

1967 Camaro Radius Rod Designs

Both of the Camaro radius rod designs used a rigid steel rod with one end thru-bolted (hinged) to ears welded below the rear axle and the other end similarly attached to a special bracket bolted to the chassis. The first design was a round steel rod with a bushing at each end, part number 3914808. The second design improved upon the first and was a stiffer 1-inch by 1.25-inch rectangular rod, part number 3920234, with a modified axle bracket interface. (Note in the Firebird illustrations shown above that the Firebird rod design differed from either of the Camaro designs, having a wide "I"-beam cross-section instead of being round or rectangular.)

1967 Camaro Assembly Manual sheet B3 for RPO L48 dated 08-Nov-66 describes the first design round rod version. The 07-Apr-67 revised sheet A8 for RPO L78 describes the second design rectangular rod version. The ears welded to the rear axle that formed the rod interface at the aft end were slightly modified for the 2nd design, and a supplemental "stop" bracket that included a new rubber bumper was added in the 2nd design to the bottom of the ears. For both designs the chassis bracket was identical: part number 3914806.

Radius Rod Applications

The first rod design was initially applied starting mid-December 1966 to all models with 290 HP or greater, as well as to the 275HP/4-speed model, the L30/M20. The second design rectangular rod was introduced in early February for the L35/M20 and L35/M13 big-block manual transmission applications, and was also applied to the L78/M21 from the start of that model in late March. The replacement of the round rod with the rectangular rod on the Z28 appears to have occurred sometime in March, and on the L48/M20 and L30/M20 circa 04C (04C L48/M20 cars have been found with both types). Despite an assembly manual cancellation notice, the round rod remained in use for the rest of the year on L35/M40, as well as (sporadically) on L48/M35.

Our reasoning and notes that justify these hypotheses follow, supported by data gleaned from the 1967 Camaro Assembly Manual and the 1971 MPC 721A Camaro-Only Parts Manual. The GM sources are in general agreement on most issues. There are a few areas of disagreement or ambiguity, and for these cases we turn to actual vehicle data to help us sort out the final answers.

Models with uncertain radius rod application include:

L48/M35: There is no indication in the assembly manual that the traction bar was not applied to all SS-350 models, but other Chevrolet documentation indicate that it was not applied to the L48/M35 SS-350/Powerglide combination. To counter this confusing background is the CRG vehicle database. For the thirty-one L48/M35 cars built 012A or later for which we have rod-related data, six are reported with round rod, and one with a rectangular service retrofit. The remainder (77%) have one or more brackets without a rod. There is no obvious pattern of application at this time.
L48/3-speed S/T: Application of a radius rod to the two 3-speed manual transmissions on the L48 was previously recorded as unproven, but doubtful - despite one GM powertrain summary promotional reference that indicates that radius rods were applied to the L48/3-speed. CRG previously had no data on 3-speed S/T cars with radius rods. However, a fairly original L48/M15 car built at NOR in 03B has surfaced that has the round radius rod. Without conflicting information, as of August 1999 we have changed the assessment to probable, based on this one datapoint.

The rectangular rod documentation in the 1967 Assembly Manual was revised after initial release; CRG has only the revised version of this sheet and does not know what the initial release looked like. The 1967 Assembly Manual UPC L78 Sheet A2 table of contents lists the radius rod sheet in the initial release as for "RADIUS RODS" (note the plural). CRG speculation is that the initial release of this sheet included both round and rectangular rod configurations, and that the April revision to the sheet was for the purpose of eliminating the round rod configuration from this sheet. Anyone with a copy of the original release of Sheet A8 of UPC RPO L78 should contact the CRG.

Chevrolet Dealer Service Technical Bulletin 67-T-30, dated 12-May-67, indicates the round radius rod was actually implemented in production on 15-Dec-66.

This same bulletin indicates that the second design went into production on 27-Mar-67. This date is a month after the initial release of the rectangular rod design in February and a week before the round rod sheet was "cancelled" (the rod itself continued to be used) on 06-Apr-67 and the rectangular rod sheet was "redrawn and redesigned" on 07-Apr-67. However, use in the L35/M20 pre-dates the production introduction date in the bulletin, since we have several L35/M20 datapoints from 02A to 03A, all of which have the rectangular rod. Also we know of a 03C Z28 with the rectangular rod installed.

On the difference in above dates for the start of the second design, our latest working theory is that the 27-Mar-67 introduction date only applies to the L30 and L48 vehicles. The scope of Bulletin 67-T-30 is to specify retrofit instructions for the rectangular radius rod for L30 and L48 4-speed complaint vehicles, and therefore it makes sense that the information in the bulletin literally applies only to these vehicles.

Note also that Bulletin 67-T-30 includes a reference to "... vehicles built after December 15, 1966, with provisions for a radius rod." Note that this quote doesn't say "with a radius rod," but says with "provisions" for one - leaving the door potentially open for application to vehicles with the brackets installed at the factory (the provisions) but without the rod in place.

Supporting Data

Chronological Data

In chronological order, here is what is recorded in the 1967 Camaro Assembly Manual, 3891775, combined with information from Bulletin 67-T-30, and key datapoints from the CRG database:

Table 2: Chronological Documentation of Radius Rods

                                     Drawn
    Document                         by
RPO Reference  Status  Date    Badge Notes
--- ---------- ------- ------- ----- --------------------------

M40 67AM UPC   new dwg 26Aug66 80921 M40 docs released include
    M40 all                          radius rod ears on the
                                     axle schematic, indicating
                                     that this change was
                                     contemplated even at
                                     beginning of production.

L48 67AM UPC   new dwg 08Nov66 80921 L48 docs released for
    L48 B3                           round rod - 1st design.

Z28 67AM UPC   new dwg 09Nov66 82586 Z28 docs released
    Z28 A2                           referring to L48 for
                                     radius rod.

L35 67AM UPC   new dwg 30Nov66 80921 L35 docs released
    L35 A2                           referring to L48 for
                                     radius rod.

--- Bulletin   start   15Dec66 ----- Round rod - 1st design -
    67-T-30    prod                  introduced on factory
                                     assembly builds.

--- CRG 7L0073 vehicle 12A     ----- Very early 12A LOS
                                     L48/M20 car with 1130
                                     engine does not have
                                     radius rod.

--- CRG 7L0082 vehicle 12A     ----- Very late 12A LOS
                                     L48/M20 car with 1210
                                     engine does have round
                                     radius rod.  This is
                                     the earliest vehicle in
                                     CRG database with a
                                     radius rod.

L30 67AM UPC   rev dwg 16Dec67 83638 L30 docs revised to add
    L30 A2                           radius rod reference to
                                     L48.  In the CRG copy
                                     this was never revised
                                     to delete the reference.

--- CRG 7N0218 vehicle 01B     ----- 01B Z28 car with 1229
                                     axle and round radius
                                     rod.

L35 67AM UPC   rev dwg 08Feb67 83999 Reference to L78 radius
    L35 A2                           rod(s) added to existing
                                     reference to L48.

Z28 67AM UPC   rev dwg 08Feb67 83999 Reference to L48 radius
    Z28 A2                           rod changed to L78.

--- CRG database vehicles 02A  ----- Several 02A L35/M20s
                                     exist with rectangular
                                     rods.

L78 67AM UPC   new dwg 17Feb67 81715 L78 docs released with
    L78 all                          reference to "RADIUS RODS"
                                     - plural.  Only sheets
                                     A1-A6 released on this
                                     date. Remainder release
                                     23-Feb-67.

L78 67AM UPC   new dwg 23Feb67 81715 Remainder of L78 sheets
    L78 A8                           released, including sheet
                                     A8 with initial L78
                                     radius rod(s). Note that
                                     L35 and Z28 have already
                                     released docs referencing
                                     this.

M40 67AM UPC   rev dwg 17Feb67 53999 M40 docs revised to
    M40 A3                           change M40 axle part
                                     number from 3910882 to
                                     3920569.  If this was
                                     for the purpose of using
                                     the 2nd design rod
                                     revised axle brackets,
                                     it didn't materialize in
                                     actual cars. Done same day
                                     as initial release of
                                     rectangular radius rod.

--- CRG 7N0219 vehicle 03C     ----- 03C Z28 car with rectangular
                                     radius rod.

--- Bulletin   start   27Mar67 ----- Rectangular rod 2nd design
    67-T-30    prod                  introduced on factory
                                     assembly builds. Scope of
                                     this bulletin may be limited
                                     to L30 and L48.

L34 67AM UPC   new dwg 30Mar67 84872 L34 docs released
    L34 A2                           referring to L48 or L78
                                     for radius rods. In the
                                     CRG copy it was never
                                     revised to eliminate the
                                     L48 reference. The reason:
                                     the L34 was not released
                                     in 1967 despite this
                                     documentation.
 
L35 67AM UPC   rev dwg 06Apr67 86318 L35 docs reference to L48
    L35 A2                           radius rod removed,
                                     leaving only the L78
                                     reference.  Despite this,
                                     all L35/M40 data
                                     uses the round rod.

L48 67AM UPC   cancel  06Apr67 86318 L48 docs cancel round rod
    L48 B3     dwg                   - 1st design - sheet for
                                     L48 (and, implicitly, for
                                     L30).  However the round
                                     rod continues in use.
          
L78 67AM UPC   rev dwg 07Apr67 86318 L78 docs for sheet A8 on
    L78 A8                           radius rod "revised and
                                     redrawn". New release
                                     shows only a rectangular
                                     rod.

Parts Manual Data

Supporting data from Master Parts Catalogue 721A, Dec 1971, Section 5.415:

L35, L78, and Z28 S/T cars initially had the round rod of PN 3914808.
L35, L78, and Z28 S/T cars later had the rectangular rod of PN 3920234 (but superseded by another rectangular rod service part number).
L30 and L48 4-speed, and L34 S/T engine used a 3920234 rectangular rod (but superseded by another rectangular rod service part number).
Note that despite the assembly manual documentation there is no record of the L34 having been released in 1967. L34 is not the only unreleased RPO to be mentioned in a GM manual.
All engines with THM400 transmission used the round rod.
CRG currently assumes, supported by vehicle data, that despite assembly manual cancellation of the round rod, the parts manual is correct on this point and there were sufficient round rods remaining for the factories to outfit the automatic transmission cars with round rods. This has been verified on the L35/M40 and L48/M35 cars in the CRG database.
Upgrade kits were 3929559 to update from round to rectangular rod, and 3929559/3929299 to go from scratch to the rectangular rod.
Note the absence of an upgrade kit from scratch to round rod.
No mention is made of the round rod on L30 and L48 4-speed applications.
CRG believes that Bulletin 67-T-30 forced the issue and caused all of these to be changed to rectangular rods in the event of a need for service.
Small-block 3-speed standard transmission references are conspicuous by their absence.
Powerglide transmission references are conspicuous by their absence.

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