CRG Research Report - 1967-69 Camaro Axle Carriers and Associated Parts

1967-69 Camaro Differential Carriers and Associated Parts

Author -

Reviewed by the CRG
Last Edit: 26-Mar-2020
Previous Edits: 11-Mar-2020
Original Release: 11-Mar-2020

Introduction

The rear axle differential allows the outer drive wheel to rotate faster than the inner drive wheel during a turn via a planetary gear train. The differential carrier case (often commonly referred to as the carrier) in this gear train is located inside the center section of the axle housing (differential carrier and axle tube assembly). It transfers motion from the driveshaft to the axle shafts using a ring gear and pinion shaft. The carrier is held in the housing with 2 caps that have 2 bolts each. The axle shafts insert into the carrier, and the axle shafts are held in with c-clips. There are two main types of stock carriers, open and limited slip. Limited slip carriers have a set of clutches that effectively connect the two axles together so both wheels will spin at the same rate. The clutches still allow for different axle speeds while going around corners. Here is an excellent video that explains the operation of the limited slip differential. GM started offering limited slip differentials as early as 1957. These were sold under several names. Chevrolet used the trade name Positraction, which is often shortened to “posi”. Standard axles typically had an open (non-posi) carrier. Positraction was available at an additional cost (option G80), and was mandatory on the 4.10, 4.56 and 4.88 ratios. Positraction was optional for Z28's with 3.73. In 1969, positraction was also a requirement on all non-Z28 cars with the 3.73 ratio. (In 67-68, it appears that positraction was not a requirement with the 3.73 ratio for non-Z28 cars.)

If an axle has an open carrier, then it can be converted to a posi by installing a posi carrier of the same series. The pinion does not have to be removed, but the ring gear does have to be swapped over from the open carrier to the posi carrier. The backlash and side shims will need to be checked and may need adjustment. Open carriers can be converted to posi by installing aftermarket parts. These would include a spool or a mini-spool or kits such the POWERTRAX Lock Right Locker. However, these are generally not as street friendly as an original posi.

The left diagram above is a rear axle cross section from a 1968 GM parts manual. This shows some of the parts that will be discussed in this report. The image on the right is an exploded view of the positraction components from a 1968 GM parts manual. The Eaton type posi was used in all 1967 to 1969 Camaros, and is shown in the lower part of the image on the right. The upper part of the image is for a Dana posi. Click on each image to enlarge.

Carrier Differences

First let's examine the visual differences in the carriers. Posi carriers have 4 springs and 2 plates inside the carrier that preload the set of clutches. Open carriers do not have these springs, plates and clutches.

The differences on the non-ring gear side or passenger side (as installed in the housing) of the carriers are shown below. The posi carrier requires extra space for the clutches. There is a casting date in Julian format on this end of the case. In the example below, the casting date of 44 = February 13. There is no year indicated in the casting date. The alternate location for the assembly date of the posi unit is sometimes located on the balancing boss on this end of the case.

The external castings of posi carriers are different. The ring gear side of posi carriers are shown below. The amount of webbing increases as you go from a 2-series carrier to a 4-series carrier. This helps make the 4-series carrier the strongest and the best for high torque use. A coded assembly date is usually stamped on the outside edge of the ring gear flange of the case. The format is month, day, and calendar year using a letter, number, letter format. The one shown below is D 11 J, which corresponds to April 11, 1969. The author is fairly certain that J was used for the year 1969. More research is in progress to verify the letter codes for other years, but it appears that G was used for 1966, H for 1967 and I for 1968. Again, these are calendar year.

COPO carriers and internal parts

The 1969 COPO 427 cars were equipped with a special axle. The 4-series carrier (GM part number 3916234) used in these axles had the same casting number as the regular 4-series carrier, but the internals were not the same. The COPO carrier was machined differently, so it could handle larger internal gears. The axle side gears used in COPO units have 16 teeth. Standard axle gears have 18 teeth. The pinion gears used in COPO units are noticeably larger than those used in standard posis, but both have 10 teeth. The pinion gears used in COPO carriers are actually the same as those used in standard open carriers. The clutch plates are the same for the COPO and regular 4-series posi units, with 18 plates (9 per side). There are some other parts that are specific to the COPO carriers. These include different springs (natural steel color - rated at 75 pounds each) and different spring plates. COPO spring plates have a round hole, while standard posi spring plates are oblong. Most COPO carriers have a "small window" on the side of the case opposite to the large opening where the clutch pack is inserted into the case. See reference pics below for examples of the internal parts.

Carrier Groups

There are two carrier groups for 10-bolts and three carrier groups for 12-bolts. Those are outlined below according ratio. 2-series and 3-series carriers were available in either open or posi. There were no original 4-series 10-bolt open or posi carriers, and there were no 4-series 12-bolt open carriers.

Chevy 10-bolt car (8.2"): 2-series is 2.56 and 2.73. 3-series is 3.08, 3.36 and 3.55 (other ratios available in aftermarket).

Chevy 12-bolt car (8.875"): 2-series is 2.56 and 2.73. 3-series is 3.07, 3.31, 3.55 and 3.73 (3.42 and 3.90 in aftermarket). 4-series is 4.10, 4.56 and 4.88 (5.13 GM over the counter and others in aftermarket).

Carriers have different offsets for the ring gear location in the housing. The 4-series carrier has the ring gear closest to the center of the housing, and thus the size of the gear end of the pinion is smaller. 2-series carriers have the ring gear offset farthest toward the drivers side, and the gear end of the pinion is the largest (compared to 3- and 4-series gear sets). See the pinion pics farther down in this report.

The left photo below shows the two sizes of the window on the backside of the case (the side opposite of where the preload pack is installed). There is a large window version and a small window version. The small window cases are associated with most 4-series carriers and some 3-series carriers. The large window makes it easier to install the internal gears.

Ring and Pinion

The number of teeth on the ring and pinion determine the ratio. Below is a chart with the individual part numbers. If new gears were needed, these were sold as a set with a different part number.

Stock 12-bolt axle ratios included 2.56, 2.73, 3.07, 3.31, 3.55, 3.73, 4.10, 4.56 and 4.88.

Not all axle ratio combinations were used in production. Specifially, in 1967, the 2.73 posi axle was of such low volume that all 2.73 axle ratio with positraction were 12-bolt axles. This was independent of other options on the car. Even some 6 cylinder cars have been observed with 12-bolt 2.73 posi axles.

The 4.56 and 4.88 ratios were available as standard options in 1967 and 1968, but not in 1969. Everyone thinks of the 427 with 4.10 axle when COPO is mentioned, but in 1969 the 4.56 and 4.88 ratios were only available with COPO 9511. One car in 1968 has been found with COPO 9511 with 4.88 gears. All of the COPO ring and pinion sets received a special heat treatment and different part numbers than the standard gears.

As noted earlier, the size of the pinion gear varied based on the gear ratio. In general, the pinion gear size gets smaller as the gear ratio increases numerically (2.73 to 4.88). See pic below.

The information stamped on the edge of the ring gear is typically in the form of part number, pinion tooth count, ring gear count, one or two digit month and two digit year of manufacture. The same information is stamped on the large end of the pinion shaft, but the order may not be exactly the same.

Internal Carrier Parts

There were different versions of some of the internal parts. Several different clutch spring rates were available. Stock springs were natural steel color and rated at 50 pounds each. Replacement springs had green paint and had GM part number 3890899 - Eaton ED 7976. Replacement springs with yellow paint were rated at 75 pounds each with part number 3890860 - Eaton ED 72043. Higher rate springs were available, including some thicker springs with yellow paint rated at 100 pounds each. The springs in aftermarket Moroso Brute Strength posi units are noticeably thicker with fewer coils, rated at 200 pounds each. The Brute Strength spring plates are also thicker. See picture below for comparison, but note that the green GM spring is not compressed. The standard spring plates were part number 3869230.

The clutch pack plates and discs are located between the differential side gears and the carrier housing. The clutch packs create friction between the side gears and the carrier housing and limit the speed differential between the side gears and the ring gear. These are stacked by alternating the two types (5 eared plates and 4 splined discs per side). The splined discs have teeth on the inner edge that lock onto the axle gear. The other plates have 2 ears that lock into the carrier with power lock disc guides (also called case bushings). The plates all have a "waffled" type surface. Reference the diagram of a positraction carrier image earlier in this report.

Clutch pack rebuild kits are available. These were originally part number 9781862, replaced by 483722 in December 1971. The kit had 4 case bushings (power lock disc guides) and the plate/disc pack. However, the replacement clutch discs had slots in them, unlike the original factory parts - the original clutch pack discs were solid.

Carbon fiber rebuild kits are also available. These carbon fiber kits have 6 carbon fiber discs and 8 metal plates. The carbon fiber discs are similar in size and shape to the originals. It appears that there were 2 different clutch packs for the 8.2 ten bolt posi units. The part number used for 1965 to 1968 units was 3888070. Further research is needed on the 10-bolt clutch packs.

Other Parts

As mentioned earlier, inside each carrier housing there are pinion gears and axle gears. The axle gears have splines inside and the axles fit into these splines. Twelve bolt side gears (and axles) have 30 splines, while 8.2 ten bolt carrier side gears and axles have 28 splines. C-clips hold the axles into the side gears and carrier.

There is a bolt on the non ring gear end of the carrier. This bolt holds the cross shaft (differential side pinion shaft) inside the carrier. The technical name for the bolt is diffential pinion shaft lock screw. In order to remove the cross shaft or pinion gears, this bolt must be taken out first. The length of the bolt for posi carriers is about 2 5/8 inches long, not including the head of the bolt. The bolt for open carriers is shorter, about 2 inches in length.

The pinion yoke is located at the most forward part of the axle housing and connects to the driveshaft. The yoke slides onto the splined end of the pinion shaft and is attached to the pinion with a washer and nut. The size of the u-joint used with the pinion yoke depended on the application. For 1967 and 1968, all 12-bolt axles, except those with the TH400 transmission, used a 1310 u-joint with a pinion yoke that had part number 3878972 (number on the yoke is 974). Camaros with the TH400 transmission had 12-bolts with a yoke that used a 1330 u-joint, part number 3879208 (number on the part is 9210). In 1967 and 1968, the 1310 u-joints were attached to the yoke with u-bolts, washers and nuts while the 1330 u-joints were attached with straps and bolts.

In 1969, a 1310 u-joint was used for all applications, and it was attached with straps and bolts. This pinion yoke - also used in 1970 - was part number 3954393 (number on the part is 4394). All 12-bolt pinion yokes have 30 splines, while 10-bolt pinion yokes have 28 splines. The part number for the 10-bolt yoke, for all three years, was 3878968.

The 9210 pinion yokes used with the 1330 u-joint are visibly wider than the 974 and 4394 yokes. The 4394 yoke used in 1969-70 is similar to the 974 pinion yoke in size, but it uses bolts and straps for driveshaft attachment. Yokes had a natural steel finish (they were masked off when the axle was painted). Hardware was normally natural steel or phosphate coated.

A fill plug is located on the passenger side of the center section of the axle housing. 12-bolt rear ends typically use about 2 to 2.5 quarts of gear lubricant and one bottle of limited slip axle lubricant additive (if it has a posi unit installed). Fill until fluid reaches the bottom of the hole with the car sitting level.

In 1967, the fill plug was about 1 inch in diameter (3/4"-14 NPT). In 1968 and 1969 the fill plug decreased in size to about 3/4 inch in diameter (1/2"-14 NPT). The exact date that the plug size changed is not known, but seems to be in the late summer of 1967. Posi plugs for both the 12-bolt and 10-bolt had a lip on the plug to retain the red plastic tag. Non-posi plugs do not have a lip. Plug part numbers for 1967 were 3866433 (posi with tag) and 3779764 (non posi). Plug part numbers for 1968 and 1969 were 3931171 (posi with tag) and 3923429 (non posi).

All axles with positraction received a red plastic tag on the fill plug. The tag used for 1967 to 1969 Camaros has the following wording "USE POSITRACTION DIFF. LUBRICANT ONLY". The hole in the tag is slightly larger in 1967 compared to 1968 and 1969, because the 1967 plug is larger.

Part Failures / Racing Use

The original differential parts held up well to normal street abuse. Failures were more common at the track with high RPM launches and sticky tires. There were incidents of axles or c-clips breaking while drag racing, including on ZL1 car #1 and at least one 69 Yenko Camaro. Case failures happened, but were not common. Some people think the small window cases were stronger, due to having a larger amount of metal in the case. Others believe the small window cases were weaker, because the hole was rough cast and could be an area where stress fractures could occur.

There is a common misconception that COPO rear ends used larger diameter axle bearings. All first generation Camaro assembly plant installed axles used the same size axle bearings. Larger diameter axle bearings (and larger diameter axle shafts at the wheel end of the shaft) were found in 12-bolt axles used in trucks, full size cars (such as Impalas) and in over the counter HD Service Package rear ends. The larger diameter axle shafts and bearings significantly improved durability.

Jerry MacNeish, who has 50 years experience drag racing and working on rear ends, says that service parts did not seem to hold up as well as original components and had higher failure rates. Problems were noted with axle shafts, pinion gear shafts, internal carrier pinion gears, and the "slotted" type service clutch packs. Because of this, Jerry turned to the November 1970 Super Sport Equipment parts book and learned of parts like the COPO-only 3916234 carrier and the 3955877 carrier (previous part number was 3915477 until May 1972). The GM 234 carrier tended to be favored by drag racers. Original over the counter HD Service Package rear ends used the 477 carrier (877 after May 1972). The GM 477 and 877 carriers had 11 clutches per side (22 total) and smaller heat treated internal gears. It tended to be used mostly by Trans Am road racers and was also sold by Moroso as their "Brute Strength" carrier. Special heat treated R-50 High Impact ring and pinion sets were available in the 4.88 and 5.13 gear ratios (GM part numbers 3961192 and 3961195).

Acknowledgements

Thanks to many of the CRG members for their assistance, review and support with this report. Special thanks to Freddie, aka "big gear head" and Jim Mitschke - JDRace - for permission to use of some of their photos and their review of some of the technical information.

If you have information that differs from what was in this report, I would like to hear from you. This report was done with the best information available, but, as always, there is room for improvement.

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

Type	2-series	3-series	4-series
open 10-bolt	3910821	3878439	n/a
posi 10-bolt	ED32118	EDB30116	n/a
open 12-bolt	3869789	3852989	n/a
posi 12-bolt	ED32088	30140PM1	EDB30174

Ratio	Ring Gear	Pinion	Set
4.10	3916226	3916228	3917971
4.56	3916230	3916231	3917973
4.88	3916232	3916233	3917972

2-series 12-bolt posi	3-series 12-bolt posi	4-series 12-bolt posi