New Zealand Transam

New Zealand Flag1986 TransAm.

A New Zealand Based Site.

Return Home A New 360 Fuel Injection Steering Column Photos Links

Transam Bird


I picked the Diff' Head up from SLP Performance for $99.00us! They were selling them as take off's from new 1990 - 1992 GTO Firebirds that were retro fitted with a clutch type L.S.D option. Having done little or no mileage the unit was as good as brand new. Special thanks to Steve of Ace Cobras in California for bringing it, and 2 new axles, to New Zealand for me, it would not have been economically viable otherwise (They only ship via U.P.S)! Original ratio on the open Diff' was 2.73:1 which was upgraded to 3.23:1 along with a change from the 26 spline to 28 spline axles from Tom's Differentials. Also fitted was an AAM aluminium cover which provides extra strength, cooling and lubrication to the side bearings.

T2Color T2Seperation T2B
T2 cut away showing gear arrangement. T2 Technical Information Here... T2 carrier in pieces.


The original Differential ratio for most GM 10bolts can be determined by reading the stamped letter/number located on the right-hand Axle Housing. Mine was 2HP which cross references to 2.73:1 Std. Although this is listed as a 1987 Diff' I assume it was also used in models produced at the end of 1986 as mine was. A list of all Axle codes, dating 1982 - 1992, can be found here... Furthermore it should be noted that GM 10bolt Diff's use 2 different types of carrier. Ratios up to 3.08:1 use the smaller Ring Gear of 7.5inch. Larger ratios of 3.23:1 and up (or lower geared), use the slightly larger 7.625inch Ring Gear and thus a different series (Series 3)carrier. All fit the same housing. Some really good information on installing and setting up of Ring and Pinions can be found here at Randy's Ring and Pinion or here in PDF.


Kick Ass Axles


As far as I can tell, all 10Bolt carriers used from 1990 onwards require 28Spline axles as opposed to 26Spline, which mine definately was. This required an Axle and Ring & Pinion change in order to fit the new T2, Series 3, carrier. Advantages are two fold...

1: Extra strength of 28Spline over the original 26Spline. Particularly with the added power of the 350 engine.

2: Up the numerical ratio. Gearing down an overly high driven ratio that was, in reality, just hell on the clutch.

In keeping with the charactor of the vehicle, the 3.23:1 gearing is a good ratio when using the original 15inch rims and redline of around 5000RPM. Acceleration and economy are both good and, strangely enough, the change in gearing actually improved speedometer accuracy!

Axles used: Tom's KA part number is KAC7-7. Spec's are 90-92, 7.5 & 7.625inch, 28 Spline, Length of 30-3/8inch.


I thought this would be a good addition to the 10bolt rebuild. The cover comes complete with bolts, magnet, drain plug, instructions and cutout templates for modifying the axle housings. The templates are used to mark the axle housing for oil passage slots. These are then ground for each side, The cover has a channel starting near the drain plug which directs oil to both main side bearings via the previously ground slots. This provides extra lubrication and cooling. Pretty damn slick if you ask me! Definately worth the small effort if you have the diff' already stripped down for a carrier change.

AAM Cover


Another mod' installed was the use of a solid pinion spacer, as opposed to using the standard crush type spacer. This helps to prevent the Pinion riding into the Ring Gear under heavy load. A lot of time was spent researching and then setting up the Diff' and gears, not to mention the aquiring of parts and special tools such as an InchPound torque wrench for checking pre-loads etc. Not a simple job but all the information and bits are out there and I found the task quite enjoyable and very rewarding in the end.

Driven daily, now, for a few years and often not so 'gently' since the job was done everything seems good. The car now has as much traction under power and cornering as you could ask for (Within the constraints of the Chassis and suspension of course but that's another chapter to come...).


Page TopReturn Home