So my 1968 Firebird 400 with stock three speed TH400 has the original 8.2, 10 bolt with 2.56 gears. But its an open diff peg leg. Now don't get me wrong. I love my 2.56 gears on the highway since I do a lot of highway cruising. Off the line my 400 has so much torque off the line that my "wheel" just spins and smokes! It would be cool if both wheels did it though...LOL.
Is there anyone that has turned their open diff into a limited slip diff. but kept their 2.56 gears?

19 hours ago, 1968BLACKBIRD said:

So my 1968 Firebird 400 with stock three speed TH400 has the original 8.2, 10 bolt with 2.56 gears. But its an open diff peg leg. Now don't get me wrong. I love my 2.56 gears on the highway since I do a lot of highway cruising. Off the line my 400 has so much torque off the line that my "wheel" just spins and smokes! It would be cool if both wheels did it though...LOL.
Is there anyone that has turned their open diff into a limited slip diff. but kept their 2.56 gears?

Well, first why do you want limited slip? If it's to get both tires to bite? That's not the function of limited slip. Like a posi rear end, those rear ends are meant to give traction to the wheel that has the best traction. So on wet roads or snow covered roads is when they are meant to excel. Not dry roads. In fact 9 times out of 10 if you repeatedly do hard excellarated starts with these kind of rear ends you will break the components that allow these rear ends to work that way & will than revert to a standard open rear end.

You can make a stock 10 bolt open rear end actually bite better on dry roads than a limited slip, but you need to know how to setup the suspension correctly.

Yes there are kits thst convert your open diff to a posi.

You have two basic choices:

1. You can purchase a complete posi unit from companies like Eaton, Yukon, Richmond, etc.

2. You can Google “lunchbox locker” for the 8.2 GM 10-bolt. You essentially replace your diff’s spider gears with the parts in the kit.

Neither solution is cheap. However the lunchbox locker means you don’t have to remove your entire ring and pinion assembly. You only remove the spider gears.

Edited August 30, 2025Aug 30 by Frosty

i stuck a eaton posi in my 72 and i love it !!!

before it was a boring one wheel spinner !!!... in fact it was embarrassing, i would get stuck on wet grass with one wheel spinning !!!

now she goes sideways on command !!!and no more ... pointing fingers when i see a bannas peel

Ceasar, i bought the eaton and used my oringinal crown and pinion with it ... cost me a grand back in ..... ummm. 98 i think it was

last indian, please explain your thoughts about setting up the geometry ? these girls are on leaf springs ... what and how can you change that ? pinion angle ?

Edited August 30, 2025Aug 30 by 64 kiwi boni

15 hours ago, 64 kiwi boni said:

i stuck a eaton posi in my 72 and i love it !!!

before it was a boring one wheel spinner !!!... in fact it was embarrassing, i would get stuck on wet grass with one wheel spinning !!!

now she goes sideways on command !!!and no more ... pointing fingers when i see a bannas peel

Ceasar, i bought the eaton and used my oringinal crown and pinion with it ... cost me a grand back in ..... ummm. 98 i think it was

last indian, please explain your thoughts about setting up the geometry ? these girls are on leaf springs ... what and how can you change that ? pinion angle ?

Kiwi, this is going to be long, sorry! Actually leaf springs are an advantage for this. I thought I had written about this before, but maybe that was elsewhere not sure. 

This all goes back to my “69” Z/28 those cars came with the famous GM 12 bolt. Mine had the bv code which was a 4:10 posi unit. So why would I change a 12 bolt 4:10 posi rear end out for a 10 bolt?! For those that have followed, remember I raced this car both drag & road course. I knew several others with GM 12 bolts. Some had posi's some didn't, but almost to a man everyone either broke the position unit, AKA didn't work anymore or broke the ring gear. The latter happening more often than not.

It didn't take me long to figure out why! When you look at a 12 bolt ring gear & then a 10 bolt there is no comparison in their strengths. The 12 bolt ring has a thinner wall section between the bottom of the tooth profile to the threaded mounting hole! Likewise the spacing of the 12 bolts mounting holes are much closer together than the 10 bolt & last the total thickness of the ring gear is thinner for the 12 bolt. All this led me to blueprint a 10 bolt rear end non posi with 4:10 gears. This, along with substantial suspension changes made a huge difference in performance. 

Blue printing a rear end is no small task. This requires everything from axle rework to carrier rework . Which includes the carrier bearings, spider gears & their shims! 

So what about posi vs non posi? Well first off a lot of people misconstrue the purpose of limited slip/positraction. As previously stated, it is not for hard acceleration, burnouts & the like. It's for what you said Kiwi. A condition where the wheel with the best traction bites. Of course when both wheels have equal bite both grip! Yet with a properly blueprinted rear end & with a suspension properly set up, the same will happen.

At LZ we did enormous amounts of axle testing of every kind. The only difference in posi vs open end are small clutch plates/spring packs. Both types of axles have spider gears. Both interface with the ring gear in exactly the same manner. When one axle is loaded more than the other the spring packs are appropriately triggered putting the clutch packs on the load side to lockup like a transmission clutch pack does thus transferring power to that wheel! When there is an imbalance, as is usually the case with dry road burnouts. The clutches load & unload very very quickly, thus producing twin tire marks, but this is very hard on the clutchpacks. Which is why they eventually break or stop working.

So the main problem is balance! Both in the axle build as well as the suspension setup. Out of the factory axles are obviously mass produced. Meaning they are not balanced. It takes an incredible amount of time & patience to balance a rear axle's internal components. Placement of the carrier, which affects pinion & ring clearance. Spider gear shimming has to be equal, which in turn affects ring & pinion gap. Axle lengths need to be addressed as does the end spacing of each axle to the carrier pin. Again suspension setups out of the factory are controlled only within a window and few if any ever fall into the perfect window & if they did odds are the axle in the car didn't. 

In any car the suspension rarely sits straight with the body & axle they are usually canted however so slightly, still canted! Leaf spring suspension are usually better than coil. Yet under hard acceleration from a dead stop leafs are worse! My “69” under that condition would rotate my rear axle upward towards the floor 7 and a half inches and hit the floor board. Literally making my leaf springs into a S shape! In this process one side of the axle was going forward farther than the other. All of this was fixed by blueprinting the 10 bolt axle internally. Gusseting the outer housing & making a torsion bar that spawned from spring perch pad to spring perch pad. And topped off with traction bars that were unlike any that were available. Leaf spring traction bars typically mount to the spring perch pad & snub out on the forward part of the leaf spring! To me this seemed like a poor design. So I built a set that mounted securely to the rear axle tubes inboard of the spring perch. The traction bars were set 1 ½ “ away from my full frame!when the axle would start to rotate it would only move that distance and stop. 

The yellow parts are the traction bars.

Edited August 30, 2025Aug 30 by Last Indian

Gees last Indian , I wish you had some pictures of how you set up the traction bars !

Please explain how the torsional bar between the leaf pads worked … I can see it in your picture

It looks to be only 3/4 inch ???

On 9/4/2025 at 5:31 AM, 64 kiwi boni said:

Gees last Indian , I wish you had some pictures of how you set up the traction bars !

Please explain how the torsional bar between the leaf pads worked … I can see it in your picture

It looks to be only 3/4 inch ???

Kiwi I will try to explain what was done, why it was done, what actually occurs & how that impacts traction & handling.

Kiwi, this seems like a lifetime ago & in those days I didn't really do a lot in the way of pictures, but I'll show what I have. The first modification occurred around 1973 when the 12 bolt started to have issues from the racing. Being 21, married & a new baby & limited funds, the bone yard became a friend. That first mod was the 10 bolt. So some of the pictures will reveal those days of meager beginnings! You will see big stones, that was my driveway! I laid on to do all my work! I was dedicated though! 

OK so an open axle lays one patch of rubber, usually the drivers side! Why? Well the standard rear ends usually have the carrier with the ring gear on the driver's side of the axle. This, because of physics, makes the passenger side axle longer. Which in turn means it's the longer lever/fulcrum. So in reality the weight on the passenger axle assembly has less weight applied to it in the fulcrum load factor equation. This causes that tire to spin due to the leverage (fulcrum) factor! AKA less force applied per tire circumference ultimately. 

What aids in changing this for an open axle is this. Believe it or not, even the axle housing itself under heavy load, flexes. And a leaf spring suspension is by far worse than a coil suspension due to how & where the force is applied. This part gets extremely involved, so I will leave that for later if you wish to hear more on it. So the first order of business was air shocks! Why? Air shocks work differently than a standard shock & I know you know that, but did you ever think exactly how that is? First air shocks have as much resistance up as down when the wheel is in contact with the road. Next you can't compress air nearly as efficiently as oil. So most of the air shocks cushion comes from the air filled rubber bladder. Next, but literally during the same initial build was, what I end up calling them, the ladder tracks. Because they were kind of a traction bar, but also kind of a ladder bar. These two components made a huge impact to the than modified 10 bolt axle. The first pic is a while after the first build, thus the red bars not yellow & no gussets or torsion bar.

Side note the rear cover I made from 8, 1" aluminum plates welded together machined into inside diameter configuration shape. Rewelded, machined to the outside configuration shape. Than rewelded, filed sanded & polished to what you see. This is about 1976-77 not the SS welded exhaust.

I'll explain more shortly.

Edited September 6, 2025Sep 6 by Last Indian

So , the air shocks added downward force and the traction bars stopped the twist action ?

On 9/7/2025 at 4:56 AM, 64 kiwi boni said:

So , the air shocks added downward force and the traction bars stopped the twist action ?

Well, yes & no! There are a variety of air shocks designs, but in basic principle air shocks do an excellent job in keeping the tires in contact with the road as opposed to a traditional shock. While, depending on psi setting vs weight, they can often give a more comfortable ride. That said, to a degree yes they create more downward force on the suspension, or more precisely they hold the suspension down in place better! Most people perceive a suspension as something that is part of a homogeneous part of the car body that simply absorbs rough road characteristics. I.E. one fluid part! Nothing could be further from true. Today, experts stress the importance of unsprung weight! Yet at the same time they stress this, car suspensions carry more unsprung weight today than ever before! If there is any doubt, just look at the weight of today's wheel & tire combinations vs the “70"s, as one example. Yet today's plain Jane cars handle better than the “70"s era dominant handling cars did. So are they right? You can guess my answer.

Yes, the simple answer for the traction bars. They do stop the axle twist! But it's more involved than that! You may remember some of my posts about torque steer. An open axle suffers from the same dilemma as fwds tend to have! In any rear end one axle assembly is longer than the other. Not the actual axle shaft, but the axle assembly. This, as I said earlier, is the axle shaft, carrier, spider gears assembly & that assembly length to the ring gear as opposed to the other axle assembly is longer. This disparity causes a deflection in the longer axle! That deflection in turn slows that axle's rotation. This action causes torque steer! It's the simple principle of a track drive! The inequality of the weaker side suspension as well as the disparity in axle lengths will cause that side to lose traction. Mostly because when the axle over rotates upwards the weaker side actually lifts! Of course this is with good traction, on slippery ground it's just axle lengths.

Now the issue as I stated before with traditional traction bars is they mount to the spring perch & snub out on the forward part of the spring. While this design has a limiting factor on axle movement it has its own flaws. Those flaws include the fact that when this type is used it is not stopping axle rotation! It's trying to stop spring wrap! Which in turn theoretically will reduce axle rotation, but does it? Some, but what actually happens is the forward part of the spring curves upward, while the rear part of the spring curves downward! Voila, spring becomes an S shape! This action shortens the spring & lifts the axle upward & it happens to a larger degree on the long axle side, aiding in less traction.

My ladder tracks functioned completely differently. They mounted to the axle tube inboard of the spring perch, inline with the frame. The rubber snubber was about 1 ½” to 2”from the frame. The portion of the ladder tracks that attached to the axle tube was two halves bored to the exact size of the tube. Then those two halves were spiral crosscut to increase bite. The halves had about a 1” gap top & bottom between the halves. The bolts used were ½” fine threaded harden Allen head & finally there were two ⅜” set screws with serrated face at 90 degrees to the through bolts. This design absolutely stopped axle rotation! Which in turn stopped spring deformation AKA S shape. All of this efficiently increased right side traction. A few years later I came up with the idea of building a torsion bar assembly between the spring perches, with connection at the pumpkin to further limit actual side to side disparity rotation. Along with left & right side gusset sleeves on the axle to decrease axle tube defection as much as possible.

Some might think this a little insane, but they would need to understand this! The car at 600 hp & a special built tranny would shift from 1st to 2nd at 50 mph & instantaneous pick the front wheels up to almost full suspension extension & lay two patches of rubber! That is a hell of a rear axle load!