Wednesday, March 20, 2013

The Diff

The Classic Revival round-tube chassis is built to take the ZF diff unit as fitted (amongst others) to the VE Commodore. There are both LSD and non-LSD options available for this unit, along with a large number of ratios. While a second hand unit can be had quite readily from a wrecking yard or ebay, the ratio options are quite limited. This is because with a manual transmission either a 3.45:1 or 3.7:1 is preferred. Manual SS VE Commodores came with 3.45:1, while manual HSV's got 3.7:1. The second-hand 3.7 units from the HSV's get snapped up quick smart, and go for (in my opinion) to much money considering their mileage and condition.

Back in the day I had a VX SS LS1 Commodore, and although the diff in the VX was different to the VE's ZF unit, it had the same ratio of 3.45:1, and I was not 100% happy with it. When paired with a T-56 manual transmission I feel that a 3.7:1 ratio is a better fit. A diff from a wrecked HSV is hard to find, as they are snapped up quickly by Commodore owners looking for a cheap and easy ratio change for their SS. I never really wanted a second hand unit anyway - I view my Cobra as a new car, and I want to use new parts wherever it is feasible. So I picked up a new 3.7:1 LSD from Holmart, new old stock.

The front pinion flange on the diff needs to be swapped out because the three-pronged flange that Holden fit has quite a large radius and interferes with the round-tube chassis.You might wonder why the chassis was not made to fit the flange, but it would have meant altering an otherwise straight (and hence strong) structural piece of the chassis - a better solution is to change the flange for one that clears the chassis, in this case it is a Ford BA flange, part number BA4851A.

After removing the pinion nut it's fairly easy to pull the flange, no doubt a well bedded-in unit may have been harder to remove.

The next problem is that the dust cover on the new flange sits a couple of mm to low, and touches the diff housing. I pressed the cover on a little bit further in my workshop press, but that made it loose so a couple of tack welds sorted that out.

The last problem is that the new flange tube outside diameter is very slightly smaller than the original, this means the oil seal does not contact the flange and it would possibly leak oil. So the old seal gets carefully levered out so as not to mark the seal seating surface.

Then a new seal SKF part number SKF21283 gets knocked in, the old VE rear wheel bearing came in useful again for knocking the seal in square over the pinion.

Even though this unit is brand new, I went ahead and purchased a new pinion nut from Holden, as these are a one use item as the nut stretches when tightened. The nut is GM part number 92194936 and costs $27.25 - no that is not a misprint - it really is a $27 nut.

Now the unit is ready to go into the chassis, but that is easier said than done. As I am not removing the body from the chassis during this build, it was very awkward to fit the diff unit up into the chassis. It doesn't look difficult, but it soon becomes apparent that it needs to go in at a very odd and very steep angle, not so easy when the car is on stands and the diff weighs over 40kg so you cannot simply lift and tilt it by hand from underneath the car. I jacked the diff up while simultaneously using a strap over the chassis to tilt the diff to quite a severe angle to get the pinion over the chassis rail and line up the three mounting locations. Then in with three M12 grade 8 bolts, and it's done. This pic is after the angle shenannigans, ready to bolt-up.

I contemplated changing out the factory rubber bushes for an aftermarket polyurethane type. Manufacturers such as Pedders and Superpro have a few different stiffness' available, but some research leads me to believe that these polyurethane bushes in this diff in the Commodore community have resulted in some people complaining about ride harshness, and in a Cobra that weights 600kg less than a Commodore this is likely to be even more pronounced. So we're staying with the factory rubber mounts for now, I can always upgrade them later if the need should arise - although that will require the tricky job of removing and refitting the unit to the chassis!

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