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 Post subject: The Gemini differential bible - brakes, LSD, swaps
PostPosted: Wed Jul 25, 2012 11:15 pm 
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Joined: Sun Aug 24, 2008 7:12 pm
Posts: 1938
Location: Windsor Gardens
Car(s): TX G180W+T, TE G180W, TG G180Z, Piazza 4ZC1T +more
This thread is intended to provide some information on factory-fitted Gemini diffs, the swapping of parts, and a little about modification and aftermarket parts. It’s been compiled from the posts of the knowledgeable and helpful of OzGemini! If you see a mistake, or have more to add, please reply or PM me.



Australian-delivered T-series Geminis came out with three distinct types of diff:

Borg Warner 68 (Petrol TX and TC models of all body type)
Small Salisbury (Petrol TD, TE, TF and TG models of all body type)
Diesel/Import (Australian diesel Gemini, Japanese Isuzu Gemini, possibly European Kadett C?)

And regardless of type, diffs are suited to either sedans and coupes, OR, wagons and panel vans.

The difference between body styles is found in the diff's shock absorber mounts. Sedans and coupes run different rear shock absorber mounting points to wagons and panel vans. There may be other differences, but this is the main thing to note. Essentially the diffs are the same internally between a given model of sedan and wagon, but cannot be swapped without fabricating or modifying mounts (and why would you bother). Furthermore, international diffs may or may not “bolt up” – accessories and mounts may differ, I don’t think many people have tried it!

For example, a Salisbury diff out of a coupe will be identical to a Salisbury diff out of a sedan, but will be different to a Salisbury diff out of a wagon.
A Salisbury diff out of a wagon will be identical to a Salisbury diff out of a panel van, but different to a Salisbury diff out of a coupe. The rule is the same for Borg Warner.

Now, to clarify the types of diff mentioned above.



Borg Warner 68

Ratio: 3.89:1
Found in: Petrol TX and TC models of all body type (sedan, coupe, wagon, panel van)
Axle splines: 23
Bolts on diff cover: 8
Bolts on crown wheel: (unconfirmed)
Factory swaybar provision: No

General info:

These cars are also pre-RTS (ie not Radial Tuned Suspension). These cars do not have the provision for a factory rear swaybar, and were sold from 1975-1977 (roughly). Remember that TD Geminis (which also have the early body style) run the later Salisbury diff! A Borg Warner diff can be identified by the rear cover/hat, which will have 8 bolts, and a very octagonal outline/shape. These diffs are generally believed not to be as strong as the Small Salisbury, but are more than ample until you start pushing big power (and/or abusing very grippy tyres).

Brakes:

The drum brakes originally fitted to these diffs were extremely prone to locking up well before the front brakes, resulting in poorer breaking distance, and potentially surprise oversteer during emergency braking. The handbrake however, is *extremely* good. The brakes can be unbolted from the axle housing and swapped with those on the Salisbury if desired, or the VB-VR Commodore 6cyl disc brakes can be bolted directly onto the diff (there’s a thread explaining this process). Diesel/import/Piazza brakes DO NOT bolt up.

Brake upgrades/swaps:

Commodore 278mm disc rear end – Disc brakes from a live axle Commodore will physically bolt onto the ends of the diff housing. There’s a little more to it than that, but it can be done with relative ease in stud patterns of either 4x100 (Gemini) or 5x120 (Commodore). Off the top of my head, these use a single piston floating caliper, and a drum-inside-disc handbrake. Keeping this legal and safe is easy since no fabrication/modifying is required – the discs can be sourced undrilled, then drilled to 4x100 (instead of re-drilling over the 5x120 pattern which is generally illegal). This conversion has been done many times over and returns a good end result. The bite of the handbrake is debateable, but should be reasonable with decent shoes. I spent around $550 for the conversion, including new discs drilled to 4x100.
More info:
http://ozgemini.com/forums/tech/viewtopic.php?t=96

Later model Gemini drums – If you have poor brake balance, but don’t want to change diffs or fiddle with proportioning/bias valves, you should be able to swap to the drums found on Gemini Salisbury diffs with relative ease. They should bolt on fine, but keep the handbrake cable matched to the drum brakes (though it may be the same anyway). The main thing to be wary of is the brake line thread in the slave cylinder/hard lines on the diff. There’s a very good chance these could be marginally different (like imperial to metric), and they could even feel fine whilst fitting, so pay very close attention to this and make sure your system holds solid pressure before driving!

LSD options:

The BW68 diff was allegedly used in a few small-medium cars in the 70’s, but details are few and far between. Some googling has suggested that they came out in 4.1:1 in some specific Ford Escorts (most ran other diffs though!) and perhaps some Datsun Stanzas (3.7:1), though both of these setups have fully enclosed spider gears, the axle splines may be different… I wouldn’t go there. About six years ago I emailed Kaaz asking if they made anything for these diffs and the answer was no. Pending (a lot of) further research, there may be an LSD solution, but as such there’s nothing readily available, and I foresee a hell of a lot of problems.
Here’s some further reading, please note that the diffs pictured in this link are NOT Gemini diffs, and have a completely different centre design. As such this information may not even be useful, but here it is anyway:
http://datsun1200.com/modules/newbb/vie ... 64&forum=1



Small Salisbury (178mm)

Ratio: 3.9:1
Found in: Petrol TD, TE, TF and TG models of all body type (sedan, coupe, wagon, panel van)
Axle splines: 23
Bolts on diff cover: 10
Bolts on crown wheel: 8
Factory swaybar provision: Yes

General info:

These cars are known as RTS models (Radial Tuned Suspension) and may or may not be badged as such. These diffs have provision for a factory swaybar –which consists of a forward-facing mount on either side, maybe halfway along the axles. These models were produced from 1977 to 1985 (roughly). A Salisbury diff can be identified by the rear cover/hat, which will have 10 bolts, but still a reasonably octagonal outline. The oil drain plug appears slightly recessed. This diff has a bit in common with the “Small Salisbury” (178mm) found in 6cyl Commodores, though don’t expect parts to directly swap.

Brakes:

The drum brakes originally fitted to these diffs are linear and offer good front/rear brake balance in their original application. They offer good overall braking (for drum brakes), however the handbrake mechanism isn’t as bitey as the Borg Warner item, and is prone to damage if abused. The culprit is the auto adjuster fork, which may bend at the tips upon repeat heavy handbrake application. The brakes can be unbolted from the axle housing and swapped with those on the Borg Warner if a good handbrake is desired, or the VB-VR Commodore 6cyl disc brakes can be bolted directly onto the diff (there’s a thread explaining this process). Diesel/import/Piazza brakes DO NOT bolt up.

Brake upgrades/swaps:

Commodore 278mm disc rear end – Disc brakes from a live axle Commodore will physically bolt onto the ends of the diff housing. There’s a little more to it than that, but it can be done with relative ease in stud patterns of either 4x100 (Gemini) or 5x120 (Commodore). Off the top of my head, these use a single piston floating caliper, and a drum-inside-disc handbrake. Keeping this legal and safe is easy since no fabrication/modifying is required – the discs can be sourced undrilled, then drilled to 4x100 (instead of re-drilling over the 5x120 pattern which is generally illegal). This conversion has been done many times over and returns a good end result. The bite of the handbrake is debateable, but should be reasonable with decent shoes. I spent around $550 for the conversion, including new discs drilled to 4x100.
More info:
http://ozgemini.com/forums/tech/viewtopic.php?t=96

Early model Gemini drums – There’s only one reason you’d want to do this – to have a ridiculous handbrake. Doing this will result in extremely poor brake balance, with FAR too much pressure to the rear. I would not recommend this for a street car (or even for most track cars). When I did this to my drift car, I ended up completely blocking off the rear lines, just to make the thing a bit safer/more driveable. However, the result is a seriously amazing handbrake. I’ve never experienced a better handbrake, ever. You so much as touch it, and it locks both wheels immediately (unless the brakes are extremely hot). Therefore, this swap has its merits for drift or motorkhana application, where absolute braking isn’t vital, but a durable and responsive handbrake is important. I’ve never even had to adjust one of these – let alone damaged or worn one out. Anyway, the drums should bolt on fine, but keep the handbrake cable matched to the drum brakes (though it may be the same anyway). The main thing to be wary of is the brake line thread in the slave cylinder/hard lines on the diff. There’s a very good chance these could be marginally different (like imperial to metric), and they could even feel fine whilst fitting, so pay very close attention to this and make sure your system holds solid pressure before driving! I can’t confirm the threads because I can’t remember how I did it, so check to be sure.

LSD options:

This diff has a few options for LSD. I’ll sort them based on my personal preference.

Altra9 aftermarket centre and axles – this is a custom made replacement diff centre, with a clutch-pack LSD and 28 spline axles. It’s extremely strong and is made from billet steel. They became available with a group buy late 2011, and are available for ongoing purchase from Altra9. Being that the centre is designed for stronger 28-spline axles, standard axles will not fit, though the kit can be purchased with or without axles. At the time of publication, these cost around $1250 for the LSD, oil, bearings and a seal, or $2050 for the aforementioned plus the billet axles. This is generally regarded as by far the best bolt-in option available for a Gemini.
More information:
http://ozgemini.com/forums/non-tech/vie ... hp?t=41186
http://www.altra9.com.au/

Pro Rally Enterprises/The Beevus LSD – using a similar principal to the Altra9, this was developed late 2009. A prototype was made and fitted, but unlike Coxy’s awesome demonstration and testing of the Altra9, media/testing of this diff have been minimal. I believe this setup uses standard axles. The last update on this was in 2011, so it’s quite possible these could still be available, but it’d be best to enquire if you’re interested. The last update listed a price for the centre at $850.
More information:
http://www.ozgemini.com/forums/non-tech ... hp?t=32165
Or call Pro Rally Enterprises - Andrew 0424 581 743 or Chris 0423 040 479 (06/09/2010)

Commodore 6cyl LSD – these items are very rare, and strength has been debated somewhat. If you happen to find an LSD centre in a Holden fitted with a *SMALL SALISBURY* 178mm diff, it should bolt onto the Gemini crown wheel (or you may be able to swap the crown and pinion too). Note that the axle spline may not be the same, so you may either need to have your axles resplined, or shorten and fit the donor diff’s axles, or have axles made. The important thing to note, however, is that these are damn rare. They were fitted as an option in some 6 cylinder live-axle Commodores and Kingswoods, and it’s important you don’t confuse this with the larger Salisbury diff fitted to the V8s. Unfortunately, most of the cars that came out with an LSD were V8s with the larger Salisbury – which won’t bolt in. By now, most of these will be worn and need a service, but they should be easy enough to get rebuilt and shimmed. Strength will be largely dependant on tyres, treatment and the condition of the LSD, with some saying they wouldn’t run more than 100-130rwkw, others swearing by them as being bulletproof.
More information:
http://ozgemini.com/forums/tech/viewtopic.php?t=5309
http://ozgemini.com/forums/tech/viewtopic.php?t=1382



Diesel/Import

Ratio: 3.58:1 for Australian diesels, unknown for international models
Found in: Factory diesel Geminis (which are believed to be only sedans), Japanese Isuzu Gemini T-series, European Opel Kadett C
Axle splines: 24
Bolts on diff cover: 10
Bolts on crown wheel: (unconfirmed)
Factory swaybar provision: (unconfirmed)

General info:

***I would only recommend running/upgrading a Diesel diff if you have a specific reason to do so, such as it already being fitted to your car, or attempting to make an accurate ZZ/R or ZZ/T replica. If you’re just looking for an upgrade, I’d strongly suggest just using the Salisbury diff***

The common belief is that Australian-delivered diesels were only sold as sedans, and as far as I’m aware, started with the TE. These can be identified by having 10 bolts and a very round appearance. The oil drain plug is not recessed, and pokes out at quite an angle. This diff is the same as those fitted to Japanese-delivered Geminis.

Brakes:

Australian diesel diffs were fitted with drum brakes. The bolt pattern on the end of the axle housing is different to those on the Australian BW68 and Salisbury diffs mentioned above. Therefore, you can't fit Salisbury or Borg Warner brakes to a Diesel diff. You can either keep the diesel drums, swap diffs, or keep reading...

Brake upgrades/swaps:

Piazza discs – It’s widely believed that all Australian-delivered Piazza discs will bolt up to the Diesel diff housing. I can’t confirm if this is dependant on the series of Piazza – as Australian-delivered Piazzas came with two distinct types of diff (see LSD options below for more info). There will most likely be more involved in the conversion, such as handbrake cable mounts, brake line positioning (and possibly the thread used), and brake bias, but as far as I know, it could still be an option.

Import T-series Gemini discs – If you source disc brakes from a Japanese-delivered T-series Isuzu Gemini, they should fit, as the diff is essentially the same. The brake lines may need to be modified or replaced, and the handbrake cable may differ, but they’re a very promising option.

LSD options:

All bolt-in LSD options for these diffs are extremely rare, and originate from very special Isuzu cars. Think carefully about this before you butcher a collectable car for your own project. There are other ways (such as the diffs mentioned above), but if you’ve got a good reason to want an LSD for this diff, read on.

There’s been a lot of talk about Piazza LSD centres bolting up, but it’s not that simple. Australian-delivered Piazzas were sold with two types of diffs. The vast majority of them have a 4-link diff setup. The diff housings un-bolt from the front, and the centres are not suitable for a Diesel diff. Some Piazzas however, were fitted with a torque tube setup, and a diff with a removable hat on the back. These centres can be fitted to a Diesel diff centre (crown wheels/pinions *may* swap too). However – not all of these came with an LSD! It was an option only, and they’re anything like the later Piazza LSD centres, they would consist of a clutch pack which can be shimmed. I *haven’t* checked the number of splines on this type of Piazza axle, but my guess would be 24, like the Diesel.

Alternatively, a genuine Isuzu Gemini LSD centre from Japan should fit the Diesel diff. Being that the Diesel diff is itself a Japanese Gemini diff, any centre from a T-series Japanese Gemini (petrol or diesel) should fit. Once again, not all Japanese Geminis came with an LSD – far from it, they’re likely to be quite rare over there, and nigh on nonexistent over here. This very much comes down to the “butchering a classic” rule – ask yourself if it’s really worth taking from a genuine classic Isuzu for the sake of your project – but, if it falls in your lap, there’s the info. Splines are unconfirmed, but I’d be pretty confident they’d be fine.

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 Post subject: Re: The Gemini differential bible - brakes, LSD, swaps
PostPosted: Thu Jul 26, 2012 8:42 am 
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Joined: Sun Aug 24, 2008 9:27 pm
Posts: 489
Car(s): TX Coupe
Nice stuff Eli. I spotted this on Ozgem last night. I think it pretty much answers any questions I've had!

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 Post subject: Re: The Gemini differential bible - brakes, LSD, swaps
PostPosted: Sat Jul 28, 2012 1:36 am 
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Joined: Sun Aug 24, 2008 7:12 pm
Posts: 1938
Location: Windsor Gardens
Car(s): TX G180W+T, TE G180W, TG G180Z, Piazza 4ZC1T +more
Cheers :D I just edited the Diesel brakes section, because there was a pretty stupid mistake in there.

The main things I haven't included or haven't yet covered are the brake line threads (mainly because I don't know which ones are different) and the torque tubes! Torque tubes are pretty simple though, there's three or four styles, it's generally best to leave them with the car but you can often modify or make them fit anyway. They're generally not a huge concern, with the most common style of course being the Salisbury type. The main (only?) difference is the bracket and rubber where they meet the body.

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Cheap car service/tunes available, can come to you. PM me.
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