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hk/ht/hg diff and brake options - hk/ht/hg diff and brake options
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Panama1986 Offline
#1 Posted : Saturday, 1 March 2025 9:54:40 PM(UTC)
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hi guys, i'm unsure what rear diff and what brakes i want to run; looking at something to suit a nice cruiser around 400hp HG sedan

am i better off going borg warner 78 or 9 inch? for 78 i'd like to look at .37 ratio but in 9 inch i'd go .36 maybe. not interested in .411s as i still drive this car country roads at 100-110.


looking at doing HQ stub axle upgrade and WB girloc calipers on the front and hq drum rear; is that the best option using holden parts?

ideally i'd like to be able to buy off the shelf parts at repco if i need to do brakes, stud pattern should also match front to rear so i only need 1 spare tyre.

what kinds of brake and rear axle combos are you guys running?

thanks in advance, any questions or context please ask
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HK1837 Offline
#2 Posted : Sunday, 2 March 2025 1:16:45 PM(UTC)
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My advice would be to buy a 9". Far easier to change ratios if you need to later. 28 or 31 spline will be fine for that hp.

HK to HG are identical stubs to HQ to mid HX. There is only a minor machining change for mid HX to WB for the alloy calipers but I don't think it even makes any difference to the calipers. So if you already have front discs just leave the stubs. No need to go chasing WB calipers, just use HQ PBR, will work fine. Just make sure you swap calipers side to side so that the bleed point is on top.

All you have to decide for the rear is whether you want to run HK-HG backing plates or HQ-WB. The drums are identical HK-WB bar the stud pattern but the backing plates are different. Running HQ front rotors its probably better to run HQ-WB backing plates. The HK-HG handbrake cables will connect straight up to them. Dr Terry might be able to chime in on whether he recommends using the HK-HG backing plates or HQ-WB. The choice can affect the width to make the diff.

Your standard HK-HG disc brake booster and master cyl should operate the HQ calipers and drums fine. Dr Terry can give you guidance if there is a better cylinder or if that one is fine to use.

Make sure when you get the 9" made that the attachment welded on for the brake line hose T is in the right spot, use your banjo for guidance.

I also recommend putting HQ-WB commercial mounts on the diff rather than HK-HG, and run HQ-WB ute/van/cab-chassis lower plates with U-bolts to suit. The shock lower mount aligns perfectly with the HQ-WB lower bolt/nut.

Its probably not a bad idea to make the diff slightly narrower than standard, but that depends on what wheels you intend to run. I made mine 3/4" narrower each side than a standard HK-HG so I can run 15x7 rear and 15x6 front, but 15x8 will fit on the rear with standard wheel arches with a tyre like a 235/50/15. I don't lower my cars though (don't like the look), so not sure what impact this might have on your tyre/rim size if you lower it.

Edited by user Sunday, 2 March 2025 1:18:19 PM(UTC)  | Reason: Not specified

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Panama1986 on 2/03/2025(UTC)
Panama1986 Offline
#3 Posted : Monday, 3 March 2025 12:30:55 AM(UTC)
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thanks heaps mate that's a wealth of info, im grateful for your response.

i hear you about running a slightly narrower one if i build, i was thinking exactly the same.

i was aware of the 2 types of mounts, thanks for confirming which ones are better.

and thanks for the hot tip about the welded on t piece. great to know. easy to miss.

i have drum stubs so change is in order.

thanks so much for the advice and info, much appreciated.
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HK1837 Offline
#4 Posted : Monday, 3 March 2025 3:32:51 PM(UTC)
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If you have to change stubs just buy them off the same car you intend to run the calipers of. Beware that WB Girlock calipers have a different hose entry to HZ. See what works best with HG.

You also want to buy a HG brake booster and bracket.
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Panama1986 on 4/03/2025(UTC)
Panama1986 Offline
#5 Posted : Monday, 3 March 2025 10:05:18 PM(UTC)
Panama1986

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thanks, it will depend on whats available, but i'll be getting adr approved braided hoses and they will make the ends and lengths whatever i want. ive used them before on another car and they worked flawlessly.

i was under the impression that wb girlock was the best available type for v8 that isnt aftermarket. how do the pbr disc calipers compare?

i have a hg v8 master and diaphragm ready to be fitted, with new lines as well.

thanks again for your advice
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HK1837 Offline
#6 Posted : Tuesday, 4 March 2025 11:34:32 AM(UTC)
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HQ calipers are the easiest to get and work on, they are in my opinion the best. WB and HZ Girlock are basically the same caliper just different hose entry points. I have used both but I reckon the HQ PBR are the best for longevity and maintenance.
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Panama1986 on 4/03/2025(UTC)
Panama1986 Offline
#7 Posted : Tuesday, 4 March 2025 12:00:18 PM(UTC)
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awesome, thanks mate

Edited by user Tuesday, 4 March 2025 12:54:46 PM(UTC)  | Reason: Not specified
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HK1837 Offline
#8 Posted : Tuesday, 4 March 2025 4:28:13 PM(UTC)
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There are people selling them recoed in HQ PBR, HZ Girlock and others on Ebay. Or buy used calipers for about $50-$75 each and do them yourself. Example:

https://www.ebay.com.au/...D%7Ctkp%3ABk9SR7SZqf2rZQ
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Panama1986 on 5/03/2025(UTC)
Panama1986 Offline
#9 Posted : Wednesday, 5 March 2025 1:14:32 PM(UTC)
Panama1986

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that link didnt work but i've seen a few pairs on there just wondering if the cast iron or alloy pbr is better or doesnt matter?

I have never had issues with the alloy PBR but for some reason people hate them, they are HX calipers. The HQ ones are full cast iron, they are what L34 used but with the hose top entry rather than bottom. You can get HJ Girlock full cast iron as well. I honestly don't think it'll matter, maybe talk to a brake place and see which ones you can still but pistons and kits for.

Edited by user Wednesday, 5 March 2025 3:24:04 PM(UTC)  | Reason: Not specified
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Panama1986 Offline
#10 Posted : Wednesday, 5 March 2025 1:31:45 PM(UTC)
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@hk1837 your inbox is full
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