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Flem Offline
#1 Posted : Saturday, 3 November 2018 11:21:04 AM(UTC)
Flem

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New post
Not a new subject.
With the 12 bolt Diff option for the HK GTS 327.
You know the story.
You get in to discussion with a friend.
I made the claim that they don’t exist.
He responded with oh ! yes do.
If you could wait for the 4 months with a firm order you could get one
It’s documented in Holden Literature and from his vast collection he come out
with it. It would have been easy to convince and he went on to say that all the experts claim they did too. Things like the floor pan was designed to provide for these diffs. etc. etc.
Ok well given all that.
I ask. Was that option ever taken up. Did they ever really exist?.
How many were ever supplied to a dealer for it to be fitted
to a paying customers car ? If I’m correct that was the way it
was supposed to be done.
If there was any. Who has ever seen one. ?
Who has one ?
Where are they ?
Or were there just a few 12 Bolts obtained and fitted to a couple of
Specialty built race cars ?
If that’s the case maybe this HK GTS 327 12 Bolt was all the time
just a fantasy.
Just one of those things that despite all the plans and
preparations for it. Our Mighty Monaro, oh ! Yes they are, with the 12 Bolt
just never really happened.
Flem



HK1837 Offline
#2 Posted : Saturday, 3 November 2018 12:29:49 PM(UTC)
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They exist, I have seen one, complete with GMH marks on the tubes and GMH made axles. Phil Bartolo has/had a NOS axle for one. The Sydney to London marathon cars had them. Norm Beechey's HT Group C car had one, not sure about the HK.

The reason they are rare is they were SO expensive. They added about 20-25% to the cost of a GTS327 or GTS350 and they had a 4month wait where they were made to order through Nasco. GMH even published part numbers for speedo drive adapters for the short 12 bolt rear axle ratios of 3.73, 4.1, 4.56 and 4.88.

Remember what the racing intention of the HK GTS327 and HT GTS350 was. The intention was to race them in Group C, where they'd have engines like the 450+hp of Beechey's HK and over 500hp in the HT. This will be why they had the 12-bolt Engineered and available originally. Once they eventually entered them into Series Production and the 500 at Bathurst then the Old Man Emu concept also needed a 12 bolt, the LT1 engine would have destroyed 10-bolts. The 10bolt was actually beyond its engineered strength behind both the manual and auto GTS350's
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abi Offline
#3 Posted : Saturday, 3 November 2018 3:19:46 PM(UTC)
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As HK has said above. The 12 bolt was application specific, for the higher numeric ratios only. If you ordered your 327 or 350 car for series production racing or for road use you chose between the 3.08 and 3.36 ratios which were 10 bolt 8.2 inch. If your application required more torque multiplication for towing or motor sport etc you had the choice of the 3.55 10 bolt 8.2 inch diff, (very rare), I've never heard of one.
Beyond this the 12 bolt was available and started at 3.73 and went all the way up to 5 point something. A 3.73 ratio with a 25.4 inch diameter D70 x 14 tyre would be a real pain on the highway with an engine that ran out of puff at 4800RPM so the 12 bolts were motor sport only due to the ratios provided by Holden.
Flem Offline
#4 Posted : Sunday, 4 November 2018 12:06:55 PM(UTC)
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Thanks HK & ABI, You haven’t quite answered the question though.
For a start we are Talking about HK 327s. That were delivered to an
average Joe customer that took up the 12Bolt option purchasing it from
the local Holden Dealer.
Not a question about if a12 Bolt Diff existed, I’m sure there would have been
many. Not about any special pre- Production vehicles were built or spare parts
that were made in the expectation of a demand for them.
Not about cars produced for another series later i.e HT.
Not about improved Production Racing Cars.
Yes HK 1837, it is quite likely you have or saw a 12 Bolt but if GMH built it or
had it built you would expect they may well put a GMH stamp on it.
Particularly to identify it from some other 12Bolt diff that may have been about.
So you may be able to see why I would still be so dubious about the Monaro
GTS 327 with the 12 Bolt option having been delivered to any Mr Average Joe








HK1837 Offline
#5 Posted : Sunday, 4 November 2018 1:43:46 PM(UTC)
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You won't find any GTS327 manufactured with a 12 bolt sold to the public, just like the public couldn't buy an L34 or no XU-1 was built with the optional M21 box or optional diff ratios or any A9X built with a T10 - they were all for motorsport only. The base vehicle was built as they were, and if the owner wished to go racing with it they purchased the 12-bolt or the different ratio gearbox or the T10 or whatever and fitted it themselves or had it fitted for them.
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Flem Offline
#6 Posted : Sunday, 4 November 2018 5:34:01 PM(UTC)
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Thanks HK, you gave me the answer in the 1st line of that reply.
Ok that’s good
Quite Simply. They weren’t available to the general public.
So for the general public the HK 327 12 Bolt may well have
been a fantasy. Fair enough they were for race cars only.
Flem
HK1837 Offline
#7 Posted : Sunday, 4 November 2018 5:56:43 PM(UTC)
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Public could buy one easily if they tried, but that was not the intent of the availability - but why would they? Who in their right mind other than a drag racer would want a 3.73, 4.1, 4.56 or 4.88 rear axle? And who would pay the massive cost of one other than a racer? Remember there were stuff all tyres available for a GTS327 that were legal, GMH had to get one specially developed in Australia and imported stuff like the Michelin XAS cost $130.45 on top of the cost of the GTS327 - remember a GTS327 was just under $3800, so $130 was like today paying $1300 for an optional tyre on the same rims on a $38000 Commodore.

GMH Engineered the 12-bolt for the GTS327 and GTS350, they developed the speedo adapters, they put the hump in the floor and the tank. But like the T10 in the A9X with its Engineered floor blister and other bits to fit it, they were only ever really intended for race cars.
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Flem Offline
#8 Posted : Sunday, 4 November 2018 6:23:20 PM(UTC)
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So would it be safe to say the ones that were used were basically in test cars.
So how many of those 12 Bolts Diffs would have got out there.
They would have simply been a GM 12 Bolt Diff.
Of the day. Wouldn’t they. Which one though ?
There were a few to choose from.
Probably still is I suppose.
Flem

HK1837 Offline
#9 Posted : Sunday, 4 November 2018 6:29:33 PM(UTC)
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They were used in competition, and the London/Sydney marathon cars had them too. I imagine less than 20 as a guess were ever sold given the cost. They were a 12-bolt centre, just like the 10-bolt centre, GMH put on the tubes and made the axles locally (or had them made). The 1967-1968 Impala and Parisienne had a 12-bolt too so they were not exactly foreign things to GMH. Plus if racers wanted other gears they were available readily overseas. Lots of Camaros were imported into Australia through dealers like Southern Motors and Bob Jane and others were racing Camaros in GroupC so the bits would be around.
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Flem Offline
#10 Posted : Monday, 5 November 2018 12:18:37 PM(UTC)
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Thanks HK, how all this come about was as I said , Monaros things.
The subject come up about an old bloke around here that for years has
Claimed he had a diff that was especially made for a GTS Monaro that
Wrecked. But we have never yet got to see it. That’s where I curious about
if there was any such thing. This old bloke also claimed that it was as a
3.89, you mean a 3.9, no the old bloke insisted. it was a 3.89. Any way I’m
Even more determined to get a look at it. What do you think this could be
if what he is right about the 3.89 bit. Oh and he reckons it is a Salisbury.
Flem
HK1837 Offline
#11 Posted : Monday, 5 November 2018 12:49:22 PM(UTC)
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GMH never offered ratios other than those 4 ratios mentioned. The 12 bolt came in the following ratios that I am aware of (carriers changed as you went up in ratio):

2.29
2.56
2.73
2.76
3.07
3.31
3.55
3.73
4.10
4.56
4.88

There are aftermarket gear sets now in other ratios including 3.9.

3.89 might be some sort of Ford diff? Like aBorg Warner?
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gmholdman Offline
#12 Posted : Monday, 5 November 2018 5:04:13 PM(UTC)
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FX-FJ were 3.89 ratio. AL.
 1 user thanked gmholdman for this useful post.
detective on 6/11/2018(UTC)
castellan Offline
#13 Posted : Tuesday, 6 November 2018 11:00:24 AM(UTC)
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Originally Posted by: HK1837 Go to Quoted Post
Public could buy one easily if they tried, but that was not the intent of the availability - but why would they? Who in their right mind other than a drag racer would want a 3.73, 4.1, 4.56 or 4.88 rear axle? And who would pay the massive cost of one other than a racer? Remember there were stuff all tyres available for a GTS327 that were legal, GMH had to get one specially developed in Australia and imported stuff like the Michelin XAS cost $130.45 on top of the cost of the GTS327 - remember a GTS327 was just under $3800, so $130 was like today paying $1300 for an optional tyre on the same rims on a $38000 Commodore.

GMH Engineered the 12-bolt for the GTS327 and GTS350, they developed the speedo adapters, they put the hump in the floor and the tank. But like the T10 in the A9X with its Engineered floor blister and other bits to fit it, they were only ever really intended for race cars.


Is it $130 divided by 4 or 5 Michelin XAS tyres.

A rag tyre back in the day $13 to $15 I think.
HK1837 Offline
#14 Posted : Tuesday, 6 November 2018 12:23:35 PM(UTC)
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Yes $130 optional tyre on the GTS327, which includes the credit for the D70 redwalls that were standard. The main point was there were no 130+mph rated tyres around in 14” other than the D70’s that GMH had to get a supplier to make locally and the expensive imports. So to go to 15” to better match the minimum 3.73 gearing of the 12 bolt would have been prohibitively expensive. If GMH had intended the 12 bolt for street use they would have offered it in 3.07, 3.31 and 3.55 ratios. This would have been easily done as these three ratios share the same carrier (or centre if you like) as the 3.73, whereas the 4.10, 4.56 and 4.88 use a different carrier. Same deal as the Aussie 10 bolt where 2.6 and 2.78 are one carrier and 3.08 up are another.
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Sandaro Offline
#15 Posted : Tuesday, 6 November 2018 1:10:27 PM(UTC)
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What ratio did the marathon cars use? I would have thought a 3.08 or at most 3.36 would have suited that sort of event. Same at Bathurst.
I can see the 12 bolt ratios that HK quoted being used in drag racing (obviously) and perhaps short tracks in the day (Oran Park etc)
HK1837 Offline
#16 Posted : Tuesday, 6 November 2018 1:55:09 PM(UTC)
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No idea, but I suspect they used different gears imported for the task. Looking at the photos they look like HK 6” rims with crossplies so you’d think 3.73 would be too low.
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Dr Terry Offline
#17 Posted : Tuesday, 6 November 2018 4:58:34 PM(UTC)
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I spoke to Doug Chivas many years ago about these cars, he drove one them in the marathon. He was a good friend of my fathers, he lived near our place on the NSW Central Coast.

They had a 350 V8 & TH400 combo, but I didn't ask about the rear axle ratio. However he related story where they held the TH400 in 2nd gear for hours @ 70 & 80 MPH, in the Turkish mountains. He said the floor which had no mat or carpet covering, got so hot you could fry eggs on it. He had so much admiration for the TH400 after that treatment, it just didn't miss a beat.

For them to be able to do those speeds in 2nd gear, you would think that the rear end must have been quite high, possibly in the 2s. No lower than 3.07 that's for sure.

These cars where prepared in-house in the GMH service garage at Fishermans Bend, so they would've had access to anything GM US made.

Dr Terry
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HK1837 Offline
#18 Posted : Tuesday, 6 November 2018 5:50:40 PM(UTC)
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Given Dr Terry’s info the centres may well have simply been 2.73 or 2.76 - whatever the 1968 Pontiac and Chevrolet that GMH assembled had.
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detective Offline
#19 Posted : Tuesday, 6 November 2018 7:04:28 PM(UTC)
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Originally Posted by: HK1837 Go to Quoted Post
Given Dr Terry’s info the centres may well have simply been 2.73 or 2.76 - whatever the 1968 Pontiac and Chevrolet that GMH assembled had.


I'd imagine the sheer bulk and weight of these big American cars might've been a good reason for having the more stout 12 bolt diff as a standard fitment for durability's sake in our Australian market....and the high initial cost to the first owner would've allowed that situation with no "grouch" or question. Just possibly, these 12 bolt diffs with tall ratios might've been exploited as a consequence, and would've definitely been very helpful for the Monaro race crews on their overseas endeavours. ???

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#20 Posted : Tuesday, 6 November 2018 7:17:01 PM(UTC)
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Pretty sure any 4BBL full size got a 12 bolt in 1968. The gearbox and diff for any given GM vehicle size was determined by both the torque output of the engine and the size of the vehicle. We saw it here in Australia where the HQ-WB got a 10-bolt for any V8 but the Torana still got a banjo for all V8. Commodore got a 10-bolt for a 5.0L but the 4.2L got a banjo in disguise.

If you step down from full size to Camaro an L30 (275hp 327) got a 12 bolt but still had a Saginaw, but an L48 (300hp 350) or an LM1 (255hp 350) both got a Muncie and a 12-bolt. It is the torque of the engine that dictates the gearbox and diff. GMH followed GM "rules" with the HK GTS327 by fitting a Saginaw and 10-bolt but should have fitted a Muncie and 12-bolt to the HT 350 manual, auto 350 should probably have got a 10-bolt although some Camaro with the LM1 and Powerglide (or TH350) seem to get a mix of 10 and 12 bolt.

I also just checked the specs for a 1968 GMH assembled Parisienne - they were a 3.55:1. I assume the Impala was the same.
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