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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

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.

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.

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.

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.

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?

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

A rag tyre back in the day $13 to $15 I think.

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)

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

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. ???