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Author Topic: Heavy duty GM 12-bolt axles  (Read 3677 times)
Sixteen Grand Sedan #56
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« Reply #15 on: May 02, 2013, 12:42:55 AM »

Not quite sure what you mean by"The Heavy Duty Service Replacement Rear Disc Brake kit" or "differrent bolt pattern".

I own a small axle bearing JL-8 rear that has no stamp codes. That tells me it was a bare housing that was sold over the counter.

I also own a large axle JL-8 rear that has stamp codes. That tells me it was sold assembled over the counter--probably dragged around the counter. Grin

When you refer to the "different bolt pattern" I'm guessing you mean the four holes on the flange that mount the caliper bracket or backing plate? This has NOTHING to do with the bearing size, as your pictures only show a flange and not the bearing area. ALL Camaro rear disc housings will utilize the larger pattern--same as the Impala you mentioned. This has NOTHING to do with the bearing size but rather the proper bolt pattern for the Corvette rear caliper bracket. Both small and large bearing axles used the big pattern because of the need to match the Corvette pieces.
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Robert Lodewyk
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« Reply #16 on: May 02, 2013, 07:06:25 AM »

Hi Robert,
I'm curious why you refer to the 'service duty' axle as 'JL8'?   Do you call it that because that's how a lot of Camaro people today refer to ALL Camaro disk brake rears?    

JL8 is the order/option code for the factory installed rear, but the 'over the counter' piece, I've only ever heard referred to as 'The Service Duty' rear end.
In my mind, only the factory installed rear is truly a 'JL8'.  The Service Duty rear served the same purpose (rear disk brakes) for Trans Am racers, but in a much heavier duty way!   Chevrolet never referred to it as 'JL8' did they?

It would add clarification if we had more distinctive terminology to distinguish the two?  There were other differences between the two rears than just the bearing size and axle diameters.   The posi unit in the Service Duty rear had more plates and heavier duty springs also, and there are probably other differences I don't know of as well.

Does anyone have written documentation for, or know the PN, that the Service Duty rear end (brake to brake) was sold as??  I can't find my copy of the 'original' Power Book; does that identify the rear by part number? or by ??
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Gary W.  /  69Z28-RS, 72 B 720 cowl console rosewood all tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe (2), 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55 Nomad, '57 Nomad, '57 B/A Sedan
Nikke
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« Reply #17 on: May 02, 2013, 03:27:43 PM »

From Untold Secrets

OPTIONS WITHIN OPTIONS...


Until the introduction of the 1969 model, Camaro owners were limited to upgrading their brake systems by using the Heavy Duty Service Package. Things changed dramatically with advent of the 1969 Camaro and the introduction of the RPO JL8 four wheel disc brake package. The Camaro owner now had the option to order his 1969 Camaro with four wheel disc brakes (RPO JL8) installed at the factory or with base equipment and later upgrade to four wheel disc brakes using the HD Service Package.


Shown below is the "basic" HD service package disc brake rear axle housing assembly. Note the Corvette caliper adapter/park brake assembly's...


 Taking it a step further... If he ordered the optional JL8 equipment on his 1969 Camaro, he then had the additional option to further upgrade the system by substituting the heavy duty rear axle from the previously released HD Service Package in place of the JL8 rear axle. The advantage in this being; the HD Service Package axle uses larger diameter axle shafts and bearings which significantly improve durability, a factor especially important in racing. Anyway you went, it was a win-win situation with this limited production performance equipment.
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Nikke
Jon Mello
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« Reply #18 on: May 03, 2013, 12:04:05 AM »

Some rear axle part number data, courtesy of Bob Morton.



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Jon Mello
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« Reply #19 on: May 03, 2013, 01:43:18 AM »


   


Hello 1969Z28-RS

YES, I was using the term “JL8” in the broadest of definitions because I'm answering a question in a drum brake forum section that the answer applies to both the RPO JL8 units as well as the service package rear ends equipped with rear disc. I was attempting to answer just the question without a complete history of the topic. Sorry for the confusion.

I suppose I should have first asked about his term Kit since that is a new term to me. I have no desire to completely hijack this section and tern it into a JL8/rear disc brake discussion that would be covered up to others looking because of the title.
 
If you or someone wants to start a discussion about disc brake rears I will gladly contribute. Indeed there are numerous differences between the two assemblies. I will state that I'm quite hesitant to show pictures of stampings because of the individuals out there that seek to defraud others in our hobby.

You are correct; the term RPO JL- 8 should only apply to units installed in a specific car at the factory. My understanding is that only the small axle bearing units were installed. That differs from what was just posted as appears in a well known book. My experience is that both small axle bearing housings as well as large axle bearing housings were available over the counter since I have a small one. I will admit that I have owned that item for probably 30+ years, the last 25+ of which it has been installed in a car that I pay no attention to. Thus I should probably look at it again before I say much more. I seem to recall the lack of stamped codes being a major concern when I first inspected it. That was before books and information were generally available on such pieces, at least to me.

This hobby certainly has MANY variants in terminology that will probably never be cleared up. The term "cowl Hood' was never used to define the super scoop hood on a 69 Camaro is just one example. Do you think if you had walked into a Chevrolet parts department in 1990 and asked for a “special ducted hood” for your 69 pace car the guys would know what you wanted? Ask for a cowl hood and then you would get it. Which brings up a question; if you install an NOS (service item) special ducted hood on that pace car does it still qualify as a “ZL-2” option?

Not sure what you mean by “original Power Book” I assume you are referring to the “Chevrolet Special Equipment” book that seems to also go by the name “Green Sheet”. Since the pages were green in the original books I suspect that is where that term/name came from. The 3-9-72 edition does indeed list a part number for an axle assembly—or not.” # 3945131 Axle (rear) (3.73:1 Ratio) ( w/Disc Brakes)” is listed at the top of page 40. That seems to be the only complete unit. Not clear if that includes ALL the brake items or not. Earlier editions may provide more options. I do own a complete 69 counter parts book that may shed more light if you want me to dig it out later.


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Robert Lodewyk
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« Reply #20 on: May 03, 2013, 07:40:47 AM »

Thanks Robert.
Your memory is the same as mine!  Smiley     I also have a copy of the 'Chevrolet Special Equipment' book - that I was referring to - I copied from the green sheets a friend of mine loaned me in the '70's (Robert Christianson, who also ran a '69  in Trans-Am and later and later a 73 Camaro in IMSA-GT).   My knowledge of the 'service duty' rear mostly goes to information I got from him, and later I purchased one of the rears that was used in his '69 Z28 TA car.
The reason I asked you those questions is because you are knowledgeable, and I was hoping that you could clear up some of the misinformation that is out there.   The 'few lines' in the available books dedicated to the service package rear do not do justice to the differences in that rear end, and seems to be mostly 'speculation' dreamed up by the authors...

Thanks for your informative response to my questions, and I for one would love to know more about the differences between JL8 and Service Package rears.
PS.  I also do not believe one could order a new Camaro for the street and have the factory install a service package rear end, although it's certainly possible (for a few months) to order the JL8 rear and then pay the almost $1000 for the service package (OTC) and pay the dealer to install it..  although that scenario certainly makes no sense!  As I understand it, the service package rears were difficult to get and likely required someone 'close to Chevrolet' to get one promptly for a TA car.  Bob told me he had a relationship with Vince Piggins when he was running that assisted him in getting some of the special parts.
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Gary W.  /  69Z28-RS, 72 B 720 cowl console rosewood all tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe (2), 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55 Nomad, '57 Nomad, '57 B/A Sedan
Jon Mello
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« Reply #21 on: May 03, 2013, 11:01:50 PM »

Production JL8 vs HD service package 4-wheel disc 12-bolt rear axle now has its own discussion started in this thread.
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Jon Mello
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« Reply #22 on: May 03, 2013, 11:04:52 PM »

Thanks Jon!

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Gary W.  /  69Z28-RS, 72 B 720 cowl console rosewood all tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe (2), 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55 Nomad, '57 Nomad, '57 B/A Sedan
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