did any 3rd gens get a 12 bolt??

by carcass Thu Feb 14, 2013 7:36 pm

Sorry if this has been covered before, I did do a search for it but only found 10 bolt swap from late model stuff threads. My short online search revealed that GM made this rearend from 1965 to 72 for cars and until the eighties in trucks, Is this true? So did any 73 to 77 A body GM's get a 12 Bolt and if they did what were they and how were they optioned, If not, why the heck not?? These cars weigh nearly 2 tons and GM put 12 bolts in cars hundreds of pounds lighter. Was it GM in their infinite wisdom and the gas crunch that killed the 12 bolt option for cars?
And if so, I must say that sucks big time. I'm tired of looking to upgrade a 10 bolt, so if theres no 12 bolt available I'm going to the furd 9. Not even Moser lists a bolt in third gen a body 12 bolt.
I dont even know why I waste my time thinking about it anyways, the stock 12 bolts had thin wall axle tubing, poorly supported bad main caps, and tapered axles with C clips anyways.
And you'd think I'd learn my lesson after building one for my Nova, it cost me nearly as much as a brand new crate Moser 9 inch to beef up a 68 12 bolt I paid 500 dollars for the housing.. with Moser axles and support cover, Eaton posi, Mark williams main cap, new 3.73's, 1350 input yoke, and have the tubes TIG welded to the center section I had 2800 total in the thing. Screw going there again, I'm switching to the 9 inch and quit trying to put a band aid on arguably the most important part in the whole car. I mean all that effort for a used cop car 10 bolt that still needs a rebuild? No way.
Tell me if I'm wrong. And why please.

by carcass Thu Feb 14, 2013 7:40 pm

Also I'm not interested in shortening and adding c clip eliminators to a second gen , or a truck twelve bolt, that seems like more of the same PAIN.

by Joe73 Thu Feb 14, 2013 7:58 pm

No 12 bolts that I've ever seen or heard of. With Gm changing from a 8.2 diameter ring gear to 8.5 ring in 1970 1/2, they increased the size of the pinion bearing which allowed it to be a much stronger rear. A properly set up 8.5 10 bolt can takes lots of power. My buddy had a grand national running low 10's with its original beefed up 10 bolt.

I say keep the rear you have, put in a nice set of gears with a good posi or locker diff, c-clip eliminators and maybe upgrade the axles. A girdle rear cover would also be nice.

by Limey SE Thu Feb 14, 2013 8:10 pm

ONLY 1 G3 came with a 12 bolt and it was the 1973 Motion SS Chevelle

by carcass Thu Feb 14, 2013 8:32 pm

I agree that the ten bolt can take some abuse when setup properly, heck I've beat on em' plenty in stock form, mine has a mini spool and stock axles and what I'm sure are stock axle bearings and I have yet to break it, although it protests loudly nowdays.
The price of ten bolt upgrades is high though,
250$ axles
250$ gears
500$ eaton
set up kit and bearings 150$
support cover 150$
proffesional set up(optional) 300$??
weld the tubes 150$
c clip eliminated axle ends$$$
gets spendy quick!

by carcass Thu Feb 14, 2013 8:33 pm

plus my ten bolt is the odd hourglass rear cover press bearing one

by carcass Thu Feb 14, 2013 8:34 pm

anybody know where to get better axles for that guy? Or a support cover?

by carcass Thu Feb 14, 2013 8:35 pm

So did the Motion SS have a GM manufactured specific rear?? Or was it a custom one off?

by Joe73 Thu Feb 14, 2013 8:49 pm

I'd like to see the build sheet for the motion SS. Just too see what kind of option code it would have been to get a 12 bolt in there. A listing for the motion says,

1973 Baldwin/Motion Chevelle-The last Baldwin/ Motion car built as Baldwin Chevrolet closed it's doors in late 72 which meant all cars built after were Motion only cars. Full paperwork includes letter from Joel Rosen & correspondence between he & orig owner as far as what parts he wanted & cost. Itemized work orders from Motion, 427 L-88 motor, 12 bolt rear w/ 4:88 gears, Hona-drive (O/D), 4-spd, Open side exhaust. All Original metal. Cortez silver with black stripes and callout letters, Like new in & out. Retains all original hard to find Motion add on parts include the alarm, yeah that's right, alarm!

So who knows? I think because it was march of 1973 when motion built it, and the 12 bolt was still the standard for heavy duty rears from gm, motion put one in with 4.88's.

Just my opinion but I would think that a few more 12 bolts would have slipped through GM in other models. Especially since the 73's have been produced since the middle of '72.

Hard to say though.

by JB2wheeler Thu Feb 14, 2013 8:56 pm

carcass wrote:So did the Motion SS have a GM manufactured specific rear?? Or was it a custom one off?

It was a one off that was by a dealer simular to the Yenko custom models. They just did not have any more customers who wanted the 73 year model except the one. JB

by JB2wheeler Thu Feb 14, 2013 8:59 pm

I have Moser axles in my 10 bolt rear end, but can't remember if they are 30 spline or 31 spline or 32 spline. JB

by carcass Thu Feb 14, 2013 9:47 pm

If the twelve bolt did make it into a 73 then we know for sure that somebody, be it GM or a one man/small shop custom rearend manufacturer (motion??) produced a third generation A body 12 Bolt. If it was GM then undoubtedly there is more on this earth. The 68 to 72's had different measurements and mounting points am I right? So this MOTION SS has either a one off custom manufactured rearend or a GM produced part number, is that the only two options??

by Joe73 Thu Feb 14, 2013 10:18 pm

A 68-72 12 bolt will go into our G3's. Although the shock mounts for the 68-72 are on the rear of the axle and ours are in front. The 66-67 12 bolt is narrower if I remember correctly. So you might have to just do a shock mount switch. Is it worth it, I wouldnt bother.

Our G3's use longer UPPER control arms and the same length lowers. I've switched around my lowers from earlier year chevelles. I've have aluminum bushings with grease fittings, Square tubular arms with steel sleeves instead of bushings and I've even ran both with ladder bars. I'm now a fan of polyurethane and thats what I'm leaving in there. 73-77 ladder bars are horrible because they go to the lower shock mount. Which I've ripped off a couple of times. I still have a brand new set of "traction bars" for our cars. They look just like ones for leaf spring cars but they're for coils.

I'll take our 10 bolt any day.

by Famstyle Thu Feb 14, 2013 11:54 pm

Joe are you sellin the slapper bars?

by pila Fri Feb 15, 2013 12:15 am

There was a guy that I heard about, running a Chevy big block with a 8.5 10-bolt on the drag strip for a few seasons already. The 8.5 10-bolt in my '77 Elco, after the built 400 SBC was installed, didn't break any gears, but was really hard on rear axle bearings, and axles where the bearings run. Finally had the scrap that rear end because the bearings wore the housing out as well as axles.
I have a spare 10-bolt 8.5, which I keep around for my Malibu, which now has that same 400 ( 408 now) in it. I don't abuse it so much as I did the Elco.

by Limey SE Fri Feb 15, 2013 12:40 am

Joe would like them bars as well if he passes

by Joe73 Fri Feb 15, 2013 7:51 am

I'll be keeping the bars. They're such an odd piece for coil cars. Dont know if I'll be using them with the new engine. I had an extra set of just the bars, no hardware, the bars were chrome, guy gave me 50 bucks just for them about 10 years ago. My current set are yellow.

by dragons_lair59 Fri Feb 15, 2013 12:54 pm

lol the high performance yellow GoFast

by Joe73 Sat Feb 16, 2013 5:21 pm

In case anyone was wondering what the traction bars for our cars look like, I have a pic. The plastic bag contains the adjustment arm that has hein joints on the ends.

did any 3rd gens get a 12 bolt?? 30wntdh

did any 3rd gens get a 12 bolt?? 30wntdh

by 1973 454 MONTE Sat Feb 16, 2013 6:25 pm

I had traction bars on my 73 years ago. They replaced the lower control arm. Sold them

by Joe73 Sat Feb 16, 2013 7:03 pm

These dont replace the lower control arms. They bolt to the lower control arm bolt at the rear and then an adjustable rod with hein joints connects from the front of the bar to the rebound bumper perch on your axle. Like this.

http://www.behrents.com/mm5/merchant.mvc?Screen=PROD&Store_Code=BPW&Product_Code=MORC2111&Category_Code=Suspension-Bars

by oldstofty Sat Feb 16, 2013 10:54 pm

My 8.5 has moser axles, welded tubes, 4.88s, full spool, ultimate girdle with support bars, and hotchkis upper and lower arms it's been 11.30s

by 2fat2fly Sun Feb 17, 2013 12:17 pm

oldstofty wrote:My 8.5 has moser axles, welded tubes, 4.88s, full spool, ultimate girdle with support bars, and hotchkis upper and lower arms it's been 11.30s

Hey ! don't forget someone here named Mr Keith Seymore runs 10's give or take with just a few modes to his 10 bolt, much respect to him also !

I ran mine without any modes with just the 540 but made sure that the tires were forgiving. I'm not out there like keith but I do have fun when needed.

With the addition of the F-2 Pro-Charger I thought it best to adjust things
Southside lower control arms and adjustable uppers, the diff.is from quick performance, Strange Detroit locker h.d with 3.89's, Moser 35 spline axles and a 2500 stall, 4L80E built to hold, power at over 1300 h.p at 5500 rpm and torque over 1400 at 4400 rpm, seems to hold very well. With the F-2 the rpm can get away at times when the tires get loose it's went to 7800 rpm.
At around 6000 the boost is growing to about 15 lbs and it's not recommended to go past that, (so the 7800 is kinda scary) been there a few times !!
[img] did any 3rd gens get a 12 bolt?? Ss_pic10

did any 3rd gens get a 12 bolt?? Ss_pic10

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[img] did any 3rd gens get a 12 bolt?? Ss_pic11

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[img] did any 3rd gens get a 12 bolt?? Ss_pic12

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[img] did any 3rd gens get a 12 bolt?? Ss_pic13

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[img] did any 3rd gens get a 12 bolt?? Ss_pic14

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by carcass Sun Feb 17, 2013 1:03 pm

2fat2fly wrote:
oldstofty wrote:My 8.5 has moser axles, welded tubes, 4.88s, full spool, ultimate girdle with support bars, and hotchkis upper and lower arms it's been 11.30s

Hey ! don't forget someone here named Mr Keith Seymore runs 10's give or take with just a few modes to his 10 bolt, much respect to him also !

I ran mine without any modes with just the 540 but made sure that the tires were forgiving. I'm not out there like keith but I do have fun when needed.

With the addition of the F-2 Pro-Charger I thought it best to adjust things
Southside lower control arms and adjustable uppers, the diff.is from quick performance, Strange Detroit locker h.d with 3.89's, Moser 35 spline axles and a 2500 stall, 4L80E built to hold, power at over 1300 h.p at 5500 rpm and torque over 1400 at 4400 rpm, seems to hold very well. With the F-2 the rpm can get away at times when the tires get loose it's went to 7800 rpm.
At around 6000 the boost is growing to about 15 lbs and it's not recommended to go past that, (so the 7800 is kinda scary) been there a few times !!

Now thats what I'm talking about!! How did you choose your nine inch rearend width?? Did you get it stock width from axle flange to axle flange?? did you compensate for any special disc brake setup for width?? and did you use the stock control arm mounts? Thanks in advance.
I'd bet a guy could spend just about as much on a ten bolt buildup as a nine inch too, well maybe a few more bucks for the nine, but well worth it for sure,
that feeling of an assured foundation for power must be great.

by WyldeBill66 Sun Feb 17, 2013 4:14 pm

I read an article yesterday that the earlier 10 bolts were the weak ones. Like 69 and older. The article said the ones in our cars were actually almost as strong as the 12 bolts. The ring gear was only a little smaller than the 12 bolt and much cheaper to build. I realize the 9" would only be a little more than building a 10 bolt, but I have a problem putting an F-word product in my car. That's just me though.

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