68-72 4.10 12 bolt posi for Beaumont, Chevelle, GTO etc

4SPEED427 · A Poncho Legend!

68-72 4.10 12 bolt posi for Beaumont, Chevelle, GTO etc

73SC · A Poncho Legend!

I should know the answer but I don't, why does this unit not interchange into the 73-77 A bodies, in particular that blue car in my avatar?

4SPEED427 · A Poncho Legend!

73SC wrote:

I should know the answer but I don't, why does this unit not interchange into the 73-77 A bodies, in particular that blue car in my avatar?

I don't know. I can ask around.

73SC · A Poncho Legend!

Thanks Carl, I don't think I would consider 4.10's but something with a little better launch than my current Highway Ratio of 2.73 would be nice. 73 GTO's had 3.42 which would be a good compromise.

65Camino · Uber Guru

Come on, go for the 4:10! Make it haul @$$...
We'll make it fit!

73SC · A Poncho Legend!

65 Camino got us the answer from his Chevelle pals and e-mailed me:

72-back = either 8.2" 10-bolt, or 12-bolt

73-77 = 8.5" 10-bolt

78-up = 7.5" 10-bolt (usually)

No interchange

4SPEED427 · A Poncho Legend!

I'm not totally convinced because I know of a guy who bolted a 68-72 diff into G body. It was an 85 Monte SS. Not sure what it involved though.

Astro Jet · Poncho Master!

I agree with Carl, I am sure I know of others who have done it. I think the issue is the mounting points for the suspension, particulary the upper A arms. I think they have to be moved and rewelded so while not a direct bolt in, it can be done.

73SC · A Poncho Legend!

Have been doing a bit of research on this topic. Turns out the '73 is an 8.5 10 bolt and everything I read suggests it is every bit as good as the '72 and earlier 8.2 12 bolt. All that needs to be done is get a gear set and posi for the 8.5 carrier. Seems that the 8.5 is widely used, especially in GMC and Chevy Trucks which attests to its strength and durability. Found an article which suggest that upgrading the 2.73 to 3.42 is a $400 deal.

http://www.chevyhiperformance.com/techarticles/48518_posi_10_bolt_upgrade_install/index.html

73SC · A Poncho Legend!

Carl Stevenson wrote:

I'm not totally convinced because I know of a guy who bolted a 68-72 diff into G body. It was an 85 Monte SS. Not sure what it involved though.

You know I had an '86 Monte SS and that was posi with a 3.73 from the factory, interesting that this guy did this.

Here is a chart that helps:

http://www.differentials.com/pdfs/40.pdf

-- Edited by 73SC at 17:31, 2008-05-08

2drpost · Uber Guru

carl,
you may have a tough time getting $1100.-1200. for that 12bolt. i sold a '70 monte 12bolt 3.31 posi a couple years ago for $600., had it advertised awhile to get that.

427carl · A Poncho Legend!

I saw a couple of them in N.B. when I was there, and they were getting 12-1500.. Carl is a good saleman He'll get the bucks...

ak 67sd · Poncho Master!

should be able to get $1000 for it around here, maybe a bit more... parts like that are just harder to find around winnipeg, but buyers might be hard to find too! ...there was a 67 diff, all rebuilt/painted with aftermarket axles for $1500 in MN last month (the 67 is worth a bit more than the later ones, since it is a one year only fit)... for comparison, there has been a non-posi 68 12 bolt diff in the buysell for $600, and its been there a while... shipping costs narrows your market a bit carl, so depending what you are trading (if they are easier items to ebay) then you might end up a bit short on the trade...

my brother has a 68 lemans convert with 10 bolt open diff, it would nice for him (i asked already but he's not that interested in putting more $ into his driver), but he'd want to change the gears to a low 3 series...

andrew

Taylor55 · Poncho Master!

The 67-69 camaro 66 chevelle and the 68-72 nova 12 bolt posi diffs are worth the most money because they are the narrowest. These diffs get $1000-$1500 depending on condition and gears. The wider 68-72 Chevelle 12 bolt bolts posi go for $800-$1200 again depending on condition and gears. These diffs are only getting rarer so in my opinion they will only continue to climb in value. The 4:10 gears would work really good with a 700R4 transmission. The Camaro 10 bolt 8.5 is a strong diff and readily available and resonably priced. At the price of fuel today I would leave the 2:73 gears in. My 1981 Z28 Camaro has the 3:42 gears and it is peppy around town but hard on fuel on a long trip.I am going to leave it that way as the car is all numbers matching.

Al

73SC · A Poncho Legend!

Taylor55 wrote:

At the price of fuel today I would leave the 2:73 gears in.
Al

Good point on the fuel, I just got back from a short highway run that lasted 40 minutes. I had just under a half tank of gas when I left and had to stop for fuel on the way home because I thought I would run out, that's about 6 gallons for a 50 mile trip. I wasn't that easy on it, cruised at 70 mph and punched it a few times, but even at 70 I was looking at my shift quadrant looking for another gear. cry

4SPEED427 · A Poncho Legend!

73SC wrote:

Have been doing a bit of research on this topic. Turns out the '73 is an 8.5 10 bolt and everything I read suggests it is every bit as good as the '72 and earlier 8.2 12 bolt. All that needs to be done is get a gear set and posi for the 8.5 carrier. Seems that the 8.5 is widely used, especially in GMC and Chevy Trucks which attests to its strength and durability. Found an article which suggest that upgrading the 2.73 to 3.42 is a $400 deal.

http://www.chevyhiperformance.com/techarticles/48518_posi_10_bolt_upgrade_install/index.html

Did you mean '72 and earlier 8.2 10 bolt? The 12 bolt is an 8.875, and after what I've seen over the years , I can certainly say that the 12 bolt is stronger than the 8.5. However, with that said. the 8.5 will stand up to quite a bit with an automatic. They don't survive real well behind a strong engine with a manual trans and any kind of a sticky tire though.

gparis7 · Canadian Poncho Superstar!

I believe the 8.5 10 bolt was used in 71-76 full size cars, as well as 77-90 full size wagons and police cars. I had two 87-88 police cars, both of which suffered broken gears in the differential while I owned them, despite no hard use from me. Each car had less than 100,000 kms on it when the diffs broke.

4SPEED427 · A Poncho Legend!

If you had a governor style 8.5 (which I think those cars you mentioned would have had) then I would agree with you. The governor posi's have been a weakness on the 8.5. The earlier design posi in the 8.5 was a lot more durable.

2drpost · Uber Guru

agreed carl.
would take a 12 bolt over an 8.5 rear for strength.
my last k1500 p/up split the governor style carrier with no real abuse. ended up changing the ring and pinion, had a helluva gear noise with the new carrier installed.

gparis7 · Canadian Poncho Superstar!

Both my Caprice cop cars had 3.08 open rear ends - no posi. In one I had to replace the whole diff case as it was worn beyond saving due to loose centre pin (used the same ring gear on the new case), and in the other, the spider gears broke into small pieces. I never did get them noise free afterwards despite everything checking out as on spec.