1977-1979 rearends

[79mine2]

What is the ring size? it do not measure 8.2 or 8.5 to me (i'm just a beginner)
Why aftermarket dealers say b.o.p 10 bolts, no talk of cadillac?
who sells posi units for 2:28 - 2:73 ratios cadillacs like it was offered in 1979 ?
I put rear disc brake on the 79 coupe , and these question came to mind.

any knowhow is helpful.

[cadiac]

What is the ring size? it do not measure 8.2 or 8.5 to me (i'm just a beginner)
Why aftermarket dealers say b.o.p 10 bolts, no talk of cadillac?
who sells posi units for 2:28 - 2:73 ratios cadillacs like it was offered in 1979 ?
I put rear disc brake on the 79 coupe , and these question came to mind.

any knowhow is helpful.
I think i read that the Cad rear is 8.75. Only came in cad's. Aftermarketr stuff fot one I can't help. I have an 81 rear, 3:42 I think (GU6 code) limited slip. Odometer says 80 something ?? I'm in Kansas if your interested. Can pull the cover and have a look -check ratio for sure.

[DaveM]

You are right on the 8 3/4" ring gear.... but the lowest ratio I know of was a 3.23 and that was only in a car with a V6 buick motor...If that is a posi that would be a great find...

[STDog]

The Sales brochure for 1980 only shows the 3.23, and that's with the V6.
http://www.modifiedcadillac.org/documents/1980s/1981/1981%20Sales%20Brochure/tn/36.JPG.html

[Ted in Olympia WA]

Didn't they chage the rear in 81 to the 7.5 inch to go with the new smaller engine? Also did the 81 come with overdrive?

In 1978 the lowest gear was 3.08

TED

[manogeorge]

'82 was the first year for the 4.1 and that was paired with the 200-4r and a 3.42 ratio. '81 was 4-6-8 (368) with a 2.41 and a th400, or a 3.23 with the v6 and the 200-4r. About the posi unit, is it possible to take the limited slip unit from one rear and use it in another? I'm thinking of doing this myself and have no experience, but it would be a lot cheaper than buying new.

[Ted in Olympia WA]

I have not put it in yet but I purchased a 1991 Rear out of a caprice cop car. They say it will go right in and it's a very common 8.5 rear. This one happened to be a 3.42 posi (I guess the 305 cars had 3.42 and the 350 cars had 3.08 or 3.23); I thought it was a 3.08 and was a bit disappointed, but a $30 3.08 gear set from eBay will fix the problem. Only thing I can see is different so far in the pinion yoke but I think (hope) it will swap over from the 78 rear.

A lot of full size GM's had the 8.5 rear and they are not hard to find and I think this is the easiest solution.

TED

[DaveM]

1981 was the last year for the 8 3/4" rear end, except for the limo.. they kept it till 84... The 8 3/4" limited slip rears , on the inside are casted different... so you can't swap parts...

[79mine2]

The limited slip rear ends are cast different?
I can not just find a limited slip rear end (77-79) and remove the posi unit then put it into my one wheel wonder (77-79) ?:mad: that will suck big time

[79fleetwood]

the 77-79 is a 8.875 12 bolt ring gear but its different then a regular GM 12 bolt no parts are available. my 79 had the 2.28 open diff and i swapped the whole rearend from a 95 roadmaster sedan. 2.93 factory posi 250 bucks from the junkyard. i had the outside of the axle flang cut down to fit in the factory cad rear rotors and used the caliper brackest from the stock rear also. the newer rearend is a 8.5 10 bolt. and it holds up to the 70 472 i have in the car

[79fleetwood]

the pinion yoke from the factory rear in my 79 bolted right up to the 8.5 10 bolt

[Ted in Olympia WA]

the pinion yoke from the factory rear in my 79 bolted right up to the 8.5 10 bolt

This is good news, I was wondering about this. I have to put this swap on the short list of thigs to do.

TED

[Eugene Cady]

Hello All, I Have a rear-end delemma too. I swapped in a '79 Caddy 425 with its t.h.400 trans. into a '79 Chevy 1-ton truck that originally had a 4 bolt main 350 small block & a 4 speed manual trans. Those parts were gone when I bought the truck. So now the truck still has the original 14 bolt G.M. with a 4.10 to 1 ratio, way too high for economical fuel mileage. The sad thing is, that type rear-end only has a 3.73 to 1 ratio for the lowest ratio. But a "lesser" 14 bolt with a 9.50 ring gear can be found with a 3.42 to 1 ratio. The question is, can the diff. with the 9.50 ring gear size be bolted into the "heavy duty" 14 bolt that came with a 10.50 ring gear? Do the axle splines match? I may have to go salvage yard hunting for a rear that is already set up with the lower ratio's that'll fit a 1-ton Chevy truck. Well, thanks. Gene

[Ted in Olympia WA]

I would think not, you are going to have to find a whole rear that will work for you. Maybe one out of a 3/4 ton.

The other option (expensive) is a gear vendor or very tall tires.

TED

[Eugene Cady]

Thanks Ted, I was afraid of that. But at least I won't have to do the "marrige" thing with ring & pinion set swapping. I'l just have to swap out the entire rear end, housing & all. Does anyone have a use for a 1-ton 4.10 to 1 geared Chevy 14 bolt rear end? I'd hate to just scrap it, as there's nothing wrong with it outside of the problem of not matching with a Caddy power plant & t.h.400 auto. trans. Thanks. Gene

[OldSub]

Eugene which rear do you have? The GM 14-bolt full float 10.5 inch rear has several options. Randy's Ring and Pinion lists 3.21, 3.42 and 3.73. I have a 3.42 under my 1-ton truck.

You should be able to swap gears instead of shopping for a new rear.

[Eugene Cady]

Hello, "OldSub". The rear end in my "79 Chevrolet/Cadillac truck is a 4.10 to 1, 14 bolt GM with the 10.50 inch ring gear. According to "LMC" truck, the lowest gear set is a 3.73 to 1 for that size differential. The 3.42 to 1 or lower would be had if I were to swap out the 10.50 rear for a 9.50 14 bolt that does have the optional 3.42 to 1 ratio. Or can I find a gear ser smaller than a 3.42 to 1 that'll bolt onto the larger differential case? The 9.50 won't fit in the axle housing due to the difference in the way the axles themselves are fastened to the rear-end. The 14 bolt/ 10.50 has the axles "bolted" on to the hubs with 8 small bolts on the axle's ends, as the 14 bolt/9.50 have the axles held in by "C" clips on the "inner" ends of the axles. "The twain don't mix" , so I've been told anyway. But, I could have been misinformed. Anyway, this type of thing is sorta new to me as I've been told that swapping out rear-end gears is a "Tricky & complicated" issue best left to professional drive train specialists. If the ring & pinion don't "run" in the correct patern, then one would end up with a "Howling mad, hot & jerky-querky" rear-end that will quite quickly fail to function properly. At the cost of a "new" gear set, I must know how hard is it really to properly set up a rear-end after swapping out the ring & pinion set to a lower ratio? Anyway thanks for the reply. I hope you or someone out there in "Caddyland" can give me the right directions with this project. I don't want give-up on driving & enjoying my "Project Cadrolet" truck because of the 4.10 to 1 rear ratio "sucking gas like a thirsty camel in a desert". Gene

[OldSub]

Gene,

LMC Truck is a good source for stock parts but I would not limit your search to what they have available.

I just checked Randy's Ring and Pinion and they do list 3.21 and 3.42 gear sets for the rear you have. The 3.21 is currently out of stock but you can order the 3.42 today. I don't work for them and have no connection to them, except I know they carry parts. Randy's Ring and Pinion (http://www.ringpinion.com/PartsList.aspx?SearchMode=Diff&TypeID=5&Type=GM&DiffID=35&DiffName=GM+14T&CatID=10&CatName=Rings+%26+Pinions&CatType=Sub-Differential).

Changing a ring and pinion is something you need to do right, but my understanding is that the 14 bolt rear is one of the easier ones to work on. I've not had a 14 bolt apart, but have had good luck doing backyard differential work on others.

I'm amused at your CaddlyLand comment. You were responding to my very first post on this forum. I don't think of myself as an expert on Cadillac's but rather as a student planning to put one into an old truck.

I have a '55 1st GMC 1-ton that has a 14 bolt under the rear and a Caddilac 500 about ten feet away from the currently empty engine compartment. With 265/75-16's and 3.42 gears I expect to run 75 mph with the motor turning about 2800.

You can see pictures and description at Caddy powered GMC 1-ton (http://www.maxwellgarage.com/MaxProjects/ServiceTruck.html).

My dream is to cross Wyoming pulling a trailer and able to pass anyone going only the speed limit. I did it once before with a 454 powered 1-ton will 4.10 gears in a Dana 70HD. That one got about 8 mpg and I hope this one will do better.

[~JM~]

....Does anyone have a use for a 1-ton 4.10 to 1 geared Chevy 14 bolt rear end? I'd hate to just scrap it....

Have you given any thought to swapping a Dana 60 or 70 underneath your truck?

Those 14 Bolts are well thought of in the 4x4 community. I would think that an Ad on Craigslist would find a new home for it in 48 hours or less.

Here are a few links that should keep you busy for longer than it will take to sell that 14 Bolt. :D

http://www.pirate4x4.com/tech (http://www.pirate4x4.com/tech)
http://www.pirate4x4.com/tech/billavista/14b_bible/index.html (http://www.pirate4x4.com/tech/billavista/14b_bible/index.html)
http://77cj.littlekeylime.com/index.htm
http://77cj.littlekeylime.com/Dana60.htm
http://77cj.littlekeylime.com/Dana70.htm (http://77cj.littlekeylime.com/Dana70.htm)
http://www.snort4x4.com/forum/showthread.php?s=&threadid=7216
http://www2.dana.com/expert/wc.dll?cvsp~deabill~2