1978 LeSabre Rear End and Transmission Question.

[SBRMD]

If one ordered a '78 LeSabre with the optional Pontiac 301 V8 and the G80 Positraction rear (and A/C):

 

1.  What rear end would it have (eg corporate 8.5", etc.)?

2.  What would the rear end ratio be?

3.  What wheel bolt pattern would it have?

4.  Which automatic transmission would it have?

 

Thanks to the assembled in advance,

Steve in Mpls.

[ted sweet]

by 77 electra has the olds 403 and has the regular buick bolt pattern

[NTX5467]

In that era, there were TWO GM transmission case bolt patterns.  The "Chevy" pattern and the "BOP" pattern.  I believe the Chevy pattern has a "round" top of the case whereas the "BOP" pattern had a "V" in the top middle of the case.  These illustrations can be found in many transmission ID publications.

 

In 1977, GM started using TWO option codes for automatic transmission options.  The basic code was "MX0" for a normal three-speed automatic, with a secondary code to determine WHICH three speed it might be, with "M38" being the THM350 code from prior times.  On the Chevy side of things, when you ordered a new '77 Caprice, you ordered MX0 and basically took pot luck of whether it would come with a THM350 or the newly-introduced THM200.  We sold a new Caprice to a customer, who really liked the car and  his neighbor/friend then ordered one for himself.  The first one (which he liked) had a THM350, yet the one he received had a THM200.  There were concerns about shift smoothness, which was attributable to the one car having a THM350 and the other one having the THM200.  All with the 305-4bbl engine.  AND no real way to orchestrate getting a THM350 on a normal Caprice, unless you might get a particular option code mix which would trigger the heavier-duty THM350 option.

 

On the rear axle side of things, it's very possible that the GM 7.5" rear axle was used in many cars.  The normal GM 8.5" (10-bolt, round cover) axle was not quite as common as in other Chevy B-cars prior to 1977.  Normal axle ratios for the Caprices were about 2.41 with the 3-speed automatics.  The downsized-1978 A-cars got a 2.29 axle ratio, but with their smaller diameter tires, the rpm levels were probably similar to the 2.41s in the larger cars.

 

I suspect the wheel bolt pattern would have been the larger 5x5 pattern on the Buicks.  Might have been the normal Chevy pattern of 5x4.75, though . . . not sure.

 

The original design intent of the Pontiac 265-301 V-8 was a light-weight, decent power, FUEL ECONOMY engine.  It became much more than that when it was factory turbocharged for the TransAms.

 

Working from memory . . .

NTX5467

[JohnD1956]

From the 1978 Buick service manual, the "B" series car, which is the Lesabre, with a Pontiac 301 could have either a 200 trans or a 350 trans.  Guess as Willis says, it would be luck of the draw which trans was installed. 

As for the posi rear axle vs conventional, both would have been a 2.41 ratio, however a conventional ring gear would have been 71/2 inches while the posi ring gear would have been 8 1/2". 

In terms of which axle housing to expect, there could be any one of three 8 1/2 units, with a Posi option.  There is a fourth on that is 7 1/2 inches but since the posi ring gear is 8 1/2 inches, logic says that can't be in a car with positraction.  Note however, that all 4 axles have 10 bolts holding the cover on.

As a side note, the manual shows that for "B" cars,  the code for the axle is on a tag attached to the rear cover.  Therefore, If there is a positraction unit in the car, there will be two tags. 

 

PS: my 78 Estate Wagon does not have any tags attached to the rear cover.  Perhaps lost over the years, or never installed at the factory, or removed for some reason and discarded. I mention this in case one was thinking of sourcing a better axle ratio rear at a salvage yard.

Edited May 31, 2016 by JohnD1956 (see edit history)

[NTX5467]

In the absence of the rear axle tag, the "birth certificate" of the rear axle is in a string of numbers and letters stamped into the passenger side rear axle tube. Location is "as if the car was on a lift and you were looking at the rear axle from under the rear seat passenger's seat location", about 1/2 way between the center of the axle and the brake backing place, facing toward the front of the car.  This string of letters and numbers will denote which plant the axle was built in, when it was built, with the last THREE letters being the gear ratio and if it includes the G80 Locking Rear Axle option.  These letters are listed in the front of the GM Parts Book.  In some of the earlier vehicles, it was a TWO letter code at the end of the string of letters and numbers.  The ONE exception to this was some of the Dana rear axles in pickups, which used a paper tag around the axle tube  . . . which usually didn't last very long after the vehicle left the assembly plant.

 

The bolt circle on the 7.5" rear axle cover is more "flattened" than completely round as the 8.5" rear axle is.  Easy to see visually by looking under the car.  As a side note, the 7.5" rear axle was also used in GM F-body (Camaro/Firebird) and DID have G80 capabilities in those vehicles (which never did use the 8.5" rear axle in the 1982+ models.  The (usually) red plastic tag for "PosiTraction", which means it would need special fluid and/or the P-Trac oil additive added to normal rear axle lube . . . would be attached with the rear axle plug, especially if it is on the front side of the housing.  The other "ratio tag" would be attached under one of the rear axle housing bolts, for the rear cover.

 

NTX5467

[SBRMD]

Thanks guys, this is very helpful.  Contemplating a clean low-mileage specimen with a mind toward replacement of the 301 with something more entertaining and Buick-y; hence interested in the stoutness of the rest of the chassis.  The car's not local, so can't inspect it easily.  It would be nice if it had the 5x5 bolt pattern, stouter rear and a THM 350.

 

It seems like GM sort of didn't want people to know what was in the cars.  I saw the window sticker, and while it calls off the fact that the engine is built by Pontiac, and the G80 Posi rear, it says nothing else except "automatic transmission" under standard equipment.

 

FWIW, it's a Flint-built car.

Edited May 31, 2016 by SBRMD (see edit history)

[NTX5467]

Thanks for that additional information.

 

Typically, when GM build a chassis for use on multi-brand cars, the front crossmember area had bolt holes to accommodate any engine family which could go onto that chassis.  Where the uniqueness comes in was in the "frame mouths" for each engine family.  So, if you want a different engine, BE SURE to get the frame mounts (which is what the motor mounts bolt to!) for the donor engine.  At this point in time, these would be salvage yard items,

 

My gut suspicion is that the Pontiac 301 would have had the THM200 transmission.  Originally, the THM200 was designed for smaller engines with low power.  BUT this is the same transmission family which went behind the Turbo Buick engines.  It takes, I believe, less power to run than the THM350 AND can be built to be as strong with aftermarket parts.  The THM200-4R (OD version) has become the darling of the street rod crowd, in "beefed" form with aftermarket parts, handling Chevy 454s reliably.  The THM200 family has a 2.74 (approx.) low gear, which can help with the 2.41 rear axle ratio, but not quite as low as the THM700s 3.06 low gear.  The THM350 is 2.52.

 

Now, you could make a pretty neat car . . . with some parts swapping and upgrades.  Put the Caprice police car sway bars on it (which might need different rear control arms).  The 2.41 rear axle ratio will mean you'll be limited to a three-speed automatic as too high-geared to use the OD of the 4-speed automatics (usually a 3.42 ratio is needed or a 3.08 if the engine is fuel injected).  The 2.41 ratio will also limit the "wildness" of the camshaft you might choose, too, as cruise rpm will be about 2000rpm at 60mph.

 

IF the rear axle is the Chevy 10-bolt, then you can upgrade the rear brakes to either 11x2 drums or the rear add-on-style disc brakes from the 1990s Caprice cars.  Wheels will need to be 15x7, which was a factory-available size on Chevies and Buicks, even in factory aluminum (or aftermarket wheels with the correct rear backspacing).

 

The transmission crossmember might not have clearance for a real dual exhaust to be installed, so it might be necessary to configure a dual-converter exhaust as some of the Camaro IROC-Z cars had (with the two converters being side-by-side, staggered, on the right side, then going to their respective mufflers.  Or perhaps the crossmember from a 1990s Roadmaster might fit, which had true dual exhaust from the factory.

 

The factory F41 Caprices from back then were great handling and riding cars.  The sway bars kept them flat in corners and the firmer shocks kept things "level" and comfortable.  The noted brake upgrades will help, too.  The Buick 350 (in 1970 specifications) is a nice, torquey V-8 (moreso than the similar Chevy V-8), to me.  Adding some modern ignition and fuel system upgrades should only make it better.

 

So, in some respects, mechanically, these cars can be a "blank canvas" to use to make interesting and neat cars from.  Problem is that resale value might not offer financial viability to the project.  But it'll be a neat and fun car when done . . .  and a "sleeper".

 

NTX5467  

[SBRMD]

An exercise in deductive reasoning:  Since the 8.5" ring gear can't fit in a 7.5" carrier, and since the G80 posi rear has to have an 8.5" ring gear (and discounting the Camaro/Firbird G80 situation, which must have been different), I think we can safely assume it has an 8.5" rear.  Does this mean that it has a 4.75" bolt pattern?  In senior series Buicks of the era, it seems to me that only cars with heavier-duty components (eg a 403) got the 5x5 bolt pattern - probably meaning an 8.75" rear -  as indicated sometimes by big-car ralley wheels; but could an 8.5" rear have a 5x5 bolt pattern?

 

I'd like to get the owner to put the car on a hoist, look at the axle tube, tags, and measure the bolt pattern, but just not feeling that demanding yet, don't want to piss him off.

 

Thanks as always in advance,

Steve.

Edited June 3, 2016 by SBRMD (see edit history)

[NTX5467]

Wheel bolt patterns were not generally dictated by the rear axle ring gear diameter, BUT other factors not related to that situation.  For example, the first year of the Cadillac Seville, it was "a Nova" of sorts, but for the second model year they standardized the bolt circle to the 5x5 pattern from the 5x4.75 pattern.  Same rear axle, different bolt circle on the axle shafts. 

 

The 7.5" and 8.5" rear axles are completely different "breeds of cat".  Only thing which might be the same is the lube they use, or the wheel studs. 

 

The '77+ GM B-platform was about the same weight as the prior '76 and back GM A/G-platform cars, so no real engineering reason for the 5x5 pattern if the smaller pattern worked on the prior cars just fine.  A few hundred pounds is not going to be that critical as to bolt circle situations.  Nor would engine torque (considering all of the GM muscle cars had the smaller circle and MUCH more torque.

 

When Cadillac went to the 5x5 circle on the Sevilles, it was more to match what other Cadillacs were using (which would include front disc brake rotors and bearings) rather than have "oddball-t0-them" parts on the vehicles.  In carlines other than Chevrolet, it was much more common to have "common parts" than not in the chassis and powertrain areas.  This meant that with fewer mechanical build variations, stocking necessary parts at the dealers was easier to do and have better coverage at the same time. 

 

IF you really want this car, you probably need to go see it in person OR get some pictures of the rear axle (from the rear of the car) so you can see the actual bolt circle roundness.  You might be wanting a factory G80 car, which is good, BUT there are aftermarket limited slip differentials which will fit the 7.5" rear axle too, but they also might require a different rear axle ratio than the car might have.

 

NTX5467