Macadoo

Found this on eBay. Will it fit my '71 Cutlass Supreme with the TH350 trans and small black 350? I currently have an open 2.56
Some pics of my rear end (insert jokes here)....

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And the link...

http://www.ebay.com/itm/151773764574...%3AMEBIDX%3AIT

Thanks guys!

m371961

I have to admit Limited slip diff. Was not my first thought of LSD.

coppercutlass

I belive you would have to swap the side gears from your current diff ., the chevy unit had a cross pin with another to hold the c clip axles inplace and I think they possibly have a diffrent width not to mention possible diffrence in axle spline. Our olds have bolt in axles which is a really nice perk. Never done the swap but my posi unit came from a 95 chevy impala . If my memory is right that's what my builder told me.

Fun71

Note that he has an open diff, so the side gears are different and won't work with the LSD. Don't quote me on this, but I think I remembered reading one of Brian Trick's posts that the c-clip side gears would work in the bolt-in rear as long as the spline count is the same. Need to verify this as my memory may be wrong. Best to check with either 507olds (Brian Trick) and Monzaz (Jim Mitschke) to make sure.

Macadoo

I'll do some more reading. Thanks guys. And say no to drugs.

oldsguybry

Since I can't get smileys to post all of a sudden .

joe_padavano

That is correct.

coppercutlass

Thats some good food for thought .

oldsguybry

This differential is the same one I have more then likely , so yes it will fit .

Macadoo

You fellas are takin' me to school Thanks.
So, from the info I have gathered here, from Monzaz, and the seller, the eBay unit is a GM 8.5" limited slip with 28 splines and 3.42:1 gears. It comes with Timkin bearings (but not the axle bearings), shims, seals, pinion nut, etc. Sounds like what I'm looking for. Now, it's just the risk of trusting the pics and the sellers answers. I'm going to make an offer. Thanks guys.

Macadoo

From the seller after I told him that Monzaz said it would fit.

"Buick Pontiac olds had GM make there own differentials which were different from the standard Chevy ones. Some BOPs came with the reg 8.5 and some didn't.
Like I said in the first email if you have the Chevy 8.5 diff this package will fit right in.
All the parts are in good condition and have been professionally checked and tested in our shop."

He also said that if I have bolt-in axles, I must have an 8.2" rear. I think he's nervous that it may not fit and doesn't want to get into a return situation. But from what has been posted in this thread, it all seems good to me. I suppose worst case scenario, I put it back on eBay. Thoughts?

joe_padavano

He would be wrong. From 1971-on, Olds used the "corporate" 8.5 with bolt-in axles in the A-body cars.

Macadoo

Thanks for the verification, Joe.

Fun71

I was focused on your questions and missed that you are planning to go from a 2.56 to a 3.42. Man, are you gonna be grinning from ear to ear.

Back in the early 80s I swapped the original 2.56 for a 3.08 and it felt like I had a much more powerful engine. Then a while later, I swapped the 3.08 for a Chevy 12 bolt with 3.55s and again it felt like I had a more powerful engine. Going from 2.56 to 3.42 is gonna be a HUGE jump in performance and significantly increase the fun factor.

Oh, one more thing - my in town mileage didn't change between the 2.56 and 3.55 gears. I didn't do much highway driving back then so no data on how the gear change affected highway mileage.

oldsguybry

I was wondering about this myself . The few times I took my 72 CS on the freeway , it sounded like the engine was revving pretty high with the current gearing ( 350/TH350 ) I'm assuming everything is original on my car in this aspect anyhow . The engine is out of a 76 Olds so I'm not sure what else was changed .

Macadoo

Mileage? We don't need no stinking mileage.
Actually, we do. That's why the next step will be a 200-4r trans. We're not going to get to places like Niagara Falls running 3k rpms.

Macadoo

I'm getting goosebumps

Macadoo

So I should mention that late last night (I can never sleep before the first day of class) the eBay seller accepted my offer. It originally had a starting bid of $395 (but ended with no bids), I got it for $375. Wish me luck that it's in as good a shape as the claim.

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Fun71

I have 3.23 rear gears and used to have short ~25" tall 245-60-14 tires and my engine would turn ~3400 RPM @ 70 MPH. The only issue I ever had with that was during the summer (110º or so) with the AC on the engine would warm up to 210-220º or so. If I backed down to under 3000 RPM the temp would drop to 190. Never had any issues during the cooler months, and I did lots of road trips.

Macadoo

Wow, I didn't realize tire size can make THAT much of a difference. With my 3.42 gears, 28" tires, and a 1:1 final gear (TH350, right?), RPMs are 2873 at 70 mph.

Fun71

That RPM looks like the calculator numbers for a manual trans (no converter slippage added). The calculator shows 2950 for an auto trans, but I don't know how they figured that. My converter slips around 150-200 RPM by my calculations..

I was surprised to find that my tires were shorter than the published height, which caused the actual RPM to be higher than the calculated RPM.

oldsguybry

The tires on mine right now are some kind of truck tires I think . I had 14" rims before ( originals ) , but now I have 15" SS2 rims with P225-75-R15 all the way around . They were on the rims when I bought them and the car sits pretty high now . Maybe that's why the motor seems to rev high on the freeway .

Macadoo

I hadn't thought about slippage. I looked at quite a few calculators and none of them accounted for that. Either way, I can live with 3k for short trips for a short time but "she who must be obeyed" likes the longgg drives. We're thinking of heading west next summer if I can get a 4 core radiator and aftermarket AC compressor. And, of course, the overdrive trans.

joe_padavano

That's because the calculators use the theoretical unloaded rolling radius. Tires compress when you load them, which changes the rolling radius and effectively makes the tire smaller in diameter. Better tire manufacturers list rolling radius or revs per mile to account for this. Radial tires compress more than bias ply.

Fun71

Taller tires would equate to lower RPM.

76olds

I'm running very close to those RPM's in my 76 with the 3:42 gears.

At first I really noticed the difference but after several drives it just seems like the norm now.
I put a 2 row BE COOL aluminum rad in that I purchased from summit and I've had no problems with overheating. The gauge stays at 190 even on the 38-40 deg humid days last weekend.
It seems to run cooler than the factory 3 row copper rad.
Can't wait to see the smile on your face after you get your new posi and gears in.

Macadoo

I had some trouble running 75-80 mph, up and down hills, with the car loaded and the AC on, 95*F day. Not everyday conditions but still.....

I didn't think I was in a hurry to get these gears in but now I'm all excited! Lol. I can wait....breath deep....I can wait....breath deep....I can wait........

Fun71

Just an FYI, your radiator doesn't care about humidity. It greatly affects us due to the lower evaporation rate with higher humidity, but your radiator is dry on the outside so it doesn't care.

Macadoo

Kind of like wind chill, yes? As defined by the wind blowing away the protective warm layer of air we generate around our bodies. I've had several debates about whether or not inanimate objects are effected by windchill.

Fun71

Many years ago I had a lengthy discussion with one of the local Olds club members who designed industrial HVAC systems and he said there was no factor in the calculations for humidity as it is water vapor (gas) and as such it is similar to all the other gases in the air (O, CO, N, etc).

BlackGold

And yet a common way to improve your engine's cooling "back in the day" was to hang a canvas bag full of water in front of the radiator.

So did this work by releasing water vapor and increasing the humidity (and presumably heat capacity) of the air flowing through the radiator?
Or did it work by releasing liquid water droplets into the air which then landed on the radiator and carried heat away?
Or did it not work at all and was just another way for dealers to make money off their customers?

76olds

Ok I'll bite ..." A" all the above as the sweat rolls off er' body.

Fun71

This.

While driving from Texas to Arizona I did a similar thing by throwing the leftover water in the dog's bowl onto the radiator while the engine was running and the temp dropped almost immediately to the point where I could remove the radiator cap to check the water level. It's amazing what low humidity does for evaporative cooling.

Macadoo

I got the LSD today. To my untrained eye, everything looks great. Bearings and races are smooth and clean, the pinion and ring gear don't show obvious signs of wear or have any chips. I know it comes down to the integrity of the clutches but honestly, I have no idea how to check that so I'll do some research. Here are some close-up pics.

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Macadoo

Well, from my reading it seems there's no good way to test the clutches until I get the unit in the car. So be it.