8.5 rear end. Custom cruiser

Old February 18th, 2019, 04:39 PM
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8.5 rear end. Custom cruiser

Could a 1984 custom cruiser have come stock with an 8.5 rear end? My 81 has a 7.5 in it now and would like to upgrade. There is an 84 in the junkyard I was pulling parts off of and saw the tell tale signs( square potrusions on the bottom) and even looked at some 7.5ís in g bodyís to make sure I was not seeing things. Are the 8.5ís the only gym rear end with the square protrusions on the bottom?
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Old February 18th, 2019, 07:17 PM
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Originally Posted by young olds View Post
Could a 1984 custom cruiser have come stock with an 8.5 rear end? My 81 has a 7.5 in it now and would like to upgrade. There is an 84 in the junkyard I was pulling parts off of and saw the tell tale signs( square potrusions on the bottom) and even looked at some 7.5ís in g bodyís to make sure I was not seeing things. Are the 8.5ís the only gym rear end with the square protrusions on the bottom?
The wagons got either the 8.5 or the 8.75. Are you sure that's a 7.5?
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Old February 18th, 2019, 08:57 PM
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I have not looked since I first brought it home but almost confident I saw a 7.5 and from my reading the wagons did not get 8.5’s until 91(last gen) that is why I was confused when I saw the 8.5 in the 84. I will take another look at mine, would be nice if it was already an 8.5 would save money and mine already has a posi.
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Old February 18th, 2019, 10:35 PM
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Usually b body platform got the 8.5 usually a wagon got the 8.75 as stated.
How sure are you this vehicle has its original rear under it?
1978 -1987 and 89-96 B-body platform cars DID get 7.5 rears. Impala sedans and low powered LeSabres v6 cars got the 7.5 rears. SO it is possible the wagon has the 7.5, BUT it just may have been put there by someone beside the factory. The fact is all the rears interchange in that b-body platform Buick Olds Pontiac Chevy all of them. Even deville caddy.

hERE IS SOMETHING YOU CAN TRY... LOOK AT THE rpo CODES AND SEE THE GEAR RATIO...tHEN OPEN THE REAR AND SEE IF THE RATIOS MATCH IF THEY DO NOT FOR SURE SOMEONE SWAPPED OUT REARS. Man sorry bout the caps... fat fingered the caps lock.

Take some pictures of the cover and post...
Jim - JD Race
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Old February 19th, 2019, 05:30 AM
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Originally Posted by young olds View Post
I have not looked since I first brought it home but almost confident I saw a 7.5 and from my reading the wagons did not get 8.5ís until 91(last gen) that is why I was confused when I saw the 8.5 in the 84. I will take another look at mine, would be nice if it was already an 8.5 would save money and mine already has a posi.
My 1984 Custom Cruiser has a factory installed 8.5. The 1978 CSM shows that the Custom Cruisers that year got an 8.75. The wagon rears are wider than sedan rears. If the wagon had a sedan 7.5 under it, the rear tires would rub on the inner wheel wells.
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Old February 19th, 2019, 08:29 AM
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I ran out and looked at mine again lastnight after reading joe’s post. The rear end did not have square portrusions like an 8.5 but it did look like if you took an 8.5 and machined a partial circle into the corners as well as it had a large round ( with scallops at 3 and 9 o clock) inspection cover with a simple “bubble”( no ring gear bulge). Definitely looked to beefy to be a 7.5 and I am fairly confident it is all original as I have receipts from original owner dating back to 82 ( I did not see anything about rear end) and the owner between us only had it a year and did nothing with it. I am thinking I must have this 8.75( I have not heard of this rear end before now) I will get a picture after work.

Last edited by young olds; February 19th, 2019 at 08:32 AM.
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Old February 19th, 2019, 08:36 PM
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I did not get a picture of mine but I looked up pictures of the 8.75 and that is what mine is. Thank you guys.
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Old February 20th, 2019, 05:25 AM
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picture 8.75

so it looks like this? But has a fill plug n the rear cover off to the passenger side of the cover. Yes?

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Old February 20th, 2019, 08:31 AM
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I will have to check on the fill plug, but I did find the build sheet above the gas tank last weekend so I referenced that and it showed rear axle 8.75.
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Old February 20th, 2019, 11:03 AM
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build sheet

That is cool your still finding cars with build sheets...lol. Everything we ever got our hands on was already messed with. That is fun stuff to see where the sheets were put... Headliners, seats, seat back shells, door panels, etc...
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Old February 20th, 2019, 11:58 AM
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I was very surprised to see it. I dropped the tank to check the sock on the pick up and after I drug it out from underneath the car I was wondering why there was a piece of paper in there, I never would have thought of finding one in an 80’s car, and the strangest thing to me is that I have a receipt showing that the tank was cleaned in the 90’s, I would have thought it would have been removed at that time. It is in decent shape too.
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Old March 14th, 2019, 01:27 PM
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A check look on inside of trunk lid there should be sticker service Parts identification which list G80 is limited slip differential GU1 -GU7 is ratio Gu4 is 3.08 also if changing from 7.5 t0 8.5 drive shaft is different length also get transmission speedometer gear

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Old March 14th, 2019, 01:31 PM
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Originally Posted by acolds View Post
A check look on inside of trunk lid there should be sticker service Parts identification which list G80 is limited slip differential GU1 -GU7 is ratio Gu4 is 3.08 also if changing from 7.5 t0 8.5 drive shaft is different length also get transmission speedometer gear
The OP has a 1984 Custom Cruiser. Olds didn't use the SPID until the 1985 model year. My 84 Custom Cruiser doesn't have one either.
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Old March 14th, 2019, 02:33 PM
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My 1984 98 has the sticker and the manual also states the same

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Old March 14th, 2019, 04:50 PM
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Mine is an 81, the junkyard car with the 8.5 was an 84.
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