9" Rear End
12-20-2018, 01:25 PM
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Car: Transam, 85
Engine: 305 HO, 4bbl Carb, G Engine
Transmission: 5 Speed
Axle/Gears: 3.73
9" Rear End
Summary:
MANY years ago, my TA's rear end went bad and replaced a 3.70 with 3.73 in a mechanic shop. I did write down the part number then. Many years later I found, I went from Posi Brog Wanrer 3.70, to Standard 3.73. Since then speedometer was off a bit and fastness reduced.
Now:
I found a third gen Camero drum brake differential for cheap price, and thinking of buying it and build a disc 3.70 posi. Presently tracking following parts.
1. Axel Shaft : 26 spline , 30-38" ($280 for 2 in Summit racing)
2. Motive Gear GM9-370 Ring an pinion (GM 7.75") 3.70 $ 200 from Amazon
3. Tru trac 26 spline pos: $400
Questions:
1. What else do I need
2. Axel shaft has 26 spline but Pinion says 28 spline. Do the the pinion and axel shaft spline has to be same number.
Any help is appreciated.
Thanks.
MANY years ago, my TA's rear end went bad and replaced a 3.70 with 3.73 in a mechanic shop. I did write down the part number then. Many years later I found, I went from Posi Brog Wanrer 3.70, to Standard 3.73. Since then speedometer was off a bit and fastness reduced.
Now:
I found a third gen Camero drum brake differential for cheap price, and thinking of buying it and build a disc 3.70 posi. Presently tracking following parts.
1. Axel Shaft : 26 spline , 30-38" ($280 for 2 in Summit racing)
2. Motive Gear GM9-370 Ring an pinion (GM 7.75") 3.70 $ 200 from Amazon
3. Tru trac 26 spline pos: $400
Questions:
1. What else do I need
2. Axel shaft has 26 spline but Pinion says 28 spline. Do the the pinion and axel shaft spline has to be same number.
Any help is appreciated.
Thanks.
12-20-2018, 06:07 PM
#3
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Re: 9" Rear End
The pinion gear is connected to the thing the drive shaft bolts to. The axles plug into the side gears. Totally different parts. Kinda like asking what threads are on the exhaust manifold bolts and then wondering if you're supposed to buy spark plugs with the same threads.
They are therefore totally unrelated except to the extent that model a rear might have x pinion splines and either y or z axle splines.
9" is a Frod rear end. Needless to say, didn't come in these cars. I'll "assume" you meant 9-bolt instead.
Your 85 L69 T/A probably came with a 3.70 9-bolt; arguably one of the most desirable stock rears for these cars, except for the crappy 88-back brakes it came with. The 89-up brakes are VASTLY better. If you took your 3.70 9-bolt and put on 89-up brakes, or took a 89-up 9-bolt and put 3.70 gears in it, you'd have pretty much the top of the line, even though such a combo never existed from the factory.
The difference between 3.70 and 3.73 gears is less than 1%. That's less than the difference you would have with the same gear going from tires with wore-out tread to otherwise identical tires with new tread; or, letting 2 psi of air out of them. "Fastness" and "speedo" didn't change because of that. Not saying it didn't change; only, it didn't change FOR THAT REASON. Now, if the new ratio wasn't REALLY 3.73, then all bets on the subject are off. Time to PHYSICALLY LOOK AT - PLACE AN EYEBALL ON - the gears THEMSELVES, and see what they REALLY are.
There no such car as "Camero". I'm gonna give you the n00b benefit of the doubt JUST THIS ONCE and assume on your behalf that you're asking something about a "CamAro".
The 9-bolt and 10-bolt are COMPLETELY DIFFERENT rears. NOT ONE SINGLE PART interchanges except wheel studs.
10-bolt axles were 26-spline from the beginning of time (77 I think... it was introduced then in other car lines besides Camaro/Firebird) until late 89, then are 28 spline from then to now. Most 89s are 26 spline and most 90s are 28 but in those years it's REAL SMART to actually LOOK AT IT before buying parts.
I would not spend A DIME on 26-spline parts, ESPECIALLY not when replacing both carrier and axles. Buy em both in 28 spline instead. NOT ONLY are the spline counts different, BUT ALSO the diameter is different. There's something on the order of a 30 - 40% difference in breaking strength between the little weenie 26 stuff and the later 28. LS1 cars come with 10-bolts with 28-spline axles. Costs the same, fits the same, don't use your own money as the weapon to shoot yourself in the shorts with.
9-bolt rears don't use 26-spline axles. They have 28 splines but are COMPLETELY DIFFERENT IN EVERY WAY from 10-bolt ones.
10-bolt rears don't have a 3.70 ratio option.
The 10-bolt pinion has 27 splines. Can't recall off the top of my head what the pinion spline count in the 9-bolt is.
There are VERY VERY VERY FEW (emphasis on VERY) 9-bolts with drums. They do exist, but I only have personal knowledge of exactly ONE (verified by a photo). Odds are above 1000:1 that if it's a drum rear, it's a 10-bolt.
The 10-bolt ring outer diameter is 7.625" in later model ones to be exact, but it's referred to as 7.5" because the larger gear bolts right onto any carrier in that model regardless of what OD gear was bolted to it originally. The ring diameter increased that slight amount in about 87 or so, not sure exactly since it doesn't really matter. AFAIK all replacement gears that you can buy for that model rear nowadays, with the possible exception of NOS ones from 86 or before, are 7.625" dia even though the catalog usually lists them as being for 7.5". The rear model is referred to as the 7½".
The 9-bolt ring dia is 7.75".
You need to identify what you are working on more clearly.
Not sure about "mechanic" and "shop". Not a language I know (and I know quite a few). It's helpful to use English on this forum.
They are therefore totally unrelated except to the extent that model a rear might have x pinion splines and either y or z axle splines.
9" is a Frod rear end. Needless to say, didn't come in these cars. I'll "assume" you meant 9-bolt instead.
Your 85 L69 T/A probably came with a 3.70 9-bolt; arguably one of the most desirable stock rears for these cars, except for the crappy 88-back brakes it came with. The 89-up brakes are VASTLY better. If you took your 3.70 9-bolt and put on 89-up brakes, or took a 89-up 9-bolt and put 3.70 gears in it, you'd have pretty much the top of the line, even though such a combo never existed from the factory.
The difference between 3.70 and 3.73 gears is less than 1%. That's less than the difference you would have with the same gear going from tires with wore-out tread to otherwise identical tires with new tread; or, letting 2 psi of air out of them. "Fastness" and "speedo" didn't change because of that. Not saying it didn't change; only, it didn't change FOR THAT REASON. Now, if the new ratio wasn't REALLY 3.73, then all bets on the subject are off. Time to PHYSICALLY LOOK AT - PLACE AN EYEBALL ON - the gears THEMSELVES, and see what they REALLY are.
There no such car as "Camero". I'm gonna give you the n00b benefit of the doubt JUST THIS ONCE and assume on your behalf that you're asking something about a "CamAro".
The 9-bolt and 10-bolt are COMPLETELY DIFFERENT rears. NOT ONE SINGLE PART interchanges except wheel studs.
10-bolt axles were 26-spline from the beginning of time (77 I think... it was introduced then in other car lines besides Camaro/Firebird) until late 89, then are 28 spline from then to now. Most 89s are 26 spline and most 90s are 28 but in those years it's REAL SMART to actually LOOK AT IT before buying parts.
I would not spend A DIME on 26-spline parts, ESPECIALLY not when replacing both carrier and axles. Buy em both in 28 spline instead. NOT ONLY are the spline counts different, BUT ALSO the diameter is different. There's something on the order of a 30 - 40% difference in breaking strength between the little weenie 26 stuff and the later 28. LS1 cars come with 10-bolts with 28-spline axles. Costs the same, fits the same, don't use your own money as the weapon to shoot yourself in the shorts with.
9-bolt rears don't use 26-spline axles. They have 28 splines but are COMPLETELY DIFFERENT IN EVERY WAY from 10-bolt ones.
10-bolt rears don't have a 3.70 ratio option.
The 10-bolt pinion has 27 splines. Can't recall off the top of my head what the pinion spline count in the 9-bolt is.
There are VERY VERY VERY FEW (emphasis on VERY) 9-bolts with drums. They do exist, but I only have personal knowledge of exactly ONE (verified by a photo). Odds are above 1000:1 that if it's a drum rear, it's a 10-bolt.
The 10-bolt ring outer diameter is 7.625" in later model ones to be exact, but it's referred to as 7.5" because the larger gear bolts right onto any carrier in that model regardless of what OD gear was bolted to it originally. The ring diameter increased that slight amount in about 87 or so, not sure exactly since it doesn't really matter. AFAIK all replacement gears that you can buy for that model rear nowadays, with the possible exception of NOS ones from 86 or before, are 7.625" dia even though the catalog usually lists them as being for 7.5". The rear model is referred to as the 7½".
The 9-bolt ring dia is 7.75".
You need to identify what you are working on more clearly.
Not sure about "mechanic" and "shop". Not a language I know (and I know quite a few). It's helpful to use English on this forum.
Last edited by sofakingdom; 12-20-2018 at 06:44 PM.
12-20-2018, 06:24 PM
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Re: 9" Rear End
Yeah, you are getting 9 bolt and 10 bolt parts mixed up. The 3.70 gear is for a 9 bolt Borg Warner rear end, not the GM 10 bolt. If you have a rear end with 26 spline axLEs then it's a 10 bolt. I would not invest any money in 26 spline axles when 28 spline parts cost the same and are about 30% stronger. By the way, it's a 9 bolt, not a 9 inch. The 9 inch is a Ford rear end.
12-20-2018, 10:14 PM
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Car: Transam, 85
Engine: 305 HO, 4bbl Carb, G Engine
Transmission: 5 Speed
Axle/Gears: 3.73
Re: 9" Rear End
-9" Rear End: Actually I was thinking it is even possible to built an end with 9" gear ring. Now I know with my knowledge I shouldn't even think of that.
-My mistake on 'CamAro'
-"https://righttorisesuperpac.org/start-manage-mechanic-shop/" ??
SofaKingdom/ Thanks for clearing the pinion spline/AxLe spline part.
-My mistake on 'CamAro'
-"https://righttorisesuperpac.org/start-manage-mechanic-shop/" ??
SofaKingdom/ Thanks for clearing the pinion spline/AxLe spline part.
12-20-2018, 10:37 PM
#6
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Car: Transam, 85
Engine: 305 HO, 4bbl Carb, G Engine
Transmission: 5 Speed
Axle/Gears: 3.73
Re: 9" Rear End
Presently car has 41/11(Counted) , 7.5", 10 bolt, standard, disc brake, from a 1984. I couldn't count the axle spline. Runs normal, but never like previous 3.70 posi one. Can I convert this to Posi, or do I need to built/buy a new one.
As from 'drum brake rear end' ....I found one in junk yard.......I have no clue what spec it has. I only wanted to buy it to use it as shell and buy other parts.
As from 'drum brake rear end' ....I found one in junk yard.......I have no clue what spec it has. I only wanted to buy it to use it as shell and buy other parts.
12-20-2018, 10:53 PM
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Re: 9" Rear End
You can add a Eaton Posi, Eaton Truetrac, Yukon Dura Grip or other limited slip differential to the 10 bolt, but I wouldn't stay with the smaller 26 spline axles. I would upgrade the whole thing to 28 spline axles.
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12-22-2018, 02:28 PM
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Car: 1985 Pontiac Trans Am
Transmission: BW T5, Hurst Short Throw Shifter
Axle/Gears: 3.70 Posi
Re: 9" Rear End
The pinion gear is connected to the thing the drive shaft bolts to. The axles plug into the side gears. Totally different parts. Kinda like asking what threads are on the exhaust manifold bolts and then wondering if you're supposed to buy spark plugs with the same threads.
They are therefore totally unrelated except to the extent that model a rear might have x pinion splines and either y or z axle splines.
9" is a Frod rear end. Needless to say, didn't come in these cars. I'll "assume" you meant 9-bolt instead.
Your 85 L69 T/A probably came with a 3.70 9-bolt; arguably one of the most desirable stock rears for these cars, except for the crappy 88-back brakes it came with. The 89-up brakes are VASTLY better. If you took your 3.70 9-bolt and put on 89-up brakes, or took a 89-up 9-bolt and put 3.70 gears in it, you'd have pretty much the top of the line, even though such a combo never existed from the factory.
The difference between 3.70 and 3.73 gears is less than 1%. That's less than the difference you would have with the same gear going from tires with wore-out tread to otherwise identical tires with new tread; or, letting 2 psi of air out of them. "Fastness" and "speedo" didn't change because of that. Not saying it didn't change; only, it didn't change FOR THAT REASON. Now, if the new ratio wasn't REALLY 3.73, then all bets on the subject are off. Time to PHYSICALLY LOOK AT - PLACE AN EYEBALL ON - the gears THEMSELVES, and see what they REALLY are.
There no such car as "Camero". I'm gonna give you the n00b benefit of the doubt JUST THIS ONCE and assume on your behalf that you're asking something about a "CamAro".
The 9-bolt and 10-bolt are COMPLETELY DIFFERENT rears. NOT ONE SINGLE PART interchanges except wheel studs.
10-bolt axles were 26-spline from the beginning of time (77 I think... it was introduced then in other car lines besides Camaro/Firebird) until late 89, then are 28 spline from then to now. Most 89s are 26 spline and most 90s are 28 but in those years it's REAL SMART to actually LOOK AT IT before buying parts.
I would not spend A DIME on 26-spline parts, ESPECIALLY not when replacing both carrier and axles. Buy em both in 28 spline instead. NOT ONLY are the spline counts different, BUT ALSO the diameter is different. There's something on the order of a 30 - 40% difference in breaking strength between the little weenie 26 stuff and the later 28. LS1 cars come with 10-bolts with 28-spline axles. Costs the same, fits the same, don't use your own money as the weapon to shoot yourself in the shorts with.
9-bolt rears don't use 26-spline axles. They have 28 splines but are COMPLETELY DIFFERENT IN EVERY WAY from 10-bolt ones.
10-bolt rears don't have a 3.70 ratio option.
The 10-bolt pinion has 27 splines. Can't recall off the top of my head what the pinion spline count in the 9-bolt is.
There are VERY VERY VERY FEW (emphasis on VERY) 9-bolts with drums. They do exist, but I only have personal knowledge of exactly ONE (verified by a photo). Odds are above 1000:1 that if it's a drum rear, it's a 10-bolt.
They are therefore totally unrelated except to the extent that model a rear might have x pinion splines and either y or z axle splines.
9" is a Frod rear end. Needless to say, didn't come in these cars. I'll "assume" you meant 9-bolt instead.
Your 85 L69 T/A probably came with a 3.70 9-bolt; arguably one of the most desirable stock rears for these cars, except for the crappy 88-back brakes it came with. The 89-up brakes are VASTLY better. If you took your 3.70 9-bolt and put on 89-up brakes, or took a 89-up 9-bolt and put 3.70 gears in it, you'd have pretty much the top of the line, even though such a combo never existed from the factory.
The difference between 3.70 and 3.73 gears is less than 1%. That's less than the difference you would have with the same gear going from tires with wore-out tread to otherwise identical tires with new tread; or, letting 2 psi of air out of them. "Fastness" and "speedo" didn't change because of that. Not saying it didn't change; only, it didn't change FOR THAT REASON. Now, if the new ratio wasn't REALLY 3.73, then all bets on the subject are off. Time to PHYSICALLY LOOK AT - PLACE AN EYEBALL ON - the gears THEMSELVES, and see what they REALLY are.
There no such car as "Camero". I'm gonna give you the n00b benefit of the doubt JUST THIS ONCE and assume on your behalf that you're asking something about a "CamAro".
The 9-bolt and 10-bolt are COMPLETELY DIFFERENT rears. NOT ONE SINGLE PART interchanges except wheel studs.
10-bolt axles were 26-spline from the beginning of time (77 I think... it was introduced then in other car lines besides Camaro/Firebird) until late 89, then are 28 spline from then to now. Most 89s are 26 spline and most 90s are 28 but in those years it's REAL SMART to actually LOOK AT IT before buying parts.
I would not spend A DIME on 26-spline parts, ESPECIALLY not when replacing both carrier and axles. Buy em both in 28 spline instead. NOT ONLY are the spline counts different, BUT ALSO the diameter is different. There's something on the order of a 30 - 40% difference in breaking strength between the little weenie 26 stuff and the later 28. LS1 cars come with 10-bolts with 28-spline axles. Costs the same, fits the same, don't use your own money as the weapon to shoot yourself in the shorts with.
9-bolt rears don't use 26-spline axles. They have 28 splines but are COMPLETELY DIFFERENT IN EVERY WAY from 10-bolt ones.
10-bolt rears don't have a 3.70 ratio option.
The 10-bolt pinion has 27 splines. Can't recall off the top of my head what the pinion spline count in the 9-bolt is.
There are VERY VERY VERY FEW (emphasis on VERY) 9-bolts with drums. They do exist, but I only have personal knowledge of exactly ONE (verified by a photo). Odds are above 1000:1 that if it's a drum rear, it's a 10-bolt.
12-22-2018, 02:43 PM
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Re: 9" Rear End
They're pretty unusual.
Now I know of 2.
Post a photo, everybody here would probably like to see it.
Now I know of 2.
Post a photo, everybody here would probably like to see it. 
12-23-2018, 10:25 AM
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Re: 9" Rear End
Thanks!! That's so bizarre.
I'd suggest getting rid of that stoooopid rubber plug in the cover... tap it VERY LIGHTLY to ½" pipe, put in a brass plug with some thread sealer on it. Then you can clean up that mess it always makes all over everything.
I'd suggest getting rid of that stoooopid rubber plug in the cover... tap it VERY LIGHTLY to ½" pipe, put in a brass plug with some thread sealer on it. Then you can clean up that mess it always makes all over everything.
12-23-2018, 02:08 PM
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Car: 1989-92 FORMULA350 305 92 Hawkclone
Engine: 4++,350 & 305 CIs
Transmission: 700R4 4800 vig 18th700R4 t56 ZF6 T5
Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: 9" Rear End
Def a odd duck to have the 9bolt and drums, cool to see proof!
12-24-2018, 03:44 PM
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Car: Transam, 85
Engine: 305 HO, 4bbl Carb, G Engine
Transmission: 5 Speed
Axle/Gears: 3.73
Re: 9" Rear End
Big Gear Head/
Finally I was able to count the spline on axle and it is 28!.
The old one of 3.70 was Borg W, 9 bolt, 26 spline, 3.70, posi. It was so good that during an uphill run in one stop light of my University town, I would beat any car and my car zoomed.
I assume all I need 28 spline posi unit. Right? Do I need any other specs.
Finally I was able to count the spline on axle and it is 28!.
The old one of 3.70 was Borg W, 9 bolt, 26 spline, 3.70, posi. It was so good that during an uphill run in one stop light of my University town, I would beat any car and my car zoomed.
I assume all I need 28 spline posi unit. Right? Do I need any other specs.
12-24-2018, 03:56 PM
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Re: 9" Rear End
If you have a 28 spline 10 bolt, and it has gears somewhere between 3.23 and 4.56, then you just need a 3 series limited slip differential of some kind. The Eaton Truetrac is a very popular choice. You will also need bearings for the differential and shims so that you can set the backlash and bearing preload.
Notice! Be sure to check backlash BEFORE you remove any parts from the rear end other than the axles. If you don't do this then you will have no idea what you need to set it at when you install the new differential.
The 9 bolt would also have 28 spline axles.
Notice! Be sure to check backlash BEFORE you remove any parts from the rear end other than the axles. If you don't do this then you will have no idea what you need to set it at when you install the new differential.
The 9 bolt would also have 28 spline axles.
12-25-2018, 08:35 AM
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Car: Transam, 85
Engine: 305 HO, 4bbl Carb, G Engine
Transmission: 5 Speed
Axle/Gears: 3.73
Re: 9" Rear End
It is definitely 3.73. I am trying to know only specification I need is '28 spline' when buying the posi piece, right? Is there any sepc for bearign and shims.
Thanks.
Thanks.
12-25-2018, 09:20 AM
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Re: 9" Rear End
Yes, you need a differential for 28 spline axles, if you are sure that you have 28 spline axles. The 28 spline axles were not used until '89 or '90, so if you have an earlier rear end with those axles then someone must have swapped them. The bearings will be the same as any of the 7.5/7.6 rear ends. I recommend getting the super shim set. This is the easiest shim set to use that I have found. https://www.summitracing.com/parts/yga-55001/overview/
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