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Question: Rear Output Shaft Transmission Seal

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,457
Location
CenCoast CA
Corvette
71 04 12 19
Third seal in five years....
You didn't give the mileage but, I'll assume it's not much. Failing rear seals could be:

1) Wrong part.
(Get the right one from your parts vendor)
2) Incorrect installation.
(See the factory service manual for proper installation procedure)
3) Groove worn in the output yoke
(Replace the yoke)
3) Worn transmission, specifically the extension housing bushing.
(Replace the extension housing bushing or overhaul the trans)
 

Vette79

Well-known member
Joined
Jan 23, 2007
Messages
1,392
Location
Millersville, MD
Corvette
1979 L-48 Black Coupe
Third seal in five years....
You didn't give the mileage but, I'll assume it's not much. Failing rear seals could be:

1) Wrong part.
(Get the right one from your parts vendor)
2) Incorrect installation.
(See the factory service manual for proper installation procedure)
3) Groove worn in the output yoke
(Replace the yoke)
3) Worn transmission, specifically the extension housing bushing.
(Replace the extension housing bushing or overhaul the trans)

Thanks Hib. Well, no groove in the output yoke. Transmission was rebuilt approximately 7 years ago. I see that there are several different looking shaft seals depending on what vendor site you go to. That's why I would like someone to verify the link I provided as a correct part for a '79 Auto T350. For example, Zip and Corvette Central's Seals look different.
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,457
Location
CenCoast CA
Corvette
71 04 12 19
I can't speak specifically to the Turbo Hydramatic 350 output shaft seal because I've never worked on a TH350, but I do buy a lot of parts from Zip Products and, if Zip's and CC's parts look different, I'd buy the unit from Zip. Also, a TH350 is not unique to the Corvette. Millions were built for all kinds of GM vehicles. Since the seal is likely quite generic, I'd also take a look at RockAuto's site. I buy a lot of parts from them as well.

I'm going to guess that unless you have a problem with installation, that you're using up seals means there's some other problem with the trans. For example, if you grab the rear yoke and you can move it up and down quite a bit, that's a sign that the extension housing bushing could be faulty and you should have an auto trans shop inspect it.
 

Vette79

Well-known member
Joined
Jan 23, 2007
Messages
1,392
Location
Millersville, MD
Corvette
1979 L-48 Black Coupe
I can't speak specifically to the Turbo Hydramatic 350 output shaft seal because I've never worked on a TH350, but I do buy a lot of parts from Zip Products and, if Zip's and CC's parts look different, I'd buy the unit from Zip. Also, a TH350 is not unique to the Corvette. Millions were built for all kinds of GM vehicles. Since the seal is likely quite generic, I'd also take a look at RockAuto's site. I buy a lot of parts from them as well.

I'm going to guess that unless you have a problem with installation, that you're using up seals means there's some other problem with the trans. For example, if you grab the rear yoke and you can move it up and down quite a bit, that's a sign that the extension housing bushing could be faulty and you should have an auto trans shop inspect it.

Any idea where I can purchase this extension housing bushing or what it looks like?
 

Vette79

Well-known member
Joined
Jan 23, 2007
Messages
1,392
Location
Millersville, MD
Corvette
1979 L-48 Black Coupe
I can't speak specifically to the Turbo Hydramatic 350 output shaft seal because I've never worked on a TH350, but I do buy a lot of parts from Zip Products and, if Zip's and CC's parts look different, I'd buy the unit from Zip. Also, a TH350 is not unique to the Corvette. Millions were built for all kinds of GM vehicles. Since the seal is likely quite generic, I'd also take a look at RockAuto's site. I buy a lot of parts from them as well.

I'm going to guess that unless you have a problem with installation, that you're using up seals means there's some other problem with the trans. For example, if you grab the rear yoke and you can move it up and down quite a bit, that's a sign that the extension housing bushing could be faulty and you should have an auto trans shop inspect it.

OK, just a couple of questions. Looks like you can purchase re-manufactured tailshaft housing extensions for a TH350 that has the new bushing and seal already installed. Is there only one type of bushing to install as well as a seal? I would also need a new gasket that goes in between the tranny and tailshaft housing extension. The other question I have is the speedo hole. Is there more then one size? And what else would I be looking at to replace or purchase if swapping housing extensions?
 

TimAT

Well-known member
Joined
Sep 8, 2007
Messages
708
Location
Gladstone Missouri
Corvette
1969 LS-7 BB
Going from memory, there are 2 sizes of speedo gear adapters- the small one is about 3/4" diameter and the large one is 1-1/2. No way to mix them up- the large one has a horse shoe clip to retainer, while the small one uses a small flat bar with a bolt hole thru it.
The bushing for the yoke is not really difficult to replace- drive the old one out with a new one and the correct size bushing driver. Honestly, if you can get a reman housing that's already done for a decent price, you're time/money ahead. The trans case to extension housing seal is a square cut o-ring.

The appearance of the seal can be as simple as different manufacturers designs. The ID and OD of the seals should be the same as GM's original designs, unless they specify them for a repair application.
 

Vette79

Well-known member
Joined
Jan 23, 2007
Messages
1,392
Location
Millersville, MD
Corvette
1979 L-48 Black Coupe
Going from memory, there are 2 sizes of speedo gear adapters- the small one is about 3/4" diameter and the large one is 1-1/2. No way to mix them up- the large one has a horse shoe clip to retainer, while the small one uses a small flat bar with a bolt hole thru it. The bushing for the yoke is not really difficult to replace- drive the old one out with a new one and the correct size bushing driver. Honestly, if you can get a reman housing that's already done for a decent price, you're time/money ahead. The trans case to extension housing seal is a square cut o-ring. The appearance of the seal can be as simple as different manufacturers designs. The ID and OD of the seals should be the same as GM's original designs, unless they specify them for a repair application.
Thanks Tim. I'm going with a remanufactured housing with teflon bushing and seal already installed. Comes without a rubber square o-ring.
 

TimAT

Well-known member
Joined
Sep 8, 2007
Messages
708
Location
Gladstone Missouri
Corvette
1969 LS-7 BB
That rubber ring shouldn't be hard to come across. Any decent trans shop should be able to provide you one. I'd be curious about a teflon rear bushing- the originals were brass/bronze in a steel shell.
 

Vette79

Well-known member
Joined
Jan 23, 2007
Messages
1,392
Location
Millersville, MD
Corvette
1979 L-48 Black Coupe
That rubber ring shouldn't be hard to come across. Any decent trans shop should be able to provide you one. I'd be curious about a teflon rear bushing- the originals were brass/bronze in a steel shell.
It is a brass bushing with teflon coating for wear resistance.
 

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