Picking the right extension housing seal stops leaks and keeps transmission fluid where it belongs.
Good seals fit precisely, resist heat and chemicals, and last through many drive cycles.
This article lists five top seals for common GM and Ford automatics and explains how material and size affect performance.
Follow the installation and fit tips to reduce fluid loss and protect drivetrain components.
| TH350/TH250 Transmission Rear Bushing & Seal Kit |
| Best for TH350/TH250 | Application / Fitment: THM-250/250C/350/350C/375B tail housing (GM Turbo Hydra-Matic) | Purpose / Function: Prevent/reseal rear extension and speedometer housing leaks | Kit Contents / Components: Tail housing seal, bronze-lined bushing, O-rings (tail, yoke sleeve, speedometer) | VIEW LATEST PRICE | Read Our Analysis |
| TH350 and 700R4 Rear Seal for Extension Tail Housing |
| Made in USA | Application / Fitment: TH350 (1969+) and 700R4 (1983+) extension/tail housing | Purpose / Function: Replace extension/tail housing rear seal to stop leaks | Kit Contents / Components: Rear extension housing rear seal (single seal product) | VIEW LATEST PRICE | Read Our Analysis |
| C4 Rear Extension Tail Housing LEAK SEAL KIT C-4 1964-1986 |
| Ford C4 Specialist | Application / Fitment: Ford C-4 extension/tail housing (1964–1986) | Purpose / Function: Seal extension housing rear area to fix leakage | Kit Contents / Components: Metal-clad rear seal, extension bushing, paper gasket | VIEW LATEST PRICE | Read Our Analysis |
| TH400 Transmission Rear Extension Tail Housing Seal Hydramatic |
| Size-Specific Fit | Application / Fitment: TH400 extension/tail housing (Hydramatic, 1965+; verify OD) | Purpose / Function: Replace rear extension/tail housing output seal to stop leaks | Kit Contents / Components: Rear extension/tail housing large seal (single seal; verify size) | VIEW LATEST PRICE | Read Our Analysis |
| GM Genuine Parts Transmission Case Rear Extension Seal |
| OEM Replacement | Application / Fitment: GM transmission case rear extension (vehicle-specific GM OE fit) | Purpose / Function: Replace rear extension seal as OE rear case component to prevent leaks | Kit Contents / Components: Rear extension with seal included (OE assembly including seal) | VIEW LATEST PRICE | Read Our Analysis |
More Details on Our Top Picks
TH350/TH250 Transmission Rear Bushing & Seal Kit
Should you’re maintaining a classic car or rebuilding a transmission and you want a reliable rear seal fix that lasts, this TH350/TH250 rear bushing and seal kit is made for you. You’ll get a bronze-lined tail housing bushing, tail housing seal, O-rings for the tail and driveshaft yoke sleeve, plus speedometer housing seals. These parts match factory dimensions, so you can stop leaks and restore proper sealing at the rear extension and speedometer housing. Installation is straightforward, and the durable materials cut service visits. You’ll feel confident that your transmission stays sealed and service life is extended.
- Application / Fitment:THM-250/250C/350/350C/375B tail housing (GM Turbo Hydra-Matic)
- Purpose / Function:Prevent/reseal rear extension and speedometer housing leaks
- Kit Contents / Components:Tail housing seal, bronze-lined bushing, O-rings (tail, yoke sleeve, speedometer)
- Construction / Material Notes:Bronze-lined bushing; high-quality seals and O-rings
- Origin / Manufacturer Info:Aftermarket exact factory-replacement design (brand unspecified)
- Installation / User Guidance:Easy to install; intended to replace worn/damaged rear extension seals
- Additional Feature:Bronze-lined bushing
- Additional Feature:Speedometer seals included
- Additional Feature:Exact factory-replacement
TH350 and 700R4 Rear Seal for Extension Tail Housing
In the event that you care about a tight, reliable rear seal for your TH350 or 700R4 extension tail housing, this CT Solutions CT7888 is a strong choice that fits straight into setups from 1969 and up for TH350 and 1983 and up for 700R4. You’ll appreciate that it’s made in USA and sold under part CT7888. It replaces the extension housing rear seal and installs without fuss. The seller shows technical details and accepts returns under a 30 day Amazon guarantee. You can check warranty with the maker and read customer feedback before you buy.
- Application / Fitment:TH350 (1969+) and 700R4 (1983+) extension/tail housing
- Purpose / Function:Replace extension/tail housing rear seal to stop leaks
- Kit Contents / Components:Rear extension housing rear seal (single seal product)
- Construction / Material Notes:CT Solutions-made seal (made in USA)
- Origin / Manufacturer Info:CT Solutions; CT7888; Made in USA
- Installation / User Guidance:Direct replacement for listed transmissions; follow manufacturer/warranty guidance
- Additional Feature:CT Solutions brand
- Additional Feature:Made in USA
- Additional Feature:Launch Feb 19, 2024
C4 Rear Extension Tail Housing LEAK SEAL KIT C-4 1964-1986
You’ll want the C4 Rear Extension Tail Housing Leak Seal Kit in case you work on classic Ford C-4 transmissions and need a dependable fix for stubborn rear-end leaks. You’ll find CT3168, made in USA, fits C-4 models from 1964 to 1986. The kit stops leaks at the transmission rear through replacing worn parts. It includes a metal-clad rear seal, an extension housing bushing, and a paper gasket. You’ll appreciate the fitment is specific to C-4 rear extension housings, so installation is straightforward. You’ll feel confident tackling repairs aware the kit targets the exact leak points.
- Application / Fitment:Ford C-4 extension/tail housing (1964–1986)
- Purpose / Function:Seal extension housing rear area to fix leakage
- Kit Contents / Components:Metal-clad rear seal, extension bushing, paper gasket
- Construction / Material Notes:Metal-clad seal and extension bushing (Made in USA)
- Origin / Manufacturer Info:CT3168; Made in USA
- Installation / User Guidance:Intended for repair/maintenance of leaking extension housing areas; direct fit for C-4
- Additional Feature:Metal-clad rear seal
- Additional Feature:Includes paper gasket
- Additional Feature:Made in USA
TH400 Transmission Rear Extension Tail Housing Seal Hydramatic
Should you’re rebuilding a Turbo Hydramatic 400 and want a reliable rear extension seal that fits most 1965-and-up units, the CT6357 is a smart choice because it’s the large, commonly used tail housing seal and it keeps leaks from wrecking your work. You’ll appreciate that this TH400 rear extension tail housing seal measures 2.702 to 2.708 inches OD. Before you buy, measure your current seal since sizes vary and some units use a smaller seal. In case your measurement matches, order confidently. In case not, contact the seller with your dimensions and ask for guidance so you don’t guess.
- Application / Fitment:TH400 extension/tail housing (Hydramatic, 1965+; verify OD)
- Purpose / Function:Replace rear extension/tail housing output seal to stop leaks
- Kit Contents / Components:Rear extension/tail housing large seal (single seal; verify size)
- Construction / Material Notes:Specified seal OD 2.702–2.708″ (material not specified; standard rear seal)
- Origin / Manufacturer Info:CT6357 (CT or aftermarket part number; verify fit)
- Installation / User Guidance:Verify existing seal OD before purchase; direct replacement if size matches
- Additional Feature:Large-size OD spec
- Additional Feature:Verify existing size
- Additional Feature:Fits TH400 (1965+)
GM Genuine Parts Transmission Case Rear Extension Seal
Should you want a direct replacement that fits like the original and keeps your transmission sealed, the GM Genuine Parts 24224676 rear extension with seal is a smart pick for GM owners who need reliability and a precise fit. You’ll get a vehicle specific, painted rear extension that includes the seal and matches GM OE specs for fit, form, and function. It’s built to the same standards used on the assembly line and backed against General Motors, so you can feel confident. The part is compact and sturdy, ships in its original package, and has warranty and return options should assistance be required.
- Application / Fitment:GM transmission case rear extension (vehicle-specific GM OE fit)
- Purpose / Function:Replace rear extension seal as OE rear case component to prevent leaks
- Kit Contents / Components:Rear extension with seal included (OE assembly including seal)
- Construction / Material Notes:GM OE-grade materials/engineering (factory-spec construction)
- Origin / Manufacturer Info:GM Genuine Parts (OEM; formerly ACDelco; GM-backed)
- Installation / User Guidance:Vehicle-specific OE replacement; install as factory part (follow GM guidance)
- Additional Feature:OE-backed GM part
- Additional Feature:Vehicle-specific fit
- Additional Feature:Includes painted exterior
Factors to Consider When Choosing Automatic Extension Housing Seals
Once you’re choosing an automatic extension housing seal, start alongside checking fitment and compatibility so the part mates perfectly with your transmission and avoids leaks. You’ll also want to compare materials and durability alongside exact seal dimensions, since those two factors determine how well the seal will hold up and how easily it installs. Finally look for products with clear installation guidance and reliable warranty and support, which give you confidence and backup should anything goes wrong.
Fitment And Compatibility
Provided that you’re swapping a seal in your transmission tail housing, start with checking exact fit and compatibility so you don’t chase leaks or stress the parts later. You’ll measure the seal’s outer diameter and thickness and compare them to the tail housing. Small differences matter. Next, confirm the seal matches the exact transmission model and year range because revisions change sizes and mounts. Also verify whether the seal is for the tail housing or a concentric rear output location since lips, chamfers, and installation depth differ. Check whether the replacement comes with bushings, O-rings, or gaskets, or get their part numbers separately. Finally, match the seal’s design type to the shaft finish and operating conditions so you won’t face premature failure.
Material And Durability
Now that you’ve checked fit and compatibility, material and durability become the next things to watch because the wrong seal can look right but fail fast. You’ll want high temperature elastomers like Viton or premium nitrile that stay elastic up to about 250°F so they don’t harden or leak. Also consider metal support at the shaft. Bronze or metal lined bushings paired with compatible seal material cut wear and extend life. Check chemical compatibility with ATF since some polymers swell or degrade with certain fluids. Prefer seals made to OEM tolerances with corrosion resistant metal parts so pressure stays even and seepage is less likely. Look for integrated backup features such as O ring lips, dust lips, and multiple sealing surfaces for redundancy.
Seal Dimensions Accuracy
Because small measurement mistakes can turn a good seal into a leak, you’ll want to check every critical dimension carefully before installation. You should measure both the inner diameter of the bore and the outer diameter of the shaft to within 0.001 to 0.005 inch so the interference fit stops leaks without stressing parts. Also verify axial thickness and lip depth so the seal lip sits with correct preload. Too thin and the seal extrudes, too thick and mating surfaces misalign. Match the seal outer diameter tolerances to the housing bore and compare lip cross section and radius to shaft finish and runout. Finally, account for thermal expansion of seal material versus housing and shaft so clearances hold across operating temperatures.
Installation Ease
Installation should go smoothly once you pick a seal that fits the job and the hands that will install it. You want exact outer diameter and lip profile so the part presses in without guessing or trimming. Also choose seals sold with matching bushings, O rings, or gaskets when the housing service pulls multiple parts, since that cuts reassembly steps and lowers leak risk. Take note whether the design is metal cladded or rubber lip and whether a seal driver, press, or pilot is recommended, because controlled seating force prevents distortion. Look for clear installation notes on orientation, lubrication points, and torque sequence so you and your helper can follow a defined procedure. Finally prefer seals that allow bolt up of the tail housing without shaft realignment to save time and frustration.
Warranty And Support
Once you pick a seal, don’t skimp on the warranty and support details because they protect your time, money, and the work you’ve already done. Check warranty length so it fits expected service life, whether 30 days or several years, and match that to your risk comfort. Also confirm what the warranty covers, not only defects but normal wear, installation mistakes, and reactions with nearby materials. Look for clear return and replacement rules with time limits and required proof like receipts and photos. Verify technical support access for installation help, part ID, and fast responses so you avoid mistakes that void coverage. Finally, make sure terms say whether labor or consequential losses are covered or only the part itself.
