1984-1988 C4 4+3 Transmission Overdrive Unit Pan Gasket
Stop your 1984–1988 C4 Corvette 4+3 Doug Nash overdrive unit from leaking with this custom cork pan gasket — more effective than OE recommended RTV sealant and made in the USA.
The Doug Nash 4+3 overdrive unit is one of the most celebrated and most specific drivetrain components ever fitted to a production Corvette — and keeping it leak-free is critical to keeping it operational. If your 4+3 equipped C4 is marking its territory the overdrive unit pan gasket is where the diagnosis starts. This custom cork pan gasket outperforms the OE recommended RTV sealant approach and gives the overdrive pan a proper, reliable seal that holds.
The 4+3 transmission was a leading edge performance advantage during Corvette racing's most dominant years — powering five consecutive SCCA Showroom Stock Endurance Series Championships from 1984 through 1988. RPO MM4 production numbers confirm just how many of these transmissions are still on the road today — over 31,000 units built across the five model years. Keeping that overdrive unit healthy starts with clean oil and a correct pan seal.
Also available as part of a filter and gasket kit — see our C4 4+3 listings for the complete service kit.
Fits the Following Corvette Models:
1984–1988 C4 Corvette — 4+3 Manual Transmission with Doug Nash Overdrive Unit Only.
Features / Benefits:
- Custom cork overdrive unit pan gasket for 1984–1988 C4 Corvette 4+3 Doug Nash transmission
- More effective seal than OE recommended RTV sealant
- Cork construction for correct compression and long-term leak resistance
- Fits overdrive unit pan — 12 bolt pan configuration
- Also available as part of a complete filter and gasket kit
- Made in the USA
- GM reference part number not provided
Material / Finish:
Cork gasket construction — 1/16 in. thick for correct compression and reliable sealing at the overdrive unit pan.
Dimensions:
Overall: 10¾ in. x 7? in. | Thickness: 1/16 in.
Tech Tip — 4+3 Overdrive Unit Oil Change Procedure:
- Raise the car on a hoist and remove the 12 bolts holding the oil pan on the overdrive unit.
- Remove the oil filter and replace with a new one. Lubricate the pickup tube and new filter grommet with Dexron II ATF before installing — confirm the grommet is not pushed into the body of the filter.
- Clean the oil pan and dry thoroughly.
- Place the magnet in the correct location.
- Confirm the overdrive housing edge is clean and dry.
- Apply a bead of RTV to the oil pan flange.
- Place pan into position and reinstall all 12 bolts hand tight. Then alternate tightening bolts to 8 N/M or 6–8 foot-pounds.
- Remove the square-headed filler plug on the left side of the overdrive unit. Note — the steel plug threads into an aluminum case. A quick counterclockwise movement helps break it loose.
- Fill the overdrive unit with Automatic Transmission Fluid until oil runs from the filler hole. Coat filler plug threads with anti-seize compound before reinstalling.
- Convertible models — reinstall the X-member before lowering.
- Drive the car and retest the overdrive unit after oil reaches normal operating temperature.
- After the test drive raise the car again and recheck oil level — refill and retest if needed.
Change overdrive unit oil at least once per year on regularly driven cars. If you drive over 25,000 miles annually and use the overdrive frequently consider changing the oil sooner. Clean oil is the single most important factor in keeping the Doug Nash overdrive unit operating correctly. Keep TPS and clutch switches properly adjusted to prevent future overdrive unit failures.
Because Details Matter:
The Doug Nash 4+3 is not just a transmission — it is a piece of Corvette racing history fitted to over 31,000 production cars. Keeping it sealed, lubricated, and correctly maintained is how you keep it on the road for the next generation of owners. This cork pan gasket is where correct maintenance starts — a proper seal, a clean oil change, and the peace of mind that comes from doing it right.
Also known under these part numbers: (For reference only)