If you’re restoring a GM TH350/TH400 automatic and want a reliable seal for the speedometer port, these plugs provide leak-proof performance and corrosion resistance. They’re best for classic GM builds where the TH350 or TH350C is present in models from 1968–1981. The top picks below suit different owner needs—from simple block-off for a no-cable setup to a robust, billet-aluminum option for long-term durability. This guide highlights five solid choices and explains who should choose each one.
| Product | Brand | Key Benefit |
|---|---|---|
| 2Pcs Speedometer Block Off Plug Kit for TH350 TH400 Transmission 1968-1981 | Acyoyava | Includes O-ring, wide compatibility, excellent sealing |
| B&M Racing 20299: Transmission Speedometer Port Plug | B&M | Billet aluminum, durable, anti-contaminant design |
| Etersec 345215 Speedometer Block Off Plug | Etersec | CNC billet aluminum, reliable leak prevention |
| Speedometer Block Off Plug with O-Ring for TH350 TH400 Auto | PFCKE | Oil-resistant O-ring, corrosion-resistant aluminum |
| Speedometer Block Off Plug Delete Speedo Port (TH350) | Jiacheng | Aluminum plug, long-term corrosion resistance |
2Pcs Speedometer Block Off Plug Kit for TH350 TH400 Transmission
Best for builders seeking a complete kit with an O-ring and broad compatibility. The Acyoyava set is CNC-machined from aircraft-grade aluminum with anti-oxidation treatment, offering strong corrosion resistance and heat tolerance. It’s designed to seal the 0.89-inch bore with gear teeth in the 17–22 range and replaces several OEM part numbers. The included O-ring improves sealing under vibration and temperature changes. Suitable for 1968–1981 GM cars and trucks with TH350/TH350C or Turbo Hydra-Matic variants.
Who it’s best for: classic GM enthusiasts performing a full TH350/TH350C rebuild or restoration who want reliable sealing and straightforward installation. Caution: verify bore and gear-teeth compatibility with your specific tail housing before ordering.
B&M Racing 20299: Transmission Speedometer Port Plug
Best for buyers seeking a billet aluminum construction with a branded finish and recognizable quality control. The B&M plug features a replaceable O-ring for easy maintenance and guards the transmission from contaminants during use or storage. It’s a straightforward, durable choice for TH350 systems and helps prevent leaks when the speedometer cable is removed. Caution: appearance and branding may be a consideration for those prioritizing a stock look.
Etersec 345215 Speedometer Block Off Plug
Best for those who want a strong corrosion-resistant solution with precise O-ring sealing. The Etersec plug is CNC machined from T6061 billet aluminum for durability and features a precision-fit O-ring to prevent leaks. It’s compatible with GM TH350 and TH400 automatics (1968–1981) and covers multiple GM brands. Caution: ensure the tail housing bore and gear-teeth match your model to ensure a perfect seal.
STPCTOU Speedometer Block Off Plug With O-Ring
Best for those who want a durable aluminum block with an oil-resistant O-ring. The STPCTOU plug emphasizes corrosion resistance and high-temperature tolerance. It fits a broad range of TH350/TH350C configurations from 1968–1981 and is suitable when sealing the unused speedometer port is a priority. Caution: verify that your vehicle’s annual use and temperature exposure won’t exceed the aluminum’s performance envelope.
Jiacheng Speedometer Block Off Plug Delete Speedo Port
Best for cost-conscious restorations requiring a simple aluminum plug. Jiacheng’s plug is CNC machined from billet aluminum, offering good corrosion resistance and long-term use. It’s designed for 1968–1981 TH350/TH400 GM transmissions and is suitable for many classic builds. Caution: look for exact bore and gear-teeth compatibility with your specific tail housing to avoid mismatches.
345215 Speedometer Block Off Plug w/O-Ring (EMIHO)
Best for those who need precise sealing with a high-quality O-ring and corrosion-resistant body. EMIHO’s plug uses T6061 aluminum alloy with anti-oxidation treatment and is compatible with 1968–1981 TH350/TH350C gear housings. It’s designed to replace OEM identifiers such as 25513247 and A44774. Caution: ensure the bore size and gear-teeth range of your tail housing match the stated specs for a secure fit.
Buying Guide: TH350 Speedometer Port Plug Basics
- Why block off the speedometer port? If you’re removing a functioning speedometer or converting to a non-speedometer setup, a block-off plug prevents fluid leaks and dust intrusion.
- Material matters: billet aluminum provides corrosion resistance and heat tolerance. Aircraft-grade alloys are common in premium plugs, while standard aluminum is adequate for most restorations.
- O-ring sealing: An O-ring improves leak protection and reduces vibration-induced leaks. Ensure the O-ring material is oil-resistant and compatible with common ATF fluids.
- Compatibility check: Confirm bore diameter (often 0.89 inches) and the gear-teeth count (17–22) to ensure a proper fit for TH350/TH350C and Turbo Hydra-Matic variants.
- Installation notes: Some plugs include retaining hardware, seals, or brackets. Use thread-appropriate sealant only if specified by the manufacturer to avoid over-tightening and galling.
What to consider when choosing
- For a no-frills seal: choose economical, billet-aluminum plugs with a proven O-ring seal.
- For show-quality builds: select a branded billet plug with a clean finish and corrosion resistance.
- Maintenance access: If you may need to service the port later, pick a plug that offers a replaceable O-ring and easy removal.
- OEM compatibility: Verify replacement part numbers and compatibility with your vehicle’s model year and transmission code.
Installation quick tips
- Clean the bore before installation to remove debris that could damage the O-ring.
- Inspect the O-ring for cracks; replace if any sign of wear.
- Hand-tighten first, then snug with appropriate tools—avoid over-tightening which can deform the O-ring or housing.
FAQ
- Q: Will these plugs work on all TH350 vehicles? A: They fit GM TH350/TH350C (1968–1981) and similar configurations; always verify bore and gear-teeth compatibility with your tail housing.
- Q: Do these plugs prevent all leaks? A: They reduce leaks by using a precision O-ring and tight tolerances, but performance depends on correct fit and proper installation.
- Q: Is billet aluminum necessary? A: It offers better corrosion resistance and longevity, especially in heat and road conditions common to classic GM cars.
- Q: Can I install these without professional help? A: Yes, most users can install with basic hand tools, following the manufacturer’s instructions and torque guidance.
- Q: Do these include all mounting hardware? A: Some kits include retainers or seals; others are plug-and-O-ring only. Check product details for specifics.