National 340847 Automatic Transmission Manual Shaft Seal

Automatic Transmission Manual Shaft Seal Overview

National 340847 is an automatic transmission manual shaft seal built for sealing performance in transmission service. It uses nitrile sealing material and is sized for a 0.500-inch shaft with a 0.750-inch housing bore, making it a direct-fit oil seal for specific Allison 1000 transmission applications.

Use National 340847 when servicing Chevrolet Silverado 2500 HD, Chevrolet Silverado 3500 HD, GMC Sierra 2500 HD, or GMC Sierra 3500 HD models that call for this seal. The part number 340847 is also referenced in transmission repair catalogs for related GM heavy-duty truck applications.

Key Features

• Nitrile sealing material for transmission oil sealing applications
• Inside diameter: 0.500 in. / 12.700 mm
• Outside diameter: 0.756 in. / 19.200 mm
• Housing bore: 0.750 in. / 19.050 mm
• Thickness/width: 0.125 in. / 3.180 mm
• Oil seal design for automatic transmission manual shaft service
• National 340847 series 340000 construction

Detailed Description

National 340847 is an oil seal for transmission sealing systems, engineered around a nitrile sealing element and precise inch-based dimensions. National has supplied sealing products for decades, and this seal reflects that focus with a compact profile, accurate shaft fit, and housing bore sizing for transmission rebuild and repair work. It is listed for Chevrolet Silverado 2500 HD and Silverado 3500 HD, GMC Sierra 2500 HD and Sierra 3500 HD, plus related Allison 1000 Transmission applications across multiple model years. The part number 340847 is a practical match for technicians replacing a worn manual shaft seal in GM heavy-duty truck transmissions.

This part cross-references Four Seasons 63158 for the same application.

What It Helps Address

• Transmission fluid seepage at the manual shaft seal area
• Leak-related low fluid condition during service intervals
• Seal wear from age, heat, and transmission fluid exposure
• Repair needs during Allison 1000 Transmission rebuilds
• Replacement of a damaged or hardened oil seal

Installation Notes and Tips

• Verify shaft diameter, housing bore, and seal depth before installation.
• Inspect the shaft surface for scoring or corrosion that could affect sealing.
• Clean the bore and surrounding transmission case before pressing the seal in place.
• Use the exact vehicle and transmission application data for Chevrolet Silverado 2500 HD, Chevrolet Silverado 3500 HD, GMC Sierra 2500 HD, and GMC Sierra 3500 HD service.
• Confirm the replacement seal matches the original dimensions and profile before assembly.

Quality Assurance

National is a long-established sealing brand with more than 75 years of experience in sealing solutions. Its product development focuses on current vehicle service needs and dimensional accuracy for transmission repair applications.