5.9 Cummins Input Shaft Upgrade Kit – South Bend ISK1.375 (NV4500)

Upgrade the weak link in your NV4500. The factory 1-1/4" input shaft in 1994–2003 Dodge Ram 5.9L Cummins trucks is a known failure point when you add power, torque, or a performance clutch. The South Bend ISK1.375 kit replaces it with a larger, significantly stronger 1-3/8" (1.375") input shaft — the same diameter used in the later NV5600 — cryogenically treated for maximum durability.

This upgrade is required for most aftermarket dual-disc and high-performance clutches designed for the larger shaft. It gives your transmission the strength it needs for tuned trucks, heavy towing, or aggressive street/strip use.

Key Features & Benefits

• Larger 1.375" diameter shaft — direct upgrade from the stock 1.25" shaft
• Cryogenically treated hardened steel for superior strength and breakage resistance (eliminates stress points and increases consistent density)
• Allows use of aftermarket clutches that require the 1-3/8" input shaft
• Restores reliability and gives you confidence under high torque loads
• Made in the USA by South Bend Clutch

What’s Included in the Kit

• Cryo-treated 1.375" input shaft
• Bearing retainer with seal
• Front bearing and race
• Pancake bearing
• Thrust bearing
• Shims and installation instructions (as required)

Note: A compatible 1.375" clutch assembly is required (sold separately). Professional transmission installation is strongly recommended.

Vehicle Fitment

Fits: 1994–2003 Dodge Ram 2500/3500 with 5.9L Cummins (12-valve & 24-valve) and NV4500 5-speed manual transmission only.

Does NOT fit NV5600 transmissions.

Important: Confirm your transmission type before ordering. This kit is specifically engineered for the NV4500.

Why You Need This Upgrade

Many popular performance clutches (especially dual-disc) are designed around the larger NV5600-style input shaft. Running one on the stock 1.25" shaft without this upgrade often leads to twisted or broken input shafts. The South Bend ISK1.375 eliminates that weak point and lets you safely run more power and better clutches.