The JIMS FAT 6spd Super Gear Transmission Cassette is a true bolt-in performance upgrade for the O.E.M. 6-speed transmission – no case mods, no drama. JIMS builds these gears from aerospace-grade 9310 American steel, chosen for its high core strength, toughness, and excellent fatigue resistance.
The FAT 6 delivers what matters most in a Transmission Cassette: strength, reliability, precise shifting, quiet operation – all in a package that doesn’t require you to modify your bike.
If you’re making real torque, the smartest move isn’t “hoping” your stock box survives—it’s upgrading to something built to win. The JIMS FAT 6 Super Gear Transmission Cassette is made from strongest possible American materials, so it’s designed to take brutal shock loads and high torque without folding.
The JIMS FAT 6 Super Gear Transmission Cassette is built from aerospace-grade 9310 American steel. Chosen for its exceptional core strength, toughness, and fatigue resistance, allowing it to handle significantly higher torque loads.
This is where the “FAT” name earns its keep. The JIMS FAT 6spd Super Gear Transmission Cassette uses straight-cut gear teeth that are up to 53% wider than stock. Delivering increased load capacity and added strength where OEM transmissions can often fail and break.
These transmission cassettes feature JIMS Quick Shift Ring technology to help reduce that annoying shift “clunk.” A lightweight one-piece steel shift drum, paired with shift ring dogs and back-cut gear pockets that actually help pull the gears into full engagement under load. Meaning when you’re hard on the throttle and torque is trying to push gears apart, the FAT 6 is designed to lock in harder — not fight you.
JIMS has been in the Harley performance world since 1967, and they’re known for making the kind of parts that builders trust when “good enough” isn’t good enough.
It started with founder Jim Thiessen, a machinist from the aerospace industry. Riders began coming to him for hard-to-find parts and higher-quality American-made alternatives — and by the mid-to-late ’70s, he went all-in on V-Twin motorcycles. That focus on precision machining is what built the JIMS reputation.
And what began as a small setup with a few manual mills and lathes has grown into a 50,000 sq ft multi-building operation loaded with CNC machinery.
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