The torque output of an engine is proportional to the bore and stroke.  Engines make torque and drivetrain components take torque.  It is torque, not horsepower that destroys clutches, transmissions and drive belts.  The torque capacity of a transmission is proportional to the gear width and the center distance between the two shafts.  All stock Harley transmissions and aftermarket transmissions have a center distance of 2.5″. By increasing the center distance 20% to 3.0″, the Torquebox conservatively increases the torque capacity to 250 ft-lbs, continuous duty! In the early Torquebox design stages, the goal was set to design a continuous duty 250 ft-lb gearbox that visually looked like a 450 ft-lb gearbox on the outside. We achieved this by taking design cues from classic race-bred American superchargers and not having any excess material, anywhere. We achieved our goal. The Torquebox is available as a 5 or 6-speed, left or right side drive. All frame, starter, and clutch interfaces are identical to stock 1990-1999 models.

- Compatible with most Evo Softails and custom builds
- Two rail shift form system with common fork used three times
- 2” diameter shift drum suspended by two radial ball bearings mounted to a
fixed spindle axis
- Ratchet pawl has overshift inhibitor (up and down) and is mounted on two
radial ball bearings. Shift lever shaft cross sectional area is 65% larger
than stock.
- Ignition kill and neutral switches top mounted for easy dwell timing
adjustment.
- Shift system lever ratio is 35% less than stock. Bearing mounted shift shaft
yields low system hysteresis. Short, tight shifts are the result. N-1-2-3-4-5-
6 shift pattern for false-neutral avoidance.
- Shaft material is super strong 9310 – the best steel made – this isn’t
standard by any means. Gears material is 8620. Very large 8.125 diameter
pitch gear teeth. The gear teeth are 2x circumferentially thicker than stock.
Tooth flanks are finish-ground after heat treat. AGMA class 12 grade gears.
- Housing is all billet construction from 6061-T651 aluminum. We start with
95 lbs of aluminum and end up with 17 lbs of case and 78 lbs of chips
- 5 year limited warranty

Materials

Starting out with a 6061-T6 billet chunk of aluminum that weighs in at 95 lbs. This creates the framework for the TorqueBox’s ability to handle 250Ft/lbs of torque.  Getting wittled down to a 17 lb case that is both structurally and aesthetically pleasing with design cues from a 871 blower. The guts needed to match this.  So both mainshaft and countershaft are cut out of the best steel made, 9310.  The gears are cut out of 8620 and are a class 12 grade on the AGMA scale.

Torquebox Top Cover Features

Shift Drum

All TorqueBox are fitted with N1 shift drums.  For 5 speeds this makes the shift pattern N-1-2-3-4-5 and for 6 speeds it makes the shift pattern N-1-2-3-4-5-6.  Along with N1 these racing transmissions are fitted with ignition kill shift drums.  This makes for clutchless, wide open throttle shifts down the 1/4 mile and stop light to stop light!  The ignition kill system will need to be wired into your current ignition system.

Shift Drum Detent

Frictionless Detent System

A 3/8” primary ball bearing is cradled by, and rolls on, 50 secondary micro ball bearings. In turn, the cradle is part of a plunger that actuates during shifting and is guided by a tertiary linear micro ball bearing system.

Shift System

The TorqueBox uses a two rail fork system.  This makes it possible to increase the width of the fork to provide more support and smoother shifts.  Also featuring 1 common shift fork used 3 times to achieve commonality.  We didn’t stop at the forks and fork rods, a heavy duty shifter pawl was in need.  The shifter pawl design makes shifts short and crisp. This system delivers a lever ratio that is 35% less than stock.  The heavy duty shifter pawl rides on two radial ball bearings for smooth, repeating shifts.

Clutch Actuator

Right and Left side drive units are both equipped with a 1.5″ bore hydraulic slave cylinder.  This provides easy holding (depending on clutch) at the drag tree before you go into clutchless ignition kill shifts.  You will need a 11/16″ bore master cylinder, brake and banjo bolt.

6 Speed Ratios for Left or Right Side

5 Speed Ratios for Left or Right Side

Fitment
1990-99 Left Side Drive Softail-Based and Custom Right Side Drive Motorcycles

Recommended upgrades

- King Kong Klutch

- 9 Plate Clutch

-Funtion Form Primary

Click on any of the images below to learn more

King Kong Klutch

SP9- Street Performance Clutch

Polished FFP

Spectro Oil BD-75140

Recommended Fluid / Level

Picking the right transmission fluid ensures the life and durability of your transmission, that’s why BAKER recommends a fully synthetic GL-5 rated gear weight oil of 75W-140. If a GL-5 rated gear lube is not available, these commonly carried viscosities (weights) will work 75W-90, 75W-140 and 85W-140. BAKER does not recommend the use of Syn3 (20W-50). For Bert’s  article on ‘Oil & Spiders’  Click Here

BAKER TorqueBox Fluid Capacity Dry: 22-24fl-oz

BAKER TorqueBox Fluid Capacity Wet: 18-22fl-oz

 Product

TB621L, TorqueBox R-Ratio, Standard Polished

TB621R, TorqueBox Right Side Drive, R-Ratio, Standard Polished, 23 tooth sprocket with .900" offset

TB621L, Left Side Drive, R-ratio, Custom Ordered Black Anodized

 Lifestyle

Kustoms Inc custom built, TB621R

LA County Choprods Super ADD bike, TB621L, with Custom Black Anodized actuator and C'shaft cover