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4-SPEED SPORTS TOURING CONVERSION GEARBOX (FOR 3-SPEED GB4 NARROW-CASE ENGINES)

Brand : YUMINASHI

11.315,15₺
SKU:
23511-GB4-4ST
Weight:
2,500.00 Grams
Minimum Purchase:
1 unit
Shipping:
Calculated at Checkout
Bulk Pricing:
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Product Overview & Rotary Shift Mechanism

This Yuminashi SPORTS TOURING 3-Speed to 4-Speed Conversion Mission Set is a complete drop-in transmission assembly engineered to convert 3-speed narrow-case Honda horizontal engines (041 / GB2 / GB4 / GEL / GCF) into a true 4-speed Touring overdrive gearbox, or to upgrade and replace discontinued factory 4-speed transmissions on classic Honda 70cc ATVs. By replacing the wide ratio jumps of the factory 3-speed transmission, this conversion eliminates the notorious 2nd-to-3rd gear RPM bog and keeps your engine operating inside its optimal torque and thermal efficiency window.

  • Complete Conversion Assembly: Includes the pre-assembled 4-speed Mainshaft, 4-speed Countershaft, 4-speed Shift Drum, and pre-mounted Shift Forks.
  • Rotary Shift Pattern with High-Speed Safety Lockout (N-1-2-3-4-N): Converts the stock 3-speed return shift pattern (N-1-2-3) into a 4-speed all-down rotary pattern (N-1-2-3-4-N). The countershaft features an integrated Rotary Drum Lock Arm (PLATE, DRUM LOCK) that engages the shift drum slot whenever the vehicle is rolling forward, physically preventing accidental shifts from 4th gear into Neutral or 1st gear at speed. Once the vehicle comes to a complete stop, you can shift directly from 4th gear into Neutral.
  • Plug-and-Play Narrow-Case Journal Dimensions: Machined to exact factory narrow-case tolerances—17.0 mm mainshaft right & countershaft left journals (for standard 6203 radial ball bearings 96100-62030-00), 13.5 mm mainshaft left blind journal, and 14.0 mm countershaft right blind journal. Drops directly into both 6V/12V plain-bore crankcases and 12V needle-bearing crankcases (91101-141-008 / 91101-121-691).

➲Available Gearbox Ratio Tiers & Camshaft Pairing

Select the gearbox specification that matches your cylinder head airflow, camshaft duration, and target RPM ceiling. Because the Sports Touring and Super Sport sets use tighter 4th gear ratios, adjusting your final drive sprockets is recommended to maintain top speed.

Yuminashi SKUSpec Tier1st Gear2nd Gear3rd Gear4th GearShift Recovery & RPM DropSetup & Sprocket Advice
23511-GB4-4TR TOURING 12T / 34T
(2.833)		 17T / 29T
(1.706)		 21T / 26T
(1.238)		 24T / 23T
(0.958 OD)		 Shift @ 8,500 RPM:
1→2: 5,120 RPM (-3,380)
2→3: 6,170 RPM (-2,330)
3→4: 6,580 RPM (-1,920)		 Touring Cam (215°–224° @ 1mm)
4,500 – 8,500 RPM.
Use stock chain sprockets. Overdrive 4th lowers cruising RPM.
23511-GB4-4ST SPORTS TOURING 12T / 34T
(2.833)		 16T / 29T
(1.813)		 21T / 27T
(1.286)		 23T / 23T
(1.000 1:1)		 Shift @ 9,500 RPM:
1→2: 6,080 RPM (-3,420)
2→3: 6,740 RPM (-2,760)
3→4: 7,390 RPM (-2,110)		 Sports Touring Cam (228°–235° @ 1mm)
5,800 – 9,500 RPM.
Add +1T front sprocket to compensate for shorter 1.000 4th gear and stretch 1st gear.
23511-GB4-4SS SUPER SPORT 12T / 34T
(2.833)		 16T / 29T
(1.813)		 21T / 27T
(1.286)		 22T / 23T
(1.045 Close)		 Shift @ 10,000 RPM:
1→2: 6,400 RPM (-3,600)
2→3: 7,090 RPM (-2,910)
3→4: 8,130 RPM (-1,870)		 Super Sport Cam (240°–248° @ 1mm)
6,800 – 10,200 RPM.
Add +1T to +2T front sprocket (or -3T rear) to regain top speed on this ultra-close gearbox.

Verified Vehicle Fitment List

1. Direct Plug-and-Play Motorcycle Fitment (Engines Equipped with 21T B-Type Kickstarter 28211-086-700)

This gearbox is machined to mesh directly with the factory 21-tooth B-Type kickstarter pinion gear (28211-086-700) found natively in the following motorcycles:

  • Honda C50 Super Cub / Press Cub / Little Cub: 12V 3-Speed Models
  • Honda C70 Super Cub / Passport: 12V 3-Speed Models
  • Honda CRF50F: 2004–Present
  • Honda XR50R: 2000–2003
  • Honda CRF70F: 2004–2012
  • Honda XR70R: 1997–2003
  • Honda Z50R (12V): 1988–1999
  • Honda ZB50 (12V 3-Speed): 1988 (US market semi-auto)
  • Honda Dax ST50 (12V): 1995–1997 (ST50S / ST50P / AB23)
  • Honda Dax ST70 (12V): 1990–1996 (AB23)
  • Honda CF50 / CF70 Chaly: 12V & Late 6V 3-Speed Models

2. Direct Plug-and-Play Honda ATV & Quad Fitment (Recoil Pull-Start Narrow-Case 70cc Engines)

Because Honda 70cc ATVs and Quads use a recoil pull-starter with no internal kickstarter spindle, this gearbox is a 100% direct drop-in across all 6V and 12V ATC70 and TRX70 models (converting 3-speed models to 4-speed, or replacing/upgrading discontinued OEM 4-speed shafts):

  • Honda ATC70 K1 (6V): 1973 – 1974
  • Honda ATC70 (6V): 1978 – 1981
  • Honda ATC70 (6V): 1982 – 1985
  • Honda TRX70 Fourtrax 70 (12V / 6V Conrod): 1986 – 1987

⚠️ Motorcycle Fitment Requiring Kickstarter Spindle Upgrade
(Engines Equipped with 22T A-Type Kickstarter 28211-040-035)

The following models left the factory with the older 22-tooth A-Type kickstarter pinion (28211-040-035):

  • Honda C50 Super Cub: Early 6V Models (C50K / C50M / C50Z)
  • Honda C70 Super Cub: Early 6V Models (C70K / C70M / C70Z)
  • Honda CD50 Benly / Benly 50S: 6V & 12V Models
  • Honda CL50 Benly: 6V & 12V Models
  • Honda SS50 / S65: 50cc & 65cc Manual Clutch Models
  • Honda Dax ST50 (6V EU/UK): 1978–1989 (ST50C, ST50J, ST50K, ST50L)
  • Honda Dax ST70 (6V EU/UK): up to 1982 (ST70C)
  • Honda CF50 / CF70 Chaly: Early 6V Models
  • Honda CT70 (6V 3-Speed): 1969–1979
  • Honda Z50A / Z50J1 (6V 3-Speed): 1969–1979

➲ Installation Requirement for 22T A-Type Models:
The 13.5 mm and 14.0 mm shaft journals drop directly into these crankcases. However, because the models listed above use the older 22-tooth A-Type kickstarter, you must swap your 22T kickstarter shaft to the standard 21-tooth B-Type kickstarter spindle assembly (28211-086-700 pinion + 28221-086-700 ratchet + 28251-086-701 spindle) so the kickstarter gear meshes properly with the new countershaft 1st gear.

⚠️Technical Disclaimers & Incompatible Models (Please Read Before Ordering)

  1. NOT RECOMMENDED FOR FACTORY 4-SPEED / 5-SPEED MANUAL CLUTCH ENGINES (CT70H, XL70, SL70, CL70, SS50, CD50 Benly, 12V Z50J Monkey, AB27 Monkey, Z50G Gorilla, JDM Monkey-R/RT):
    Unlike misleading online listings, this kit is strictly a 3-Speed to 4-Speed Conversion Set (and ATV 4-speed replacement). Installing this kit into a factory manual-clutch bike (such as a CT70H, SS50, CD50 Benly, or 4-speed Monkey) will permanently convert your manual return shift pattern into a step-through Cub rotary shift pattern (N-1-2-3-4-N), and will bind against factory 22T A-Type kickstarter pinions unless the kickstarter assembly is completely swapped.
  2. NOT DROP-IN FOR LATE 4-BALL-BEARING CUB CASES (GB5 / GK4 / GBO-J ~1987–1999):
    Late-model JDM (Japanese Domestic Market) and Asian 12V Super Cub C50, C70, and C90 crankcases (GB5 / GK4 / GBO-J) use four radial ball bearings—replacing the 13.5 mm and 14.0 mm blind shaft bores with 12.0 mm ID 6001 and 6201 ball bearings. Installing this kit into a 4-ball-bearing GB5 / GK4 crankcase requires turning the 13.5 mm and 14.0 mm blind shaft ends down to 12.00 mm on a lathe.
  3. DOES NOT FIT WIDE-CASE 100cc / 110cc / TRX90 ENGINES (GN5 / KFL / KFM / KEV / HF7 / HP2):
    For wide-case engines—including Honda TRX90 (1993–Sept 2005), TRX90EX (2006–2008), TRX90X (2009+), C100, EX5, Dream 100, Wave 100, Wave 110 (carbureted), and Nice 110—please order Yuminashi Wide-Case Close-Ratio Gearbox SKU 23511-GN5-4SP (23511-GN5-4SS).
  4. MANDATORY THRUST WASHERS & END-PLAY VERIFICATION DURING INSTALLATION:
    Always reinstall the two factory blind-end thrust washers before closing the crankcase halves: 90451-141-000 (13.5 mm Thrust Washer) on the left blind end of the mainshaft, and 90451-121-000 (14.0 mm Thrust Washer) on the right blind end of the countershaft (outside the rotary lock arm). Verify 0.10 mm – 0.25 mm of axial shaft end-play after torquing the crankcase bolts.


● DESIGN / GEARBOX NATURE & HORSEPOWER LIMITS:

● For standard narrow-case architectures (C70, Dax, CRF50), we recommend a safe maximum of 12 to 15 horsepower for this specific gearbox.

● To give you a baseline for comparison, a stock Honda engine using these crankcases produces roughly 5 to 7 horsepower.
This means your new 4-speed conversion assembly is built to safely handle double the original stock power.

● The Hidden Cause of Gear Failure: Many builders assume transmission gears break simply because the steel is too weak. In reality, there is a physical limit to how much torque standard factory aluminum crankcases can take. When pushed beyond 15 HP, thin OEM crankcases flex and distort under load. This flex pushes the transmission mainshaft and countershaft out of alignment. The gear teeth lose their perfect, deep mesh and ride up onto the fragile tips, causing the teeth to shear off instantly.

● The High-Horsepower Solution (25+ HP): If your build targets 20 to 30+ horsepower, you must eliminate crankcase flex. For these extreme builds, we engineered the #11150-KFL-690 Yuminashi Heavy-Duty Wide-Case (Nice110 Platform). Cast with almost double the aluminum wall thickness of a stock engine, these reinforced cases physically prevent the shafts from pushing apart.

● When you combine our #23511-GN5-4SP Wide-Case 4-Speed Gearbox (LINK) with our Heavy-Duty Crankcase, the perfect shaft alignment unlocks the true strength of the gears, safely handling 25 to 30+ HP. Furthermore, this Heavy-Duty case utilizes a crankshaft-driven Nice110 oil pump (LINK) and is designed with the Ø69mm crankcase bore to accept modern, large-displacement Wave125i and GROM125 crankshafts, cylinders, and cylinder heads for ultimate power generation.

● Conclusion: As long as your current narrow-case build is producing under 15 HP, this 4-speed gearbox will perform beautifully and give you snappy, reliable acceleration. To get the maximum life out of it, we recommend smooth shifting and avoiding aggressive "clutch dumps" from a standstill.

※ This 4-Speed Conversion Assy. can be installed in combination with both the factory semi-automatic centrifugal clutch and aftermarket crankshaft-mounted manual clutches. 

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