A drawback with original 3-speed transmissions is the large gear ratio difference between 1st and 2nd, i.e. from 2.47 to 1.57:1 with a consequent big rpm drop. If you ever rode 2-up in the mountains you will know how annoying this can be. With the installation of this 4-speed conversion kit one can achieve a big improvement here because its extra gear allows you to stay in the power band much better for an overall riding performance unknown before. Additionally, all gears are machined from modern materials for higher strength and feature an updated tooth-design for smoother and quieter running. Solo machines, sidecar rigs and Servi-cars - all likewise benefit from these features.
The conversion kit includes all gears and the shifter drum to transform an old 45” gear box into a 4-speed transmission. All other stock 3-speed parts, like shafts, shifter forks, etc. can be retained or must be purchsed separately if not present. The original primary drive with clutch and the chain sprocket with cover fit without problems. After installation the 4-speed tranny will look exactly the same as its 3-speed counterpart so nobody can tell the difference from its outer appearance. The only extra part required is a 4-speed shifter gate. The stock side cover has to be machined for clearance. As an option a pre-machined version is also available.
Stock 3-speed gear ratios:
1st - 2.47:1
2nd - 1.57:1
4-speed gear ratios:
1st - 2.67:1
2nd - 2.03:1
3rd - 1.42:1
4th - 1:1
This conversion kit should be installed in a professional workshop by an experienced mechanic.
- fits 45”/750cc Flathead solo models 1941-1946
- NOTE: These transmissions are designed for →1946 shifter gates (1st gear forward).
- gross weight: 3.36 kg
HMB Shifter Ratchet for 750cc/45cui
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