Guzzi Smallblock Tranny

Gear cluster
















Smallblock Tranny Primary Gears

The Moto Guzzi smallblock transmission is much better designed than the earlier big block tranny. The smallblock design uses straight cut gears instead of the common helical gears on big blocks. The smallblock is much lighter and more efficient. Instead of the extra shaft (layshaft) running the entire length of the tranny, the smallblock uses only two main shafts with a a primary reduction gear set up front. In theory, yes, the primary gear set is a third shaft, but it's a very short one. That extra shaft in the big guzzis introduces more lash, weight, inertia. The result is the unnecessary clunky shifting demonstrated by many Guzzi big blocks. Small blocks also lack the internal cush drive that I am convinced is unnecessary and redundant on the big blocks.

Note that the Guzzis in general require more than the standard two tranny shafts found in most Japanese sport machines, Ducatis, etc. Why? Well, the tranny has to move the axis of torque tranmission from the center line of the engine output (i.e. the middle of the clutch) to the drive shaft axis. For reference, this distance is around 6 inches on big blocks, and a shorter ~3.5 inches on smallblocks [the latter design limiting rear tire width somewhat]. Accomplishing this change in the torque axis this with just two shafts would require a gear set with large diameters. This is also undesirable as it adds weight, inertia, etc. Large gears can introduce dynamic problems and lower shaft natural frequencies into the standard operating range, too, which can cause poor behavior and failures. So for the smallblocks, Guzzi obviously decided to try to make do with a augmented two shaft tranny....they accomplished this with the primary reduction gear set shown in the second photo.

The one bummer about the smallblocks is that the rear drives are all the same gear ratio. Changing gearing requires swapping out the primary gear sets, which come in a few different ratios. Thus, to change your overall gear ratio trackside, you would basically need to change out the entire tranny! Pity the poor Smallblock racers such as myself then.