Small Frame Cranks

Original Piaggio small frame crankshafts differ in six main areas between models:
  1. The stroke 43mm on all 50cc bikes & 51mm on all the others.
  2. Connecting rod length is 87mm on all 50cc bikes & 97mm on all others.
  3. The removed area of the crank side web (intake duration).
  4. The taper of the flywheel cone (thin = tapers from a 13mm diameter to 19mm over 29.5mm, thick = tapers from a 13.5mm diameter to 21mm over 37.5mm or XXXXXX)
  5. Flywheel side bearing diameter (20mm or 25mm)
  6. Flywheel side oil seal diameter.(19mm, 20mm, or 24mm).

The diagram below shows each of the areas that differ on the various crank designs for smallframes. Since the 51mm crank is the most popular, this page will show the differences in the design of the small frame 51mm crank throughout the smallframe models.

Standard crank type 1 - Vespa 90 to 1966 only: The small end is a brass bushing instead of a needle roller bearing, the inlet duration is only 101 degrees, the crank taper is different and runs a smaller flywheel. The crank measure 143mm from end of clutch thread to end of flywheel taper. Since this crank is pretty old and a little obscure, I don't know too much about it but below is a picture of a sad one pulled out of a very wet motor by Chris Morris.

Standard Crank type 2 - Vespa 90, 100, and Primavera: 20mm flywheel bearing, 19mm oil seal, thin taper - This type of crank is the same from the 1966 Vespa 90 to the Primavera ET3. With my crappy calipers I measured 71mm in a straight line between the two crank web openings on the clutch side (intake duration). This works out to be 110 degrees out of the 360 for a full crank web. The flywheel bearing diameter is 20mm and the oil seal diameter is 19mm. Click here for a PDF of the crank. * Note the 100 Sport crank has an inlet duration of 137 degrees.

Standard Crank type 3 - Vespa PK Series: 20mm flywheel bearing, 20mm oil seal, thick taper - The introduction of the PK line introduced a different crank. The flywheel taper became the same as the P series bikes, and the oil seal increased to 20mm and the flywheel bearing remained the same.

Standard Crank type 4 - Vespa PK125XL and PK125ETS: 25mm flywheel bearing, 24mm oil seal, thick taper - The last large displacement of the PK line introduced a third design of crank. The flywheel taper remained the same as the P series bikes, and the oil seal increased to 24mm and the flywheel bearing increased to 25mm. The intake duration is longer but I don't have the actual figures yet. The intake duration is longer at 149 degrees.