Reduction Ratio Measurement

Measuring reduction ratio can be done as follows:

First check it is not marked on the side of the gearbox - usually on a small brass plate labeled  A:x.xx   B:y.yy   where A is the ahead ratio and B is the reverse ratio.

If this is not available proceed as follows:

1. Mark the shaft with an indelible marker.
2. Engage AHEAD on the gearshift.
3. Turn the motor over in an Ahead direction X times using a spanner on the nose pulley until the output shaft has completed 1 exact turn.
4. You can generally turn a diesel over compression if you just take your time.
5. Then X is the reduction ratio of the gearbox in Ahead.

NOTE:     This will not work for hydraulic gearboxes where they require the motor running to energize the clutch to engage Ahead and Astern. These require a digital tachometer if the ratio is not marked on the side of the gearbox. A small reflective sticker is attached to the shaft and the meter then shines a light on this as the shaft rotates and counts the reflections.