Crank Rotation to Piston Drop
Some manuals show how to time an engine using degrees of rotation at the crankshaft, some show how to do it measuring piston drop down the bore. Few, if any, give figures for both. If your manual gives one method but you prefer using the other, look no further. This page shows you how to convert one to the other. If you don’t fancy doing it yourself just skip to the bottom and download our spreadsheet that will work it all out for you.
Required Information
The method shown on this page uses trigonometry to do the conversion and all you need to know before starting is:

The length of your conrod (between centres of the big end and small end)

Stroke of the engine
Notation
a = third side of the imaginary triangle made up with the conrod (c) and line between the big end and main shafts (b) in mm
b = half stroke in mm (Stroke/2)
c = conrod length in mm
C = angle of engine rotation from top dead centre in degrees
d = piston drop in mm
S = engine stroke in mm
Draw the imaginary triangle
Whichever way you want to do the conversion it helps to draw an imaginary triangle made up by the half stroke, conrod and a vertical line from the main shaft to the small end.
Convert Piston Drop to Crank Rotation
Measuring a distance down the bore can be less accurate than putting a degree disc on the crankshaft but is often the only method given in the workshop manual. As such it can be useful to convert piston drop to degrees to allow very fine setting up to be done when an engine is being assembled with a degree disc on the end of the crankshaft. It is also the easier conversion to do so we'll show this one first.
Calculate the length of the third side (a)
When the piston is at TDC the half stroke and conrod make a straight line and the piston drop is 0. With this in mind it is simple to calculate the length of the third side by adding the half stroke and the conrod lengths and subtracting the piston drop.
a = b + c  d
Calculate angle C using the cosine rule
Now that you have the lengths of all three sides the Cosine rule, shown here, can be used to calculate the angle before or after TDC (C).
Convert Crank Rotation to Piston Drop
As previously stated, measuring down the bore is often a less accurate way of setting timing. It is however quicker and easier to do if something slips whilst at an event. So if you are doing it in the paddock or a hotel car park this is probably the best way forward.
Calculate missing angles using the sine rule
The first thing to do is calculate the two missing angles. Angle B can be calculated using the Sine rule shown here.
Angle A can then simply be calculated by subtracting angles B and C from 180.
A = 180  B  C
Calculate the length of side a
Now that you have all three angles and two sides the length of the remaining side (a) can be calculated by further rearranging the Sine rule as shown here.
Calculate piston drop
Now you have the lengths of all three sides the same equation as used to calculate the third side earlier can be rearranged to give the piston drop.
d = c + b  a
Feeling baffled?
If you have read all of the above and are left feeling confused, or would rather be out on your bike than sitting indoors with a pen and paper, we’ve created a spreadsheet do all the hard work. Simply input the information from your bike and get the answers calculated for you.