The timing gears of diesel engines are marked. There are roughly several forms of marking, such as the letter ABCD, the number 1234, or the dots, etc. Whatever marking, we need to look at it when we are on the timing. Take the digital marking as an example to illustrate that it is easy to understand that the same number exists on the meshing gears. The marks of the two gears, one has two adjacent teeth with `1', the other has a `1'. Next, the key point of the marks is to insert a 1's teeth in the middle of two 1's teeth. That's good. Other gears are also in this way.