Ignition adjustment made easy: Measure in millimeters before TDC

One of the most frequently asked questions in the workshop is: How do I set my ignition precisely? Today we will look at one of the simplest and safest options: Determining the ignition timing based on the piston position in millimeters before top dead center (mm before TDC).

The basic requirement

The cylinder head must be dismantled for this method and is therefore best suited for completely new bodies where the engine is still on the workbench or in the engine stand.

The cylinder head must be dismantled, because we need direct contact with the piston crown in order to determine the exact height in the cylinder with the dial gauge.

In our example, we are using a Vespa wideframe engine with a stroke of 57 mm and a connecting rod length of 110 mm.

However, the principle can also be applied to all other engines.

Understanding markings (example: Vape)

Modern ignitions such as the Vape, for example, make things a little easier for us. We have:

1. A marking on the base plate (ignition point).
2. A marking on the rotor (pole wheel). If these two markings are congruent, ignition takes place.

Step by step to the correct value

1. Determine TDC: Use the dial gauge to find the highest point of the piston (top dead center). We set this point as "zero" on the dial gauge. This measurement is the basis for all settings and must be exact.
2. Calculate the value: Let's assume we want to set 23° pre-ignition. Converted, this corresponds to exactly 2.83 mm before TDC for this engine setup.
3. Determine the ignition point: After setting the ZERO at TDC, we turn the pole wheel back against the direction of rotation of the engine until the dial gauge reads exactly 2.83 mm.
4. Determine ignition timing: The pole wheel remains in the position 2.83mm from TDC and we turn the base plate until the markings on the plate of the rotor are congruent.

That's it! The ignition is now set exactly to 23° before TDC, but should always be checked using the existing or attached markings.

Note: The 23° in this video is only an example value. Always check the specifications for your specific cylinder!

You can find a table with the most common "ignition timing in millimeters before TDC" specifications listed by stroke/pulley length, for example, with our holder for the dial gauge, the caliper gauge and the degree disk in the download area.

If you want to strain your brain, you can use this formula: s = r + l - (r - cos α + √(l² - (r - sin α)²)) where, s = mm before TDC, r = stroke / 2, l = connecting rod length, α = degrees before TDC.