Renewing Cambelt - Pug 206 1.1ltr

[sooty]

I'm to replace the cambelt on my wifes 51plt 206 1.1.
What is the purpose of the Tool shown in the Haynes manual for positioning the Rockers etc. when renewing the belt????
They also say a electronic device should be used to measure the tension when setting the cambelt, I've been told of a method to set the belt tension without the device.
But haven't a clue about the Tool for the Rockers. [;)]