SCALE is a modular, non-contact, in-line measurement system. It measures the diameter of the countersink directly after the drilling, the amount and distribution of sealant in the open hole, and the head height of the fastener as well as pressed out sealant (cf. Figure 1). The system is fast and reliable and the out coming information is reliable and trustworthy.Until now the system could not measure the inner diameter of the hole. The reason for this is that it is not possible to detect the inner diameter with a camera that looks only at the top of the component.But as our customers make the request to us, we decided to develop an optical hole probe system which is fully integrated in the auto fastening process. We think that a mechanical system cannot fulfill the customer expectations in terms of reliability, low maintenance, precision and speed. Only a non-contact system can measure permanently safe and fast the inner diameter of holes. The development objective was to create a system that is capable of to measure one inner diameter in less than 200 milliseconds with the precision better than 10 microns (0.0004 inch) at six sigma confidence level.The paper starts with a short description of different non-contact sensor types that are available on the market and that can be used to measure the inner diameter of a drilled hole in an auto fastening machine. The main part of the paper reports the results of our intensive investigation regarding the applicability for fast and non-contact optical measurement of the inner wall of the drilled hole. We have done more than 30.000 measurements in carbon fiber and metal.