The correct tool for measuring a small gap with reference to a tolerance is a feeler gauge, which will either slide through the gap or it won't.
Typically used with a straightedge, surface plate or a square for checking the condition of surfaces, feeler gauges can of course be used to measure any small straight gap.
Our feeler gauge sets come from Moore & Wright of Sheffield, they are available in either imperial or metric, marked in thousandths of an inch or mm to -2 decimal places respectively. They are lovely precise tools and have that wonderful oily smell when you open the packet.
Looking for something in particular? Call our team in Banbury on 01295 678941 or email us.