How to check flatness using an engineer’s straight edge?

How to check flatness using an
engineer’s straight edge

Shop for Engineer’s Straight Edges

Other equipment you will need:

Set of feeler gauges used to measure gaps between two objects

Feeler gauge

Feeler gauges are thin strips of metal used to measure very small gaps between two objects.

Fluorescent light used to see gaps between the straight edge and workpiece

Light

Fluorescent glare free light is used to check for gaps between the straight edge working face and workpiece.

Spirit level placed on top of a straight edge can be used to check the workpiece is level as well as flat

Spirit level

Placed on top of an I section or rectangular straight edge, a spirit level can help check if the workpiece surface is level as well as flat.

Checking an engine block or cylinder head is flat

Straight edge placed on top of a cylinder head to check for flatness An engineer’s straight edge can be used to check that an engine block or cylinder head is flat by being placed across its surface.
Using a light behind the straight edge helps to show gaps between the straight edge and cylinder head. By shining a light behind the straight edge, any gaps between the cylinder head surface and straight edge will be visible if they are more than 0.002mm in size.

The light is easier to see, the thinner the straight edge is, so knife edge straight edges are the most accurate for this task, although you do have to hold them, as they will not stand up on their own.

Gaps between the engineers straight edge and cylinder head being measured with a feeler gauge Any gaps should then be measured using a feeler gauge.
The six positions on a cylinder head that need checking for flatness This process has to be repeated across six areas of the cylinder head. If gaps in any area are outside of the permitted tolerance for that engine, then the cylinder head will need resurfacing. This process then has to be repeated on the cylinder block in order to achieve a fit within the tolerances for the cylinder head and block combined.
It’s important to always check what manufacturers tolerances are for a cylinder head, particularly if it is made of aluminium. As these have much stricter tolerances!
Typical head and block combined maximum tolerance for a cast iron cylinder head