The flatness of a casting can be measured by supporting the surface in question on three widely separated points to establish a datum and measuring the deviation from that plane by the use of feeler gauges.

The flatness tolerance is the total deviation permitted from the datum and consists of the distance between two parallel planes within which the entire surface so tolerated must lie.

The values shown represent production practice at the most economical level. Greater accuracy involving extra work or care in production should be specified only when and where necessary.