Thickness Gauge
For fine measurement in thousandths of an inch.
[.0015 in. = .0381 mm]
From Wikipedia
Why is one leaf in some feeler gauges brass?
The brass gauges had two uses, both cases because the gauges had to be non-magnetic. On small engines, and many years ago on larger engines, the spark was created by magnetos which contained strong magnets. You couldn't check clearances in the magnetos with a steel gauge it would stick and probably bend if you tried.
Thickness Gauge
For fine measurement in thousandths of an inch.
[.0015 in. = .0381 mm]
From Wikipedia
Why is one leaf in some feeler gauges brass?
The brass gauges had two uses, both cases because the gauges had to be non-magnetic. On small engines, and many years ago on larger engines, the spark was created by magnetos which contained strong magnets. You couldn't check clearances in the magnetos with a steel gauge it would stick and probably bend if you tried.