The diesel engines as compared to petrol engines require

ignition of the charge before the passage of flame front

Auto ignition is defined as the self-ignition of the vapors emitted by a liquid heated above its ignition temperature.

allow for lengthening of the valve owing to the heat of combustion

It is necessary to maintain the valve clearance as they allow for lengthening of the valve owing to the heat of combustion.

incomplete valve closure

feeler gauge

Feeler gauges are used:
– To check the gap between the mating parts
– To check and set the spark plug gaps
– To set the clearance between the fixture (setting block) and the cutter/tool for machining the jobs
– To check and measure the bearing clearance, and for many other purposes where a specified clearance must be maintained.