What size screens shall vent caps have?

The requirement for vent caps to have screens of No. 4 mesh or coarser is rooted in practical considerations regarding airflow and preventing the ingress of unwanted pests or debris. A No. 4 mesh indicates that the openings in the screen are large enough to allow free airflow, which is essential to prevent pressure buildup in the venting system. At the same time, a coarser mesh prevents the entry of larger contaminants that could infringe upon the system's operation or even lead to safety hazards.

This balance ensures that the vent cap functions properly, maintaining efficiency while safeguarding against potential blockages and contamination that could arise from inappropriate mesh sizes. As a result, the specification of No. 4 mesh or coarser provides an effective means to achieve these operational standards in oil fuel systems.