This inspection cap is a locking retro-fit cap that can be fitted to the 4" or 4.5"  visual inspection point on all Harlequin oil storage tanks.

Generally, this cap is used to replace either the black 1/4 turn locking cap or the 4" black screw cap with a female thread on the older model Harlequin oil tanks. This cap is manufactured so it fits loosely over the inspection point so that it doesn't split or crack over time like the original used to do. 

This cap is supplied with fitting instructions (which can also be downloaded from this page prior to purchase) and is now supplied as standard on all new Harlequin oil tanks.