Although they are called "radiator caps", the FC3S can have as many as three of them in use.  These filler caps can be further divided into two different types - sprung and unsprung.  The unsprung cap is just a plain metal cap with a rubber seal.  The sprung cap is more common and has a sprung center section for bypass.  You can technically use a sprung cap where you normally use an upsprung one, but you CANNOT use an unsprung cap if you normally use a sprung cap.  The sprung caps have pressure ratings which tell you when the sprung center will collapse.  This is used for bypass systems into overflow bottles.

We've found there are badly made radiator caps being sold.  The caps are normally made out of stainless steel.  The inferior caps are made of very thin stainless steel that tend to splay when under heat and pressure.  This causes the cap to flare at the bottom and lose sealing force.  The rubber seal can be perfectly fine, but if the cap can't hold onto the flange, it will leak.

The rubber seal tends to disintegrate or harden and warp when they get too old.  Under visual inspection, it should be obvious that the rubber seal is bad.

Although rare, a sprung cap can have a bad center section.  If this sprung section goes weak, it will bleed pressure too quickly and cause overheating.  The cooling system needs to be properly pressurized to be effective.

Questions?  Comments?  Send mail to: