Originally Posted by Stu @ M Developments

Yes they do Steve. They stop any downward movement caused by the fact the differential pressure across the injector means that the pressure is higher at the fuel injector end.

Aftermarket rails are normally badly manufactured and mounted and have a slightly different rail to plenum mounting distance.

With everything just slotted together they often appear to work, however, its very common once everything gets nice and warm that the fuel injector is pushed downwards towards the plenum by fuel pressure, sliding the injector into the plenum as far as it can go and exposing the upper O ring that seals the fuel rail.

Result = fuel firing everywhere at high pressure and a potential for a huge fire.

Seen it not a couple of times but TENS of times. Even had it happen on the dyno with a cossie we hadn't worked on that just wanted a power run... The potential for a deadly inferno is HUGE with some of these badly thought out fuel rails.