On Mercury's web site they show a restrictor fitting on the Headers. Go's from the header to the tail pipe. On top it's the u shaped jumper hose and on the bottom it comes off the Distribution tube to the tail pipe. What are the purpose of the fittings?

To get enough water through the closed cooling system at the proper flow rate to cool the engine properly, especially after hard runs. The restrictors are in there to keep the water from flowing through to quickly and not allowing the water to remove the engines built up heat in the heat exchanger. The restricted orfices also keep to much colder water from quickly flowing into the headers when hot to help stop quick quenching of the header, especially at the welds where they are prone to cracking from embrittlement.
Do not remove or change this specially designed and developed flow pattern and rate as you may do damage to your beautiful HP525. If you are having cooling issues or heating issues, go after the real problem and don't mess with this designed setup unless its damaged , scaled up or restricted.

Best Regards,
Ray @ Raylar

Is the restrictor fitting a separate piece? My jumper hoses connect to the male bung's. When we removed the Headers from the tails there we're no restrictors.