Cooling system bypass header

[Keith]

My 1985 Catalina #431 fresh water cooling system has what I understand from the web forms as a bypass header. This hard piping has the supply and return lines to the fresh water cooling side of the heat exchanger and the supply and return lines to the water heater with a valve in the middle. A mechanic suggested this header should be removed and fresh water cooling lines connected directly from motor to heat exchanger. Also remove the bypass line from the fresh water pump and connect hot water heater lines.
Has any one had this header arrangement on their boat and have you removed the header as per my mechanics suggestion?
Also the fresh water cooling pump has 3/8 inch outlet and the heater hoses are 5/8 inch. How was this transition from 3/8 to 5/8 inch done?
I have attached a picture of the header removed.
Keith
SV: Victoria dos

[TomSoko]

Keith,
My '87 had the exact same bypass valve. I also checked with a few places (well known Universal facilities), and they all said to remove the extraneous plumbing, so I did. When I did mine, I could not find a suitable reducing fitting to go directly from the circulating pump to the 5/8" hose to the water pump. Instead, I plumbed the water heater just as it had been before, but without the bypass valve. Page 44 of the M25 Owner's Manual shows the diagram that I used. It's on the Tech Notes CD. I left the short 3/8" hose between the circulating pump and the thermostat intact. In theory the water heater should be plumbed between these two points (as new engines are), but my setup (as in the Manual) has worked well for the past 15 years or so.