Jim,
Congratulations on the new boat. I too just purchased an 84 C27 with an M18 and I too had a leaky HX. I just completed the project of installing a new one, my old one was beyond repair. While completing the install, I also added a vented loop in the raw water line from the HX to the exhaust. As I understand it, this will prevent water from siphoning up into the exhaust when the motor is not running and ending up inside the motor. I purchased a new HX, hoses, clamps and a stainless vent for $500 and did the install myself. I found that removing the screws in the starboard side access panel and moving it out of the way made access to the HX much easier than working over the hatch door. Before you undo any hoses, make sure you drain the fresh water side (antifreeze). There is a drain valve on the starboard side of the motor, I ran 2' of clear hose out toward the front of the motor and drained the coolant into a container. If the boat is in the water, make sure you close the sea cock for the raw side before removing any hoses on that side. When you do your reinstall, make sure you run the engine for 1/2 hr or so with the cap off the fresh water side to burp it, advice given to me by the yard. This will prevent an air bubble which could cause overheat and engine damage. The mechanic told me that if drafting off of a bucket, to remove the hose from the sea strainer outlet and put that in the bucket, not the hose attached to the seacock/ strainer inlet. I had to prime the raw water pump and it took some doing to get it to suck water while the boat was on land. Once in the water and hydrostatic pressure was enough to let the pump do its thing. I just brought my boat home yesterday under power and ran the motor for 2 1/2 hrs, she ran at between 130 and 140 degrees f. I also used some old bike tire tube between the HX and mount to prevent the finish from getting damaged from vibration. Good luck on the project!
Jeff
