This is how I set up a cooling system. I am sure there are other ways to it that work, I just haven't found them yet. One word of advice, stick with one guys mods. Doing a few things from 3 or 4 different peoples ideas will most likely not work. Everything I am going to tell you works together. It all has to be done or it won?t work right. You guys with 6500-RPM motors, I do not recommend modifying your stock cooling systems. There is nothing worse for a motor than to run too cold and ONLY high RPM motors need these mods. Find the problem with your stock cooling system and fix it, don?t just go to a 260 HP cooling system. 260s have a life span of 100 hours, I am sure you want your fishing motors to last longer than that. Ok, enough of the lecture.
I don't do much to the lower unit but check to see that the two or four holes drilled up from the nose do not intersect each other before the enter the one large passage leading to the water pump. Mercury had a problem with this and it should be checked. From there up to the exhaust adapter plate I do nothing. Seal the water pump gaskets and seals with heavy grease. Either a ½ or a ¾ inch feed tube is ok to use. Nose cones seem to need two-½ inch hoses from the cone to the pump to really make good flow and pressure.
The water now goes up the center of the block and into the exhaust divider plate. No mod here but check the plate for cracks and don?t over torque it. Mercury has switched to a new cheap gasket, which does not seal properly especially when over torqued.
Now the water goes into the cylinder water jacket. All you need to do here is install the diverters exactly how Mercury says to. These are VERY VERY important to install or your block will be damaged. Not installing these will allow the cylinders to warp from all the water going on one side of the cylinder. This will not blow your motor and you will not its happening but power will suffer.
The water now goes to the top of the heads. In the top of the head is a good place to install a water temp gauge. This is not a head temp gauge it only measures the water temp in the block. Here is where the thermostats are. I install 1/8 stainless steel fender washers in place of the thermostats. 1/8 washers have a hole in them just a little smaller than 1/8. They will bleed off just enough air and water to keep the block full without compromising head temperature. As long as you are using the 1/8 washers it will not mater if you run two hoses off the heads or go from one head to the other then run the hose down (the way Mercury does it).
Now the water goes down the heads and into the exhaust adapter plate. I only run a one-piece, 240, 260, 300 style plate, because it's stronger and easier to modify. If you have a two piece plate I would recommend drystacking it to control the flow of water. There are other ways to block the dump on a two piece plate with epoxy and redrill the right size holes but I never mess with that. Now on the one-piece plate the 2.4 dump hole were pretty close to the right size but I still modify them. The 2.5 plate dump holes were all over the place. I have seen some that were so big the motor could not get more than 5-lbs. water pressure at 9000 RPM. I weld up or tap and brass plug all the dump holes in the exhaust plate. The 2.4 will have 4. Two in back and two in front. The 2.5 will have two ridiculously large dump holes in the front. Keep in mind the worst thing for a motor is to run it too cold so don't worry about closing these holes up a little. After I block the stock holes I redrill two new holes in the front of the plate, right next to the other holes, that are 19/64. This will allow proper cooling, good water pressure and still let some heat develop. You are looking for a minimum of 15 lbs. at full speed but no more than 30lbs max. Dry stacking is also an option. I block all the dumps and install two 90-degree 3/8 barb fitting in the side of the plate. Then I drill about a 1/16 hole down in the front of the plate to cool the water pump.
Now the water will exit the motor through all the dumps and hoses. One of the places it can dump is the popet cover. Do not run a popet. With a one-piece exhaust plate a popet will do absolutely nothing. From the popet plate you should run a 5/16 dump hose for Nicasil blocks or Nicasil blocks with 4 steel sleeves or less. If you have a Nicasil block with 5 or 6 steel sleeves run no dump at all off the popet. This will get the water in the motor a little hotter. The reason for this is that Nicasil can transfer the heat to the water very fast and efficiently pre heating the water for the other steel sleeves. If you have all steel sleeves the water in the block will not heat fast enough and will contribute to a cold seize situation.
7. The final cooling mod for steel sleeve production motors is plugging up the two cylinder water jacket drains and redrilling them in the output side of the water jacket with a 1/16 drill bit.