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I just replaced my water pump about a month ago and I just used the gasket that came with it. No sealer. As long as the mounting surface is clean and you torque the bolts to spec, you should be fine.
I think ya'll are right however I always use copper spray gasket.....not only does it make it easier because it's tacky and sticks to the surface......it gives me a littleless worry and stress.
I use monkey snot (weatherstrip adhesive) to keep the gasket in place on the pump..apply to both, let dry, stick it right the first time, you don't get a second shot most times if you did it right.. then install..
But if you cleaned the surfaces well, and aligned the gasket right, your fine.
The factory doesn't use any sealant on the water pump or timing cover gaskets (lots of others too). My water pump has never leaked a drop. I replaced my timing cover gaskets last month and I used Ford gaskets which are very noticably higher quality than Napa or Carquest's Victor brand. I was very tempted to not use any but chickened out and did use a small film on them.
I've had trouble with my thermostat gasket (Carquest) leaking and I blame the RTV. It actually blew out last summer and I could tell the RTV had really weakened the gasket. I put another Carquest gasket in with RTV as that's all I could get when I needed it. It's seeping again now. It'll be Ford gaskets if possible for me from now on.
The factory service manual (Helms) recommends the use of a sealant for both the timing cover and water pump gaskets along with thread sealer on the threads of any water pump bolts that penetrate into a water jacket.
On a used motor, you do not have the same advantages as Ford did on the sparkling new motor. I ALWAYS use Hi-Tack on Water pumps and hoses. I NEVER have leaks.
I ALWAYS use high tack. I just got my F-250 so not a bunch of wrench turning yet but I have always used hign tack prior to my new toy. Except for intake manifolds, thats a whole 'nother thread..........