To expand on what Chuck wrote--the return pipe for the heater is secured with one bolt--slip the lubed new "O" ring over the pipe and insert in pump. I get all the bolts/nuts started except for the bottom most one. I run the nuts/bolts in gently - not all the way to tight. Then I use the bottom hole to insert a rod (screwdriver or whatever you have that is less than 8mm in diameter) and lever the pump up to squash the upper seal and simultaneously tighten the bolts/nuts and then add the remaining lower bolt. I've used GMB pumps many times--never had a problem. -- Dave
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