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They are a pain to put back together, but I've never used a press on them. I'd be too scared of breaking one of the housings. I use the slickest grease I can find on the o-rings and use a big dead-blow plastic mallet, and I've sat on a few also. Just be very carefull not to let any dirt into the oil part of the cooler. I put some black tape over those holes until I'm ready to put the ends on (don't forget to take it off though).
You can take it on and off the motor easily. The ends come off fairly easy too, I used a rawhide mallet to tap the ends off. When I put mine back together, I put motor oil on the O-rings, lightly clamped the center section in a vise, use a rag or wood to be gentle. Then I used two of those long clamps for gluing woodworking projects to pull the ends back on. It takes three hands to keep the clamps from slipping off, but I prefer this to pounding.
Also- I installed it back on the block first without the gaskets to realign the ends, then took it back off and installed it with the gaskets