I'm not sure what a 'rubber hammer blade' is but here's how I've always separated the cases. I'm not sure if you're asking about removing the cylinders from the transmission case or separating the transmission cases but the same principles apply to both.
Use a hard plastic mallet. Don't use rubber; all of the force of your strike against the engine is deflected back through the rubber. Don't try to put anything like a chisel, flathead screwdriver, or other prying device where the pieces mate up; they are aluminum and you risk destroying the mounting/sealing surface. While securely holding one portion you strike the other portion (with the plastic mallet) at an angle that pushes it away from the one you are holding. It is best to start by first striking the selected piece all the way around the mating area to assist in loosening the bond between the two pieces prior to actually trying to separate them.
Once they begin to separate work your way all the way around the unit continuing in the same manner. Separating them can be a slow tedious process as there are aligning dowels that can hang up and cause binding so you have to go a little at a time. Be patient, they will eventually come apart.
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