>>
>> So why would one not buy a pump head every time? what's left in the
>> pump body and what's in the pump head?
>>
I think this is the equivalent to the repair I carried out on my 3-port valve. (i.e. leave the casting alone and replace all the "innards")
Essentially, the pump body is simply a casting, containing nothing but air (or water ;-) ), and to which all the rest of the pump (impeller, motor, control box) bolts.
It is entirely possible, therefore, to replace all parts that might fail/wear without having to undo pipework and remove the body.
Of course, the ease of doing so will depend on how securely the pump head is fastened to the body, but it is likely that this will suffer considerably less from seizing than the body/pipework connections.
My pump, fitted some time ago by myself is also a Grundfos. I'm not sure that you can get a replacement head (only) for that model, but it would seem entirely possible, and perhaps sensible, to buy a full unit and cannibalise, not least since the installation process allows for various positioning of the pump head (4 x 90 degree options) which involves removing the head from the body (4 hex bolts as above) and retightening after the desired position is achieved.
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