How can I re-fit the bottom concrete block on my hoover?

Lift it back into place and screw the bolts back in. (May need an extra pair of hands).
You should be able to see where the blocks were fitted from the bolt holes and marks on the mounting surfaces.
Once you have them in and are happy the machine is working OK, take them out one at a time and re-fit with some threadlock on the bolt threads.

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June 2015