Mounting this coolant pump horizontally

[Noplace]

What stops coolant making it's way through the dust seal on the shaft, through the bearings & into the motor?
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If you want to run a pump attached to the tank you've designed you should use one where there is an air gap exposing the rotating shaft between the motor and the mounting plate, that way a coolant leak will be visible when it starts rather than only being detected when it's either made it's way through the motor or caused the motor to fail.
The pump is centrifugal, the flat round thing at the bottom of the cage in your linked product, this needs to be fully immersed if you wish to avoid airlocks, another reason why suds pumps are almost universally designed vertical.
You're on a hiding to nothing with the pump you linked to in a horizontal tank.

- Nick

Thanks for the detailed explanation, all notes and will redesign