If you're going to describe this process, be clear about it (otherwise damage to the meter, motor, or user is a possibility):
I didn't write using hobby motors i assumed that fact , as per the tutorial he is following most probably the motors would be Hobby grade(its very rare but not impossible to destroy multimeter's with these type of motor's) not to expect a RC Heli Motor 
,well you have been specific im fine with that.
stacking them means the ones on the bottom can't effective cool themselves properly...
I had a remedy to it too!, inserting a Aluminium plate that i custom cut from some scrap at my workshop and made sure it fitted in tight so the transference of heat to the plate happens and then giving the shape of a wing to the excess of plate getting out of the bottom IC's can be worked out.