The wire inside Most heating elements are blanketed with silicone, so they are okay to bend a bit... Rigid ceramic heaters should not be bent, like bakerg said....
But if it is silicone & you mash them too much the resulting pinch could shorts it out.
You might check the other core plugs for better fitting... or go with a tank type heater.
Are your glow plugs working? I have never needed a block heater with kubota diesel engines..:smile:KennyV