I have a P6KE51A Glass voltage suppressor.
I am busy repairing a 18V drill charger. I measured across the P6KE51A and the resistance is zero ohms out of circuit, so its basically a short.
Is this correct?
And another thing, if its blown, i cant source this part, will a P6KE68A be to much of a higher voltage?
No, it isn't correct, a short means that it's blown and you need to buy a new one. Using a higher voltage rating is a bad idea as it will reduce the level of protection.