Originally Posted by
PlanoFuji
If I am reading the OP's statement correctly, the fact that the wrench is wide enough to prevent the other nut from tightening properly is the problem. I think the bolded text you are writing is referring to the diameter of the spacer and not its thickness which I believe needs to be thick enough so that the wrenches are not preventing the nuts from being properly tightened. Of course the fork will need to be long enough to allow such a spacer to be added while still allowing enough threads for the lock nut to engage sufficiently.
Either way, while the OP may be able to 'make do' with what he has, the right tools for the job are always preferable.
I mentioned the diameter of the spacer because in some instances I have seen it interfere if too large a diameter, but in general there should not be a problem, and yes, there needs to be sufficient threads left for an extra spacer to be used.
Of course the right tools for the job are preferable - but many people have only one headset wrench, and functionally there is no difference when it comes to the locknut.