most likly your Jukar is going to be a 6x1mm. it wont be a 1/4 , 28 unless someone changed it over . folks used to try and use a 1/4, 28 as it would wobble then tighten down . but more times then not it would also end up blowing out at some time and then the breech would need to be rethreaded .
as such it could be now larger . you will have to check it and see .
Also, the thing about metric , factory nipples is that they are all metric , not just the threads . IE the necks and seat are also metric .
unless you really like the metric threads . i would have retap it to a proper standard thread . but thats just me