How much tread is enough

[Rob 3]

I got a set of alloys for a fiat,the pcd was 98 i think but with the help of a round file they fit.
only problem is the length of the studs,getting about 3/4 of the way up the nut wont wont go any further.anyone know if its enough or ever done this? the fronts are nearly at the top its just the back ones im worried about

 

[markbognor]

1.5 x stud diameter into the fixing, so with a 12mm stud you want 18mm or there abouts into the nut.

It's best not to force the 4x98 wheels onto the 4x100 studs, even if you have enlarged the holes the seating surface will still be incorect. It will put a sideways loading on the studs - they are not designed for this - and could lead to you being overtaken by your own wheels. Wobble nuts and good centre location are the answer.