Nismo Gas Shocks

[R31Heaven]

I was under my car today doing a thourough check on bits and actualy had a look what type of shocks are on it, they are Nismo Gas Shocks i think the number is P18 they are not a sealed unit they seem to have a screw top. Any way my question is does any one know if these shocks are rebuildable or it cheaper to buy new shocks...by hte way there is nothing wrong with them at the moment this is for future reference...