They are 19mm heads, same as the factory ones mate.I like the Trident wheels
Are your new bolts 17 or 19mm spanner size?
I've resorted to just blowing a bit of satin black over the head of my bolts now until I can find some caps that fit in the holes. Its a pity that Supermetal didn't make the holes big enough originally to accept the factory caps especially as they were offered as a VW dealer accessory!