Shimano Ultegra 10 speed wheels - what size cone spanner?

Michael S. Roper

I'm trying to adjust the cones on the rear hub and I seem to need a very large spanner to work on the left side. I bought an 18mm cone spanner thinking this would do the trick but no luck, even it is too small. Shimano service diagrams are unhelpful, they give everything but the cone dimensions.

Does anyone out there know what size cone spanner I will need?