The sketch included in the AlKo document that JohnG references above shows 67mm minimum from centre of towball to the mounting face. On a standard towball this dimension is 55
That could be corrected with a spacer but the length/height of the neck is critical to allow full articulation
The RHS top drawing shows min 60mm from centre of ball to horizontal on towball(why they don't show to bolt holes??) and a requirement for 25deg from the vertical articulation
I have seen a standard towball gouged round the neck and the owner in question borrowed a file from me to file it down when in France
Amazingly he was still using it when we met again 3 years and lots of Europe later!
I recently got the genuine Alko ball complete with black painted ball for 50p at a car boot
Yes, there is no standard dimension comparable to the 60mm that you refer to in the AlKo sketch. For a standard towball, the centre of the ball should be 70
In the interest of knowing I measured the distance from the bolt hole centres to the centre of the ball on my Alko towball as above and it is 80mm
More interesting facts??
Using my digital callipers my new Alko ball measured 49.74mm at the centre from side to side and the 6 year old one it replaced measured 49.68mm so a wear figure of 0.06mm or 0.01mm per annum