Scooterotica

the gap between the bottom of the headset and the steering lock.( shown on p.139 stickys book ).

on my 1965 SX [125 Special] headset, distance is about 9mm.
It's the original Itaian headset bottom.
[measured with a child's ruler in the dark]

11.19mm on my SX150, measured from the bottom of the headset casting to the bottom of the recess for the steering lock. [Daylight, cheap Vernier calipers from eBay]

Thanks for the replies, much apprieciated.