It’s a common issue, for 26″ and 29″ bikes alike. A bit more for 29″ though.
Modern suspension forks seem to have a tower of adjustments and flags on top of their crown, to the point that they won’t clear the downtube, for instance in case of a crash. Your older frame may not yet be corrected for these new “improved” fork dimension. Bad for your frame finish and even warranty.

Ventana already invented a special lower race to fit the Chris King headset with higher stack. But not everyone can afford that, so now On-One makes a whole headset specifically to solve this issue!

The On-One Smoothie line of headsets consists of the Light, Regular and Grande models. The regular also comes in +5mm. The lower cup is simply made taller by 5mm, keeping the frame safely away from any knobs attacking your precious powdercoat or even compromizing downtube integrity. No need to file away material from the fork, replace screws, etc. Just fit this nifty headset.

More info on www.on-one.co.uk