I inquired of the jacobs chuck company regarding rather inconsistant runout specifications for their chucks, here is the answer
We apologize this confusion. The runout specs of both chucks (superchucks and professional keyed chucks) are measured at 1.5" from the nose at half capacity test bar.
- Superchucks T.I.R. spec is MAX. 0.003" at half capacity (1.5" from the nose).
- Pro keyed Chucks T.I.R. spec is MAX. 0.004" at half capacity (1.5" from the nose).
We are updating our PDF file accordingly to avoid this kind of confusion now.
Again, thank you for your feedback.
Sincerely, Jacobs Chuck Manufacturing Company--A Danaher Company
as an aside, if you have one of the later jacobs ball bearing chucks, be very careful to never pound on the jaws - it causes brinellling of the bearing and destroys the chuck accuracy - I just looked at one that must have been dropped on the jaws - looked perfect from outside, but there are nice ball sized dents in the race