I have talked to Paul, that is were I got my info. see bellow:

"James,

I assume we are talking Commercial motors. The dividing line for yes or no has to do with motor case integrity. If your modification compromises the case it is not allowed. This could either mean a hole through it, taking out the temper, or a thinning of the case enough to cause a bursting.

Not sure why knurling is that big of a deal if left on, but if taking it off, I would recommend not grinding it off and milling it off, instead

I would say a Redrill and an eyebolt threading is fine, except you have to make sure the temper of the case is not compromised, there is no drill through AND the guy doing the modification knows what he is doing.

That's the crux though, how can we be sure someone did a good job at any modification. That's why it is frowned upon.

When a manufacturer offers any changes they are supposed to come to me for approval, like the foreword fill/staging closure I approved for Skyripper motors. Then they take the liability if something does not work.

Paul"