Some changes have been made to the ER16 Collet Chuck, which are needed before assembly

The shoulder behind the head of the ER16 Chuck, needs to be machined true to the shaft & Head, most are not machined/ground when manufactured, They are rough machined & then heat treated, After heat treating most parts of the ER Collets are Ground but not many manufactures do that face

You put the ER Collet Chuck between Centres, see that it is running true, & a light cut on that face to clean it up is all that is needed

The next part to the mod is to put (2) more flats on the head, to match the (2) that are already there

Putting (2) more flats on the head will give it better balance