Just to let you know what to expect from these commercial items.
The supposedly 'accurate' 5C collet chucks I have fitted previously, and I have done a few. Every one had to have the internal nose taper reground to get them to within tolerance, which they say should be 0.001", my findings were well above that. By doing the grind myself, I can get that to well less than 0.0005", in fact I would expect 0, but a bit of dust on the collet can knock them out somewhat.
I have heard thru the grapevine that these bolt on ER jobbies are worse, hence, that is why I made my own.
It is just the law of averages on cheap tooling, that they aren't made to the same specs and quality control as expensive ones. If they didn't clean a tiny bit of swarf off the holding jig during manufacture, you will get a variation in their tolerances.
You might be lucky and get a replacement that is better, but on the other hand, you could find you end up worse off.
The chuck requires mounting onto it's backplate, then fitting to YOUR lathe, followed by truing up the taper. If you are lucky, and the ER chuck isn't fully hardened, you might get away with turning the taper true with a razor sharp cutting tool, rather than having to grind it.
I am sorry to say, that is the price you have to pay for precision.
Bogs