What do I need to know about chucks before buying a four-jaw?

Picked up a (well-loved, by all appearances. Looks pretty good, used, but taken care of) Skinner four-jaw six inch off the fleabay! Hopefully everything works out well when I get it!

Comes with a backplate that should fit. If I understand correctly, I'll have to screw the backplate onto the spindle and machine it so everything is concentric/true and then put the chuck on, right?
 
If the backing plate has already been fitted to the chuck and the backing plate has the correct diameter, treads and register for your spindle, I'd just screw the hole assembly on and check the chuck for trueness, before machining anything.

If the backing plate and chuck have never been mated to each other before, then yes, most likely you will need to go through a truing and alignment process.

I was fortunate with a screw-on backing plate/chuck assembly I picked up, that it aligned almost dead on when I first screwed it to my spindle. All it took was some fine adjustment of the chuck to the backing plate, and I had it holding less than 0.001" run out.
 
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This picture may help. V threads are typically non-centering hence the register on the lathe spindle to keep it centered. You do not want to alter it because then all the chucks will need to be altered to fit. Machine or polish out the bore on the backing plate so that it fits snug, any play will allow it to not center correctly. (this is more important on self centering chucks, i.e. the part). You may encounter both types of backing plates, one (A) will have to have the register machined in it or the other (B) may need to be trimmed to match your chuck. The closer the fit the better off you are. The other reason not to alter the spindle register or the chuck internal register is than the backing plate becomes sacrificial and is easier to replace. Making a proper backing plate for a beginner is quite an undertaking. Hope this helps
 
Okey doke, the backplate is already attached to the chuck (at least the pics on the fleabay showed it that way), and the threads are supposed to match. If they don't actually match my machine, I'll have to get a new backplate and make it fit the chuck, but I have great hopes and expectations (whatever that's worth). We'll see in a week or so! Well, I'll see it in a week or so, but I probably won't have time to jump right on it and see if it is perfect.

And no, absolutely nothing on the spindle is getting touched with anything more abrasive than a toothbrush.

I was thinking I would be cutting a couple thousandths off the register on the backplate just to make sure it is concentric. If the runout is not bad, I'm not even going to worry about it for now. It's an import lathe from the '80s. I'm not going to be doing any REAL high precision work on it.
 
It's a good idea to use antiseize on the spindle register as this fit is usually really close, you don't want to seize the backing plate on the spindle. The fit needs to be tight for two reasons. one to center the plate and the other is to transfer the moment (load) to the portion of the spindle beyond the threads other wise the spindle can develop cracks at the last thread.
 
Hi Mauser,

I don't use anything other than oil on my lathe spindle register and threads. It is very important to keep those areas completely clean.
I use a fresh clean paper towel and a toothbrush first on the threads of both the spindle and the chuck backplate, then liberally oil them both. Screw the chuck on and then wipe off the excess oil. If you don't wipe afterwards you will get an oil bath from the thrown off spray.
 
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