I've always made the hole as snug as possible because it seemed to me that was how it should be. One of my original Myford supplied chucks was so tight it needed a light scrape before it would fit the spindle. I read several arguments about the fit, more than one stated that the hole should not be a close fit because the register was the back face and not the bore, the thread took care of centralising the chuck. Several of the aftermarket backplates I've bought are a good rattling fit despite which the chucks seem to fit with acceptable run out (for a 3 jaw chuck).

I wouldn't worry to much about a smidgin but you need to keep it below a gnat's whisker.

Daveb

Hi Paul I would bore out and bush the hole as it is the location that matters. Use the same material, cast iron and Locktite as well as a life press fit then Finnish bore after finishing.

David

Now it is revealed in its true form, I'm in the sleeve and bore camp, it should be fairly straightforward. If the sleeve is to be thin walled when in place, it would be easier to bore the backplate to the required size and make a fairly thick walled bush. Fit the bush and bore to size. The backplate then keeps the bush round when you finish bore it. Thin walled bushes are difficult to hold without distorting them.

Do check first that the projecting spigot on the spindle is parallel and not tapered.

Edited By old mart on 11/11/2019 15:44:03