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I think I see the "plate with 4 jaws" on the shelf above your lathe.
But is that not a conventional 4-jaw chuck next to it ?
Is that not big enough to take the 3 1/4" ally bar ?
You should be able to take each jaw out and reverse it, creating the scope to hold bigger diameters.
This is not possible with a 3-jaw, self-centring chuck because the jaws are driven by a single "scroll gear" so their driven teeth are curved and can therefore only fit one way round.
On the 4-jaw chuck, each jaw is driven independently by its own worm gear and the driven teeth on the jaws are straight-cut, allowing them to be reversed.
Sorry if the above is all old hat ... but better safe than sorry, eh ?
ps. ....just to dot the i's and cross the t's .....
If it does fit in the 4-jaw, you may need to also use a centre to steady the free end of your 3 1/4 inch bar.
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