4 jaw independant chuck

scored a 4 jaw chuck that was advertised as a 6 inch but is in fact a 61/2 inch Munro chuck.It seems in reasonable condition after a cleanup in the vinegar bath for 24 hours; some questions if i may
is going to be too big ( depth of chuck is also 65 mm)it weighs 8 kg
how do you remove the scroll retainers shown in pic2 & 3. i tried some gentle persuasion after i removed the grub screws but it didnt move (i dont have to it was only for a clean)
im going to try and use the steel boss( i think its called) for a back plate after a few practise runs with the weights
any suggestions and info on the chuck most welcome especially making the back plate i have another 4 jaw chuck to hold it

Would advise you not to use the steel plate for a backing plate,you run the risk of it not wanting to come off easily.
In regards to the scrolls are the grub screws you mention only there to hold the scroll retainer in place or were they used for another purpose,you may find that the scroll retainers are a push in fit or size for size and you may have to knock them out with a punch.
If the chuck jaws move freely and there is no damage to the scrolls leave them alone.

if you look at pic 3 the lighter circles are the retainers with the hole the grub screw fits in half in the retainer and half in the chuck body ok on the steeel back plate too

I have a different four jaw to yours but it has that arrangement with grub screws holding the retainers in. My retainers come out pretty easy once the grubs screws are removed with some light hammer blows using something like a bit of scrap round brass for a punch.

Edit: actually they might just come out, memories a bit fuzzy on it.

i might try making a brass drift with a U shape so it hits on both sides of the retainer and keeps it moving straight and see what happens but in any case it probably doesnt have to be taken out atm every thing looks clean and it turns freely the back plate is the next challenge
what sort of clearance from the face of the back plate to the back of the chuck is required
the recess on the back of the chuck is 10 mm

Minimal

That chuck looks like it may be one where you can have a small back plate with no step. If so you can just make the O.D. of the backplate fit inside the register on the back of the chuck. Hard to say for sure without seeing it.

a pic is worth more than what i can describe:)and also a pic of the swarf from the intended final back plate material it come oflike grey powder so i think its some sort of caste
comments welcome
it would be better if i could just make the back plate to fit in side the recess would cut down on the weight i can drop on my toe
the recess is 10 mm deep

Is the Chuck to be mounted with Bolts through the Chuck or are you going to Drill and Tap the Chuck and attach the Back Plate that way.
The more clearance you give behind the Chuck to the Locateing Spigot will increase the amount of rubbish that can sit in the cavity.
The Rule of Thumb for Thred engagement is 1 1/2 x the diameter of your bolt.

Sure looks like cast iron swarf Tanii. The spigot diameter in that chuck is pretty big. Probably best if you make the full diameter, and do it with minimal gap as Pipeclay suggests. Looks like it may also be possible to accept a backplate just the diameter of the spigot. First you'd need to make sure the surface for it to sit on is all ok though.

i plan to use the exiting way of mounting it, the bolts go thru the chuck into the back plate
one bolt of the supplied ones is missing so maybe a bit of improvising they are 1/2 whitworth or very close (1/2 w nut screws on easily )
the back plate is 17.5mm thick where the thread will go thru
will cap screws be ok for mountiing in place of the bolts that came with it

Are the Socket Head Cap Screws that came with the chuck Fitted or just Extended Head Cap Screws.
If they are Fitted I would suggest to Retain the 3 you have and just buy an additional Cap Screw of the correct Length and Thread.

Did you purchase the Material for your back plate,if so how much was it and what diameter is it .
Also after you cut your Thread and Machine the Register mount the blank on your Lathe Spindle to complete the Machiniig.

the bolts that came with it look like they were machined for the chuck ( they have centre bit holes in the thread end) and the head had a square internal but fortunatly the sqare shaft from a a old door handle fits perfectly 5 /16
ill check the thread for angle to see if they are whit or unc and then probably use an extra cap screw for the lost one and ill leave the extra on the back to fit the outside of the chuck diameter
thanks guys for your help