What the heck?

Look closely at the back edge of this Buck chuck:

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I got a great deal (free) on a 6" adjust-tru with no backplate and found one with the same oversize condition you have. Rather than using the original holes in the backplate I rotated the backplate hole pattern and made my own mounting holes then filled the originals with JB weld. Once mounted on the chuck I turned the extra metal off leaving the exposed JB weld segments. That was about 15 years ago and I use the chuck nearly every day. Don't underestimate JB Weld.
 
In my opinion before resorting to JBweld or any other ideas you have in mind, bite the bullet and order another properly sized packplate that will fit your chuck , I know it's extra expense but this is a hobby for you and should not be a source of aggravation, plus you can always use the new one as your template and/or a tool to measure your machining prowess by comparing it to the one you're fixing .

I know LMS (as arron-W ) mentioned may have exactly what you need
Just a thought.
 
@francist I absolutely agree. I knew it was going to be close. :-(

@Ken from ontario I might just do that. I'm going to take some photos and detailed measurements tonight.

@middle.road Everything lined up perfectly. Keen eyes on the bolt sizes. Sitting at my desk I'm betting they are M10.
I searched for 202-6579A and there is no product. What I received is definitely a 202-6579.
This is the relevant line from my invoice.

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Looking at the details I found this:
It is a 6" self-centring 3-jaw that uses the same backing plate as my 4-jaw.
This one mounts with M8 cap screws.
In the reviews there's a photo of a completed setup. The backing plate is turned to the chuck diameter and there's no breakout of the counter-bores. My assumption is that the M8 counter-bores are small enough that they are not exposed when turned to diameter.
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I've bought two d1-4 back plates from CDCO and if I remember correctly with shipping they were the cheapest. I know shars shipping prices are on the high side. Back plates were of good quality and fitment didn't have to adjust D taper for correct register to spindle. Actually have to tap chuck to release after pins are released.
 
I've had chucks where the back plate was slightly larger than the chuck body to cover the counter bores. Not Shars fault that the bolt circle on the chuck requires a larger backing plate.
On the extreme side, I wanted a smaller 4 jaw fir the Summit. The 16 inch will only close to 1 inch. Some one gave me a D1-8 back plate that was probably part of an integral mounted 3 jaw ? Faced off a lot of material to get the face down to mount an 8 inch one.
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I could take some material off the OD but if I took it to the size of the chuck there would only be 1/8 th of an inch left past the mounting pins.

Greg
 
If the 4-jaw is using m10's from the backside then that's why the C'Bores broke through. Even m8's are very close to the chuck body.
Grab a picture of the back side of the 4-jaw with a scale laying on it if you can.
If I'm reading the charts correctly, then:

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@middle.road that looks to be exactly the right dimensions and about how much of the counter-bore is exposed.
Looks like an M10 has a 16mm head so .75mm clearance if I'm interpreting the drawing and cap screw specs correctly.
 
The backplate is advertised as being 6.3” in diameter. Naturally I assumed that was to allow for finishing the outside diameter to exactly match the chuck.
David, I have several older chucks to fit threaded spindles that are considerably SMALLER in diameter than the diameter of the chuck O. D. None of that seems to matter much if you can get the two pieces mounted together solidly and without compromising the integrity of the chuck and the back plate. With what you have done, I hope you did not leave sharp (or even close to sharp) edges that will bite you if your hands touch them. I would heavily round the sharp points to a decent size radius to eliminate that danger. Pics?
 
Here’s what I have.
I do need to pull the cap screws, clean up and deburr the edges.

That is an M10 cap screw.
The counterbore is 17.75 mm
The head of the cap screw is 14.85 mm
So the counterbore is certainly larger than it needed to be.

I wish I had quit while I was ahead
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Sent from my iPhone using Tapatalk
 
If this was my situation I would remove the entire edge of the backplate that contains the counterbores. Make a step there and just have the bolt heads exposed. You could change to a different style bolt head, like a button head. This would be as safe as the chuck itself. The other option it to buy another backplate for a do over, and use the current one for a smaller chuck. A 3-4" or even a collet chuck.
 
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