I need to replace the door knob for this interior door:
The door knobs I've found at the stores require a 2⅛ inch hole. Mine is only 1½:
Did door knobs used to be smaller? Is there some place I can find a replacement door knob that will fit my door?
If I need to modify the door to fit a modern door knob, will the new hole completely overlap the old? and can I reuse the latch hole?
Two other options. If you need to expand the hole with the exact same center, you can get a hole saw kit that uses an arbor with multiple attachable hole saws. Place the larger size hole saw with the smaller size inside of it, and tighten the arbor on both. The smaller hole saw acts as a guide in your existing hole.
For this to work the smaller hole saw must be the same or longer height, as depicted in the picture above. If they are the same length, the thickness of the base of the larger hole saw will cause it to protrude slightly. You can also use a washer to give it more depth as long as you can still secure the bit to the arbor.
The second option is similar to Chris's suggestion. Clamp a thin piece of plywood over the hole on the door, and drill through that. Pay special attention to keeping the drill level. The plywood keeps you from sliding all over the door. Just make sure you clamp it on well.
i used a piece of plywood clamped to the door as an outer-guide. Cut a hole in the plywood, the right size (2 1/8) and then clamp it where you want the new hole in the door. Then using it as a guide on the outside of the hole saw, drill on through the door. voila. no additional tools required beyond the 2 1/8 hole saw, and well, a c-clamp which you should have.
Yes, door knobs used to use smaller holes.
You probably can find a replacement, but if you want to use a modern door knob, yes you can reuse.
Get yourself one of these babies:
It's a door knob jig - you can find them at the major box stores and they come with the appropriate drill bits. Align the jig with the existing latch hole, and then use the larger hole to enlarge the bore of the main hole.
DO NOT try to enlarge the hole w/o the jig. Your hole drill center bit won't have anything to bite into (since there's a hole there already) to hold it steady when the main cutting circle hits the wood and it'll slide all over the place.
This is a fantastic investment compared to buying a new door. :)
If the new hole location completely encloses the old hole, then it's relatively easy.
With a 1.5" hole saw, cut a piece of 2x4 and plug the hole. You may need to sand or wrap it in paper to make it fit snuggly. Then you can use the 2 1/8 hole saw to cut the new hole.
If you need to move the location completely, then you need to plug the hole, leaving about 1/8 to 1/4" recesses on each side, and putty/plastic wood over it. Sand and paint.
You can usually buy a cheap set of 4-5 hole saw bits for under $20. That's sufficient for occasional use.
The Schlage B60 series allows you to very simply remove a non-functional internal cuff which enables the large lock to fit into the smaller hole. No cutting or enlargement necessary. Just did it. Very simple and I understand many other lock manufacturers also offer this option in order to insert modern door locks into older door holes.
I was in exact same situation replacing a 1970 door lock got a name brand lock and the barrel was protruding while hitting back of knob hole.
Wondered same thing,
it's like my new lock was John Holmes wtf.
Anyways I went back to home Depot there was a $25 Cdn deadbolt Lock called Defiant there , I recall also using one last year at a friend's apetment it's pre configured for the smaller hole and adjusts by spining it to longer hole.
VIOLA no drilling or or Jig headache required.
DEFIANT brand $10-15 USd or $20-$25 Cdn Walmart, Home Depot and Lowes all sell it. Installed that and a pull handle on both sides, last year I simply drilled a hole for old liock above deadbolt which was in existing hole. That way I had a pull without buying anything extra. I may do that here and return gate pull.
Defiant Single Cylinder Stainless Steel Deadbolt DL61 at The Home Depot - Mobile http://www.homedepot.com/p/Defiant-Single-Cylinder-Stainless-Steel-Deadbolt-DL61/100352212
Also Google how to adjust the backseat and your brand of lock. 
I just added a 1/4” hole to fit the door handle through screws. Also did a little trimming on the inside of the 1 1/2” hole for the latch to be able to seat all the way in. Tiny bit more trimming to allow the handles to fit and done!
I inserted a bit of wood I found laying around into the existing hole so the pilot hole had something to grab onto.. That's it
I did this today on a wood door using a Dremel with a sanding attachment on it.
First I outlined in pencil where the screws hit the door and sanded out that part, first on one side of the door, and then the other. Once the screws would go through, I traced a line where the door knob would be inserted into the door (this was on the lock side only.)
Since this doorknob only had semicircular pieces - top and bottom - that inserted into the door, I just reamed out the top and bottom of the existing hole until the doorknob would fit in. Not as elegant as using a hole bit, but this was the tool I had and it worked just fine. Took a bit longer, I'm sure, but it worked. (I think next time I'll buy a hole bit.)
As Chris said. If you are only doing one door, you can cut a plug, same thickness as the door, that you can glue into the existing hole, flush with the surfaces. Then mark the center of the old hole, or, the new backsets (latch) measurement, usually 1-3/8" or 1-3/4", but most are adjustable for either, from the edge of the door. Use the block for the pilot drill to center the hole saw and cut the new hole around it
I did this today by using a Dewalt door lock installation jig. I bought this at Lowes. It cost more than the Irwin model but I bought the Dewalt because of the clamp on the jig. It was about $30 but I have several door knobs that are over 50 years old and need replacing. The Dewalt jig worked perfectly. Line up it up, clamp, and drill. It only took about 10 min. to drill and then mount the door knob. This tool is worth the money!
Now my wife want to replace more worn door knobs. I've already been back to Lowes to buy more......
