In this case I have need for one gear with a 1/4" o.d. attached face to face to a 5/8" od spur gear.
You don't measure gears in o.d., but in pitch and number of teeth
Take a look at this set
. It may be expired, but I have the precise same set from a local hobby shop and Googling "EXPO A26220" should find some.
From the eBay page (which don't give pitch, but this can be calculated from the data they give):
EXPO A26220 - 9 PIECE NYLON GEAR SET
WITH 2mm HOLES
CONTAINS 1 x EACH GEARS
9mm Outer Diameter with 16 Teeth
16mm Outer Diameter with 30 Teeth
22mm Outer Diameter with 42 Teeth
31mm Outer Diameter with 60 Teeth16mm Outer Diameter with 30 Teeth & 10 Inner Teeth
22mm Outer Diameter with 43 Teeth & 10 Inner Teeth
6mm Diameter Worm 6mm Long
6mm Diameter Worm 10mm Long
2mm Steel Shaft 75mm Long
I've considered trying to drill the center hole of the 5/8" o.d. gear just a few thousandths short of 1/4" and press fit the 1/4" gear into the center. Not sure if that would work. The last detail is the shaft size is .080"
That sounds like a bad idea.
Much better to keep both at an the same axle and then lock them together in some way (bolts, heated needles or whatever).
I've looked a little bit and it seems to be a tough time finding those gears. The 5/8" gear would need to be 1/8" thick and the 1/4" diameter gear would need to be long enough to be able to pass through the 1/8" thick 5/8" gear and still have a good 3/8" beyond that for the motor's worm gear to mesh with.
Check out the EXPO set.