[Note: this is #11 of a series of 20]
Don't let stuck cogs interfere with wheel rebuilds.
When rear wheels need new rims and/or spokes, stuck cogs (or freewheels or cassettes) can prevent introducing new spokes. Gears sit near the drive side hub flange and you can't get spokes in or out when they're in place.
With multi-speed hubs and coaster brakes, the solution is to disassemble the hub. Remove the axle and all guts. Out comes the driver with the cog. The empty hub shell can easily be laced with a new rim. Now, reassemble the hub and finish the build. Reassembled, the hub will rotate accurately and you can true your wheel.
Interfering cog removed.
For stuck fixed and freeewheel-type gears, the problem is solved another way. Best is to remove all gears while the previous wheel is still built. It's easy to remove cogs when the rim is still laced up and can be used to hold the hub still.
If, however, your rim has been removed too early, you can reattach it for the sake of cog removal. This requires a dummy rim and spokes for one side of your hub. The spokes must be "dog leg" shape rather than traditional elbows. "Dog leg" spokes are rare these days but any length will do. If you have none, bend some up for the occasion.
"Dog leg" end on the right.
Dog leg spokes can be inserted from the hub end because they have no enlarged head, just the "Z" shaped bend. Insert them into the flange nearest the stuck gear. Attach all of them to the rim with nipples. Twist the rim so all the spokes are in the cog removing direction. Voila, the cog or freewheel can be removed with standard tools.
Don't let stuck cogs or gears prevent your rebuild!