Making a double ended Allen key of different sizes.

[johnny2007]

What's wrong with having two separate allen keys?

I'm pretty sure Somalians mothers who don't know if they'll eat or drink on any given day have this exact same "problem"

 

[Tigercubrider]

Looking at the coupling though, it seems solid at the end, so nothing can connect?

No, the ones I have are drilled right through.
you might need to buy a selection and open the larger hole up yourself.

 

[Old Salt]

The couplings have holes in them to connect the motor to the shaft and are then held in place by grub screws. Lots of couplings of different sizes and materials are available. I just gave you one example.

 

[Old Salt]

Here is another example

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