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I tried using a pulley puller with moderate force to remove the tachometer but it didn’t seem so budge (although I didn’t use heat to soften potential adhesive inside).

Looking at the shaft I can see two slots similar to what I have seen in threaded optical tubes with filters in them. Is there an analogous tool used to thread or unthread tachometers onto shafts? I don’t want to be trying to pull something off that is threaded on. Hopefully this looks familiar to someone and they can advise.

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UPDATE: I have tried pulling harder using a pulley puller and a cast aluminum disc that I specially machined for this. The disc buckled and broke leading me to believe the tach rotor is attached pretty tightly and possibly is indeed threaded onto the shaft.

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    $\begingroup$ square pin spanner wrench amazon.com/Orion-Motor-Tech-Adjustable-Spanner/dp/B09QPNP762/… $\endgroup$
    – Phil Sweet
    Commented Jun 8, 2023 at 0:24
  • $\begingroup$ Ah I see now how that type of pin spanner wrench could work. A pin would hook into one slot and then a detent does the same on the other side! $\endgroup$
    – user391339
    Commented Jun 8, 2023 at 2:43
  • $\begingroup$ It is a tachometer rotor I have removed the brushes but I need to replace it with an encoder. Will update with details about the attachment once I get it off. $\endgroup$
    – user391339
    Commented Jun 8, 2023 at 2:44
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    $\begingroup$ Used to make those on a grinder using an old socket... $\endgroup$
    – Solar Mike
    Commented Jun 8, 2023 at 5:43
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    $\begingroup$ Search for "pin wrench" which is what Mike is describing. "Wrench" isn't really the right term - a "socket" seems more correct. @PhilSweet, the hook wrench you've linked is for nuts with the slots on the side like this. It's not suitable for this job. $\endgroup$
    – Transistor
    Commented Jun 8, 2023 at 20:46

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I saw those notches and thought of this wrench I used years ago to remove the sprocket from a bicycle wheel. Bicycle Sprocket Wrench
I'm not sure something of that design would work in this case because you don't have enough the plane where the ears would fit into those notches to allow turning it. One idea is to make a tool like this by grinding down (as per @SolarMike) an old socket and bolt the result into the tachometer to keep it in place while unscrewing. I don't think you will be successful removing this thing unless you turn both notches at the same time.

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