Pentair Whisperflo. I need to remove the impeller but the set screw is not a screw of any kind. It's just a flat piece of metal. Every manual says it should be a phillips head screw. It's not a screw. Model is W-24 1 hp.

Take the Diffuser off and run it if the impeller turns then might be tightening the Diffuser too much, it just needs to not move, doesnt really need a super waterproof seal or anything like that. If it still doesnt turn with the Diffuser off it might have locked up when replacing the Impeller, you can try to turn the impeller manually or remove the cover plate on the back of the moter and turn the shaft manually back there. OOoorr you broke it

The impeller has a reverse thread locking screw through the center. You can secure the motor shaft by putting a 9/16 open end wrench on the other end of the shaft, which is under a cap in the middle of the rear of the motor.
Check your seal plate carefully. Whisperflo is a great pump but they don't take heat very well. It is very common for the seal plate to also be damaged when the pump runs hot, particularly the lower inserts where the bolts screw into.

Separate the plumbing portion of the pump from the seal plate which is attached to the motor. The bolts will be a 9/16 inch hex head, that are visible facing the motor. Remove those bolts. That will then allow you to pull the motor and the seal plate away from the plumbing portion of the pump. Remove the diffuser which is covering the impeller, and then the impeller will need to be unthreaded off of the motors shaft. The motors shaft, will need to be locked. You can do this by removing the Cap on the backside of the motor and you can use whatever tool required to keep the shaft from spinning. Once you are able to lock the shaft, you can read the impeller... Note* some styles will have a screw in the middle of the impeller securing it to the motors shaft? This screw is Backwards thread, not normal thread. If your pump has this screw, need to remove it first, then you will be able to unthread the impeller off of the motors shaft.
once the impeller is removed, you can then remove the two halves of the shaft seal. Very carefully reinsert the new shaft seal into the proper places, just the way they were prior to you removing them.

Reassemble in backwards order as you disassembled everything.
good luck

Remember to remove the impeller locking screw (left hand thread) before attempting to unscrew impeller.

Yes the impeller is screwed on right handed normal thread. *** Info below pasted form one of my other posts *** You need to lock the motor and unscrew impeller. Depending on the motor, you remove a cap in the middle of the tail piece and hold with a wrench, (usually 9/16") or remove a cap that covers the whole end and grab the shaft with a wrench (usually 7/16") by sliding it under one of the components (not sure exact part off top of my head have to look at it) and then turning impeller. If the impeller has a bolt or screw in the middle remove that first. (you already did this.) It should be reverse thread so make sure you don't turn the wrong way and break or strip. If there is no screw, or after you've removed it, just spin the impeller off (it should be counter clockwise standard thread). If you can't do it by hand and assuming you don't have an impeller wrench, be very careful and use a large pliers, pipe wrench, or strap wrench to loosen it.

Disconnect all power. Remove the six bolts holding the seal plate to the wet end. Remove the two small allen screws to remove the diffuser. Remove the dust cap from the end of the motor and secure the shaft from moving. If installed, remove the Phillips head screw in the center of the impeller by turning clockwise (left hand threads). You can now remove the impeller by turning counter clockwise, note that you may need an impeller wrench to remove it and a specific tool to hold the shaft while you remove the impeller. Once the impeller is off you can remove the four bolts that hold the motor to the seal plate. This process just needs to be reversed to re-install but you will need a new shaft seal (PS-1000).

Remove the bolts that hold the 'wet end' of the pump to the 'dry end'. If the impeller is held on with a screw, it is a 'left-hand' tighten which means it turns the opposite way of normal screws. Be careful not to damage the impeller. Any scoring on it will effect its performance and it will only be worth doing if everything goes back together cleanly and properly. Replace and lube all O-rings with silicone lube. Make sure all bolts are tightened and do them in an alternating fashion to ensure that you get a good seal.
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Stick a large screw driver on the motor in the area of the bearings and on the pump housing ... stick the handle to your ear (notice I said to your ear ... not IN your ear) Listen. Often you can determine the source of the noise in this manner.

My money is on the impeller. You will need to replace the seals when you disassemble the pump motor from the pump housing. I found a dime in the impeller recently ...it made a racket until the impeller wore down in that area.

Remove back cover from motor. Hold shaft stationary with wrench or screwdriver(depends on motor) Unscrew impeller. Some impellers have a screw in the middle securing it to shaft. The screw is reverse thread.