For an M3 size nut you need a 6mm wrench size, or a 6mm hex driver, but be careful that the socket driver isn't too shallow for the screw you're putting it on to. I would recommend a small set of metric wrenches for regular use like this and drivers in specific applications only.
The nut is made of steel, sometimes ***r quality and ***r quality control. I've had them with the shaft about 10pcnt off center or some with no thread. The top edge is crimped over a nylon washer which locks the nut after it's mated with the bolt. Great if worried about vibration loosening them.
Certainly not Aluminium - just polished steel, not high quality steel, but will still hold anough newtons of force to be safe for whatever you are trying to hold together with such a small nut and bolt...i.e if you really want to be sure, use something bigger anyway!
It's Very good Hex locknuts. It's enough quality. I am satisfied this nuts.
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Nice quality item. Fits perfectly M3 screws and locks them with no problems. With this locknuts you do not have to use a double nut anymore. M3 size is difficult to find in hardware stores but as HC is stocking it I will be buying some more.