For a 1.5" bulk head you need a 2 3/8" hole. Your 50 mm is a 1/16" th smaller than 2". So you need an additional 1/2". Now what I have done is I have a 1 1/2" bit and I have done 2" holes no problem. I drill out the 1 1/2" and then as you see the diamond bit will have a rough diamond edge on the outside of the bit. I use this to slowly (by going in a circular fashion) to ream out the hole larger. I keep the bit perfectly straight so as not to bind and slowly grind out the sides. You will be surprised how easy it is, just make sure you always have cold water pouring on it. I set it in my back yard and fasten a running garden hose to it. Stop once in a while and check your bulk head to it. Just remember to ream out all the way around the hole so you grind all sides evenly. Any questions feel free to ask.
Kevin
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