New Thimble in Chimney

I have a lined 8 " round flue, and need to cut in for a new thimble for new efm boiler, has any one had any luck drilling and fitting new 8 inch round thimble into chimney,i am thinking that I can drill with diamond hole saw and trim with diamond blade on 4 inch grinder then cement with refractory cement.. I built the chimney 17 years ago...My present wood coal unit is on the other side, I will plug that hole when this unit is up running
Thanks for any input advice, The end of project is very near,,the boiler is full of water and only a couple of zones left to plumb and then electrical till I turn into a girlyman
Ted

Hi Ted that is exactly how I cut my opening for my thimble/flue pipe. Just take your time, and have a vacuum nearby to clean up the masonry dust. Wear eye protection and a mask.. the dust bothers me..

Greg L

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You could just drill a bunch of small holes (1/2") in a circle and punch it out. Then just cement the ends in place. Two things, make sure you pitch the thimble up (1/4" per foot or so) and that it doesn't extend into the chimney.

It appears you have two flues, with one being used for the hot water heater. ? Anything else?

Could you do some rearrangement, so that you don't need to punch in a new thimble?