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Robert B
Robert B, General Contractor
Category: Home Improvement
Satisfied Customers: 12
Experience:  20 years in commercial general contracting plus numerous home improvement projects
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I just noticed that I have a crack in my chimney masonary thimble.

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I just noticed that I have a crack in my chimney masonary thimble. It is an 8" thimble, but my stove has 6" outlet. Can I insert some sort of smaller thimble/pipe as a substitute? Can they be repaired without tearing out the old thimble. Is there any type of caulking type material that can be put into the crack? Sources of such materials?
Hmmm, this is a tough one. I presume the thimble connects to a metal chimney that goes up and out and that you are concerned about smoke or a spark getting into the masonry chimney. Do I have this right?
Customer: replied 8 years ago.
It is a masonry/ceramic thimble that goes through a garage wall to a masonry chimney. The crack is where it goes through the garage inside wall into the outside chimney (of block with ceramic liner). The Thimble has an 8.125" inside diameter and is about 12" long. I really think that I ought to be able to insert a smaller thimble within the larger thimble, but I don't know. THe stove I use has an 6" outlet, so there is no practical reason I can think of that it must be 8" at the chimney. For the stove hookup, I use 6" all the way then had a 6" to 8" adapter that slid into the end of the thimble.
Customer: replied 8 years ago.

I agree. My only concern is if the crack opens into the garage wall cavity you could have a problem. Is there a way to make a tight mechanical connection between your new thimble and the masonry chimney? You don't want any backdraft to cause a fire in your wall.


One idea would be to seal the crack with Dap High Heat Mortar caulk #18854 available at After doing this you may or may decide that it's unnecessary to install the new thimble.


Good luck with this and let me know if I can be of more help or if this didn't answer your question.


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