"dpb" wrote in message

Virtually any RTV silicone is good to well over 200F.

I don't think there's any chance at all the temperatures at that point
will be a problem for any high quality silicone caulk.

How deep is this crack and is there anything backing it to help fill the
void?

It's not a crack, but a 1/4" (or so) gap, the depth of the crown, left by
the mason, between the flue and the crown. When the cap mortar sets, I want
to fill that space with some flexible sealant.

Some people recommend using this method to prevent crown from cracking.
Sounds good, but now I'm having problems finding something easily available
to use for that purpose. DEFY product is expensive and sold 1gal
quantities.

Regards,

RichK