I want to use a canister muffler on my DLE-85 and I can get a "flex" header and canister from popular suppliers.

But since the "flex" header has ridges in it, how much flex, or more precisely, BEND, can I put into it without damage?

tI has been my experience that you can bend / flex it, till you can not flex it anymore. Once it stops flexing, my personal feeling is that it is at its breaking point.

all of the flex headers I had (two) had broke where the flex joint meets the solid header pipe.
I now just buy the header kits and braze my own up to fit where I want it.

Thats the same place mine broke.

I wonder if that break could be silver soldered?????

Yes you can silver braze the joint after a thorough cleaning; sand blasting the area would be best.

But let me suggest that you use a coupler. I make all of my own couplers for gas applications (all piped) as follows:
I use ceramic fabric from Mc Master Carr, usually a braided tube. But regular silica fabric works fine; just cut it on bias.
I add Permatex Ultra Copper hi temp sealing compound and rub it into the fabric, inside and out, until saturated and fully sealed.
For the size engines I run, up to 55cc, 1 layer is enough plus a layer of glass on the outside. But for an 85cc you should add a second layer on top of the first and the add a layer of 3 oz glass cloth, again bias cut, saturated in Ultra Copper also and wrapped around the braided tubes.

Then you may simply add the wet lay-up directly to the pipe stubs and let it cure for 48 hours. To hold things neat and tidy, wrap the assembly neatly with some plastic tape (I use teflon tape but that's not necessary). After the assy cures, remove the tape. Wrap a couple plastic tie wraps on each end and you should be good to go.

Side note regarding these style headers: The convoluted headers are made from thin stainless steel sheet, maybe 0.025", bump convoluted to produce the flex and that's it. These are never heat treated to remove stress from the convoluting process and hence don't last more than a couple dozen runs. Adding the coupler I developed and discussed above, eliminates much of the harmonics that cause the damage and actually really make the assembly flexible and forgiving. Try it!

If you like it, send me royalties of 10%. Seriously, it's simple and most effective way to solve this problem. Been doing it for years and my original couplers are still in use today, some 120 hours of operation later. You see the Ultra Copper inner layer simply blows away and exposes a ceramic inner layer impervious to the exhaust of a gas engine. And i am well aware that some enterprising whipper snapper would want to patent the idea. Don't bother.

BTW- DA sells flex couplers that are wrapped with Teflon unsintered tape at the flex joint. If anyone has one of this style and have found leaks through the teflon tape, cut it all off and treat the flex joint with my ceramic treatment. It works great!

If you need any further help, contact me direct at my email addy