Coming in late (sorry) but since it's not my bike, I advise to "just do it!"
I was just talking to a framebuilder (who I won't name because this may have been meant for my ears only) who says he has often crimped titanium chainstays for more tire clearance on built frames. Some of you may know, 3/2.5 tubing was developed for hydraulic lines in aircraft. Something about the pilgering process that reduces the diameter and wall thickness from the pierced billet, combined with Ti's hexagonal-close-packed crystalline structure (cf. steel's cubic structure) gives it anisotropic properties. Great for bending, like to run hydraulic tubes thru a wing, but terrible to try to make fluted chainstays. Therefore a lot of builders that want fluted stays get their chainstays in the annealed state, much weaker than the more-common CWSR (cold-worked, stress-relieved).
Anyway, long digression, hope you don't mind (I just think it's interesting) but now I'm finally getting to the relevant part: On CWSR stays, that you can't crimp without them cracking, this builder I was talking to just crimps them, lets them crack, then welds the crack shut! Says he's done that years ago and those bikes never had a problem afterward.
I think that might be pretty analogous to crimping 853. If it cracks, no prob, just braze the crack with brass ("bronze"), which
I think will be strong enough, and should simultaneously stress-relieve and probably get some strength back from the air-hardening.
Obviously, "I think" isn't very convincing, and it's not my bike so I don't expect you to take all that risk on my say-so. Just sayin', food for thought?
I like to crimp with a fairly sharp radius at the bottom of the indent. I think this is actually better (stronger or better fatigue life) than some indents I see out there with a large radius at the bottom of the indent, typical on many Chinese made bikes I've noticed like Rawland, Crust et al. I won't go into why I think that's preferable, not at length anyway, but suffice it to say this shape has been well-tested for many decades. I have seen Barra aluminum frames from before WWII with that sharp crease down the middle of the indent. Here's a pic.
Barra frames do break sometimes, but not there, down in the bottom of the chainstay indent. Or think of the sort of leaf-shaped indents in older Columbus SL/SP c-stays, before ROR became fashionable. Let me know if you don't know what I mean and I can come up with some photos. If ever one of those cracked down in the bottom of the indent, it's news to me.
Oh
here are some pics of a chainstay denting tool I made a couple years ago. Not very sophisticated -- the one @
Andrew R Stewart showed off earlier in the thread is much nicer. But I think it at least shows what I mean about the crease down the middle.
Anyway, if it cracks there while indenting, you should be able to float some brass down the crease, which will look very normal after painting -- it won't look like a repair.
Mark B in Seattle