My summary...
For anyone who found this post like I did: looking for an answer to my 2016 Mazda 3 2nd gear grind...
i found this thread after searching the net for a solution to my 2nd gear grind. I spoke to the dealership about it at around 5k miles, telling them that every time I rev first gear above 4.5k rpm and shift to 2nd I get a terrible grind and it won't go into gear. Their answer was that I either don't know how to shift or I shouldn't rev it so high in first.
Both being worthless cop out answers. I'll spare you my driving credentials.
This epoxied rear motor mount mod has fixed that problem for me, but here are some things I found out in my experience that I didn't really find searching this thread:
1) I used loctite plastic binder epoxy. I can't speak for longevity yet, but this seems to have the highest psi rating and highest temperature and chemical resistance to any other cheap epoxy I could find. (If you google "loctite psi" you can look at all the different figures yourself in the tech data pdf's)
2) I got 3 different levels of stiffness/vibration balance with three different methods I used.
>First I filled the mount all the way up to the lip of the poly in the mount with epoxy. To the point that you couldn't see any black poly anymore. This was great to drive! No more grind issues at all! Rev the nuts off first, and slide it into second like a hot knife through butter... but terrible vibrations at idle, especially with AC on. This might be a good cheap alternative to a solid mount, but I have too many stop lights in my day for this.
>Second I dremeld out all the excess epoxy and left only the parts that filled the gaps in the poly. So basically like the Corksport inserts (that they don't make for 2014+ years.) this was the best compromise between vibration and stiffness. I only really got vibrations when the AC would cycle, all other times were fine. And still no grind issues. But it was still too harsh for my tastes.
>Finally I started watching the Corksport video they have for their RMM, and I realized that my shifting issues were probably caused by deformation of the mount in tension from the 1st gear torque. Filling the engine-side hole only strengthens against compression forces, and filling the frame-side hole strengthens against tension forces. So what I did was I hammered out the bit of epoxy in the engine-side hole and left the frame-side hole filled. Boom for real! No more 2nd gear grind, and I don't notice any difference from stock vibes. Just what I was looking for from this mod.
Sorry for the huge post, but if anyone else is going through this then 5 minutes of reading might save you a day or two of work.
(And I still have that solid piece of epoxy from the engine-side hole if anyone wants it. Lol beat it out with a hammer and flat-head and it still came out in one piece. This epoxy is no joke.)