To keep the answer at a high level here is a summary type answer to your question.
Grade 8 bolts have a UTS of 150ksi, grade 5 120ksi ( assuming 1" or less bolt diameter). The plain carbon alloy required for both of these has a carbon content of 0.28 to 0.55% placing these bolts into a medium carbon type steel. If these are high strength stainless steel bolts they are likely a 400 series martensitic alloy. Whie the composition is different, the following discussion still applies.
First you would need an alloy appropriate filler rod. That's a big subject.
Then you have the problem of the HAZ - heat affected zone. The HAZ will contain a region of untempered martensite that is hard and quite brittle. Unless you heat treat this, you will find your welded bolt will snap off quite easily right next to your well.
In order to make a high strength bolt even close to it's original design requirements would require a full heat treatment for the entire bolt. Simply not a practical option in this case.
dj
Thank you for the eloquent & well supported answer to my question. Your concerns are all valid. They are also issues that I was previously familiar with. If I were to only rely on my “book learning” & I did not also have a fair base of real world experience welding motor mounts & suspension components, I would have agreed with your opinion & considered it to be gospel.
In the case under discussion, I suspect that welding the broken parts in their current (out of line) positions will result in more evenly distributed stress loads across the entire set of fasteners. I have found from experience that tapering the fillet allows pretty close to full structural strength when using soft (60ksi) rods on higher class bolts. I once welded a grade 8 bolt to a socket extension, to make a temporary tool to get me out of a bind. The tool ended up being far stronger than I had expected possible. As you suggested, I expected the edge of the weld to snap off like the bolt ears on a 30-40 Krag. I would later use cheater bars that were more than 20 diameters long to put heavy stress on my temporary tool. I was never able to break that tool after more than 15 years of trying. I never expected that.
I agree that filler rod selection is a fairly deep subject if you want to cover all the bases. I will just stick with a Cliff’s Notes type response here. 6011ish rods can get you what you need, in or out of position, in most cases when welding bolts if you taper your filets & watch you cherry (majority of the heat affected area). Even on stainless bolts, you can get good structural strength out of mild steel rods, but of course you don’t get any of the corrosion resistance, so that would be a weld of limited lifespan. When welding stainless bolts of unknown alloy, I have had good luck running 308 hi sil wire, when I wanted to keep the corrosion resistance intact.
I also agree that welding with the correct alloy filler rod, followed by full heat treat (you might want to use a little x-ray inspection in between) would be needed to gain 100% full certified performance from a repaired fastener like this, but I have field tested a lot of welded bolts that lasted me for a lot of years, back in the dark days when buying new parts was not in my budget.
Of course, replacing all motor mounts & properly aligning them is the best option. There is no doubt in my mind about that. Having the right guy hit the broken parts with a 6011 stick is a move that would likely buy you a season or two with just a few minutes of time invested to make it happen. It’s not a Bristol move, by any stretch of the imagination. Having the wrong guy hit that with a stick would likely result in a quickly broken weld, as you would seem to expect.