Getting ready to put a new tire on the rear of my 93 softail springer. Stock solid wheel, stock everything. What is the widest tire i can stick in there without changing anything, except maybe trimming the belt guard? Stock is mt90b16 so im hoping i can cram a 140/90/16 in there.

 

You can.
FWIW that is a mu90b16.

 

I installed a 140/90 in my 1990 Glide, but had to trim the rubber curtain off the belt guard. Installed width of tyres varies depending on brand, so actual clearance is not predictable, hence you may not be able to get away "without changing anything".

I also think you will find that the MU is 140/85, not /90 profile. That makes a slight difference as an /85 is similar in rolling diameter to a stock MT, whereas a /90 is slightly larger in diameter. And yes, I could tell the difference in ride height with my /90!

 

I was going to put a 150 on my Fatboy but traded it in....I think it will fit with a pulley spacer....the "MU" looks quite a bit bigger than a 130 even tho it's only 1 size up...I put a 170 under my old softail but had to buy a 1" pulley an belt...it was a lot of extra work but I liked the look.