Torque-wrench + 8mm hex-bit socket

It may be too late, if the cranks are ridden loose just once, they get buggered and will end up wobbling no matter how tight you screw down the bolts. This wobbling continually backs out the bolt and you have to repeatedly re-tighten them. You may need a new crank soon...

I discovered this little gem of a BB from Suntour about 3 weeks ago. A matching crank set got sent to me from overseas so I set out to install it on my old XC mtb to test. Much to my annoyance, I discovered that my "early" BB spline tool which has a small bore, was being prevented from engaging by these @#&@! raised sleeves (green arrows). I had to buy a modern spline tool with a large inner bore that can go over those sleeves just for this one install.

My annoyance soon turned to delight however when I realized that these sleeves acted as "crank stops."

I greased the square tapers and the crank-bolt threads & flanges, and simply torqued away until the aluminum crankarms stopped and seated against these sleeves. At that point I gave both sides a decent final tightening and that was it. Usually a square-taper BB needs a few re-checks (being careful not to over-tighten) before it settles down. This one doesn't... it's done once and it stays that way for good!

The cartridge looks like the modern style construction seen on modern BB units with power-spline drives, but this one just has a square taper. I marveled at the fact that this little overdue innovation was about 25 years too late!

Perhaps you are over torquing the bolt? That's something else you want to avoid. Also, you would get a #8 hex fitting for a socket wrench, and put that on a torque wrench instead. Alternatively, torque something else to that torque and compare feel.

Not a #8, but an 8mm is the correct size for most bb bolts.

I'm aware of that. However, the OP used that nomenclature earlier on so I followed along as I felt it would make things easier for him/her. A #8 is just over 5mm (0.199 in)and would clearly not turn an 8mm hex.