ASK THE MXPERTS: ODE TO THE REARMOST NUT
The rearmost nut on my KTM 150SX shock linkage fell off. This is not the first time this has happened to me. I lost the exact same nut two years ago on my 350SXF. What am I doing wrong?
Obviously, you are not tightening the rearmost linkage nut enough. But, we sympathize with you because we have also lost the rearmost shock linkage nut often enough that now we carry a spare nut with us to every race. Luckily, we have never had the bolt itself walk out of the linkage when the nut falls off. Have you been using a torque wrench when you tighten the shock linkage nuts? They require 59 pound-foot of torque. We check this nut along with the head stay bolts, chain adjuster bolts, top shock mount and shift lever bolt as part of our pre-race checklist.
When a nut or bolt is properly tightened, the bolt itself is actually stretched to the point where its threads act like rigid springs that pull the mating surfaces of the bolt and nut together. This is why a torque wrench is important—because too much torque can cause the bolt to break, but not enough torque will allow the nut to spin. The size of the bolt/nut combo is the determining factor in how much torque should be applied. The KTM shock linkage bolts are 14mm in diameter. As a rule, larger nuts require more torque, but torque can be tricky. You would think that your bike’s 25mm rear axle would require more torque than your 14mm shock linkage bolts, but both use the same 59 pound-foot of torque.
Be forewarned, if you lubricate the bolt to make the nut spin on easier, it will break at 59 pound-foot of torque because you have reduced the coefficient of friction. As for Loctite, MXA has resisted using Loctite 243 on the linkage nuts because KTM does not recommend it, and Loctite is often just a crutch to avoid actually checking the nuts and bolts on your bike. Check your linkage bolt at regular intervals and carry a spare nut in your toolbox.