Had the replacement on my '02 640a for years.
It smoothed out the power and a seemed to have a few fewer vibes.
Coupled with the Comp muffler the BST40 pulls flawlessly from Zed to redline.
And runs great above the treeline.
It's how it wants to be.
'02 KTM 640 Adventure-lowered
"On the road there are no special cases."
Cormack McCarthy-The Crossing
The faster it goes the faster it breaks.
And high performance=high maintenance.