KTM 640 Adventure fork revalving

The 640 Adventure 2003-2007 all use the same 48mm WP Open Chamber fork. The fork is also mounted to SX/EXC bikes from the same era and 660 Rally bikes. The fork uses a 14mm damping rod that was used on SX/EXC until 2006. Fork length is 910mm with a 275 suspension travel, this is lowered compared to the SX/EXC that is 940mm/300mm.

Rally Tuareg 2009 or 2010

Making the OEM forks taller:
For serious offroad riding getting the full 300mm suspension travel would make a difference. It should however be combined with removing the lowering spacers in the rear shock to make the lift symmetrical.(separate blog about that here)
For increasing the fork travel you need:
2x 48600409 PISTON ROD D=14 L=551
2x 48600092 ADJUSTMENT TUBE L=517MM

Swapping forks:
SX/EXC fork fits right into the triple trees and already have the sweet 300mm fork travel. For bikes with dual discs you however need to take the complete EXC front end with you as brakes and wheel axle is different. 300mm brake discs and brake calipers wont fit EXC forks.

Springs:
The 640 ADV is severely undersprung from the factory, the original 4.6N/mm spring is plainly stupid for such a heavy bike even before you load it up with 25L of fuel. For comparison a 525 EXC uses 4.4N/mm and the 660 Rally 4,8N/mm. If your weight is around the 75kg mark 4.8N/mm is ok and for 100kg rider a 5.2N/mm is ballpark right.
Since the fork is lower then the rest of OC forks the springs are supposed to be slightly shorter 485mm compared to 505mm for the standard forks. However it is way easier to find 505mm springs both new and second hand, get the longer ones, scrap some plastic spacers and have some extra preload.

2nd place in  Swedish 4-days enduro 2010

Valving:
Again KTM/WP made the suspension valving much more street oriented then what most people actually use the bike for. The shim stack is softer the the much lighter 450EXC. Most people experience excessive brake dive and when riding offroad at a faster pace the forks blow through the stroke bottoming severely.

So i have compiled a specifications sheet with more offroad oriented fork valvings:
WP OC valving sheet

640ADV by boris is the stack i prefer to use on the bike. All shims added compared to the original stack is marked in green. In the base it uses an extra face shim, a crossover and a single 22x.10 shim. The bigger difference is in the midvalve where an extra 0,15 face shim is added and flexible backers used for closing up the MV float. This transforms the forks to something much more offroad oriented.

660RFR is the front stack from the factory rally bike. I have one of those as well and the fork is just amazing if you are fast enough. For regular use it is too stiff and non compliant on small stuff when not going at race pace.

640ADV soft is the stack for those who use the ADV for more enduro style riding. The valving will be too soft for fast pistes of Morocco but works well for rocks and ruts in 2nd and 3rd gear. Even if the base valve is really soft the decreased MV float makes the bike stay up much better in the stroke so the excessive brake dive is gone. This stack is inspired by stacks from KTMLew@ktmtalk.com.

Happy fiddling with your forks!