Three-Shoe Automatic Clutch Information
How to setup clutch and how to do regular Maintenance
The proper clutch is so important on all 50cc motocross race bikes that have the standard three shoe clutch. Cobra King 50cc, Cobra CM50cc, Polini X1, X3 LEM R2, R3 or KTM50 Pro Jr, KTM 50 Pro Sr.All use the same type of clutch design. YES these bikes are very different, but the clutch design and theory are all the same. Our specs, measurements and setups are used for the KTM 50 L/C engine. However owners of other brands of bike may find some of these tips helpful.
Looking to put “maximum power to the ground” then, clutch adjusting procedures on your KTM 50s is maintenance and setup. In proper working order with clutch shoe engagement adjusted to optimum RPM is your key to the best performance possible.
Is your bike purchased used? Is your bike over six months old?
Bikes with a clutch that's worn out or not adjusted correctly will make the bike feel slow, sluggish, bogg or give it a slipping feel. If your bike is not brand new then think about replacing the spring washers, clutch shoes and possibly the clutch basket with new parts. This will give you the best performance possible because the clutch tolerances are very important to maintain. Also improperly maintained clutch or worn clutch could confuse people in thinking that the need different size sprockets, carb jets or it may have bad ignition system but most of the time its due to a clutch not engaging correctly maybe too-early or too-low in the working RPM range.
The three shoe clutch design seems to remain a mystery to people who are afraid to maintain, clean, adjust or inspect them. With knowing a few basics these clutches will not be so confusing or frustrating.
Clutch Basket Measurement
The measurement of the inside diameter of clutch drum/basket should be approx., 84mm new. Difference between drum/basket and shoe assembly should be no less than 1.5mm, no more than 2.0mm.
Remove and measure the diameter of three assembled clutch shoes (approx., 82mm new).
Clutch Spring Washer Kit
$29.99 – $39.99
Stainless Spring Washers
Stainless steel spring washers are considered a performance part, because they are not AMA stock class legal. Many people use these stainless washers because they are much stronger then the normal carbon spring washers. They also resist fading less then standard carbon spring washers when the clutch heats up. Keep in mind that a these stainless spring washers will use a different stack compared to the standard steel spring washers. USA made STAINLESS STEEL Belleville spring washers.
When you reassemble spring washers, use Loctite "Thread Locker 243" and torque to 96 lbs/inch - 11 Nm
Martin & Slater uses Belleville spring washers that are USA made and have design standards based on DIN 2092 specifications and are manufactured to DIN 2093 specifications. This quality supplier is a USA OEM supplier of spring washers since the late 1960s. USA made carbon STEEL Belleville spring washers
Morini Clutch Puller
Clutch & Flywheel Puller
Use a clutch puller tool and piston stop tool for clutch removal. Reinstall drum/basket and clutch shoes onto crankshaft and tighten clutch assembly nut to 22 lbs/ft - 30 Nm. WARNING: Do not use a AIR impact wrench or exceed maximum torque specs or center hub will fracture or you could damage the crankshaft.
Loctite Thread Locker 243
Piston Stop Tool
The following is some basic advice received by a very experienced racer dad that we would like to share with you…. If you have any tips or ideas please email us so we can post them here for everyone to see.
There are several things to consider when setting initial stack set for these KTM 3-shoe clutches.
First and foremost: Skill and ability of the rider.
Second: Type of terrain, “motocross” hare scambles” “flat track” or recreational trail riding.
Third: How the bike is presently geared. You have to treat gearing and clutch set up independently. Example: If you were to make a rear sprocket change to increase top end speed, (Less teeth on rear sprocket) then you loose some bottom end power. Now with the clutch set up you can change your power curve to compensate.
My advise, buy a good dial/electronic caliper. This is the most important tool in your toolbox. Next; for your initial stack setup go conservative. I recommend the following as a base line setup for a beginner rider on flat terrain…..
Legend for clutch washers: (HH) = head of clutch block bolt.
( = Bevel spring washer –notice direction.
/ = Flat washer in various thinknesses so all three of your stacks are within 0.003” of each other.
Beginner setup is as follows. HH (( )) (( )) (( )) ( ) /
- add flat washer if needed to bring stack in tolerance. = 0.585” – 0.590”
Alllow your child to ride the bike. Is he/she blipping throttle? If so allow child to ride bike longer to see if child can adapt.
If your child is comfortable and riding more aggressive, this is my preferred stack……
HH (( )) (( )) (( )) ((( / = 0.610 This setup is more aggressive. The flat washer can be taken away or made thinner or made thicker if needed for fine adjustment.
Again flat washer in stack up to adjust overall engagement speed.
Some basic pack thicknesses.
Motocross: anywhere from 0.590” – 0.670”
Hare Scrambles: anywhere from 0.590” – 0.640”
Recreational: anywhere from 0.580” – 0.610”
Remember change oil after every hard practice seesion and after every race. At motocross races change between each moto.
Use only Maxima MTL 75 wt Ultra light trans oil.
Clean & inspect spring washers.
The most important advice I can give is; Don't be afraid to experiment. Keep a notebook and record your findings. Some stacks work better then others at different tracks, conditions, weather, sand, hard pack, mud, hills long starts etc…
Good luck! – Tex by Ken Crews, from Crews racing
Got some ideas about KTM50 clutch info or set up you want to share with us? Please send us a email [email protected]