Post by honestjohn on Apr 10, 2009 7:24:12 GMT -5
As every rider of the thumb-start Ducati knows, the weak point in the starting system is the sprag/ freewheel bearing. This is most annoying, and is a part that usually fails at regular intervals. Also it is relatively expensive to replace.
After years of replacing these items, I have (finally) not only found the dark science of why they fail, but a method of repairing the non functioning unit. The best bit is that you don’t actually have to buy anything to fix it.
The sprag is a one way bearing, when the starter is engaged, the bean shaped nodules all stand up and grip to the (crank) shaft, turning the engine, once the engine fires, the beans all sit down and ‘free wheel’ around the shaft.
The main cause of failure seems to be that the spring looses tension, so the beans cannot get purchase on the crank, and the bearing just spins. So by restoring spring tension, the beans will do what they were originally intended to do. Here’s how it is done.
With the bearing removed…
Carefully remove the perimeter spring.
Unscrew the spring apart, (don't try to pull the ends apart or you will loose the tension). You will find there is a tapered end (male), and a thick end (female).
Cut about 20mm off the thick end (female). Don’t cut the tapered end (male) or you will have nothing to screw back into with.
Wind some backward tension into the spring and screw the two ends back together, then simply refit it around the bearing.
Replace the unit into the engine and enjoy the pleasure of the bike once again starting using only pressure on the starter button.
You should be able to do this 3 or 4 times until the spring is too flogged out to re-use.
After years of replacing these items, I have (finally) not only found the dark science of why they fail, but a method of repairing the non functioning unit. The best bit is that you don’t actually have to buy anything to fix it.
The sprag is a one way bearing, when the starter is engaged, the bean shaped nodules all stand up and grip to the (crank) shaft, turning the engine, once the engine fires, the beans all sit down and ‘free wheel’ around the shaft.
The main cause of failure seems to be that the spring looses tension, so the beans cannot get purchase on the crank, and the bearing just spins. So by restoring spring tension, the beans will do what they were originally intended to do. Here’s how it is done.
With the bearing removed…
Carefully remove the perimeter spring.
Unscrew the spring apart, (don't try to pull the ends apart or you will loose the tension). You will find there is a tapered end (male), and a thick end (female).
Cut about 20mm off the thick end (female). Don’t cut the tapered end (male) or you will have nothing to screw back into with.
Wind some backward tension into the spring and screw the two ends back together, then simply refit it around the bearing.
Replace the unit into the engine and enjoy the pleasure of the bike once again starting using only pressure on the starter button.
You should be able to do this 3 or 4 times until the spring is too flogged out to re-use.