several times the 2 blade fan loosens on the water pump
 

I am at a loss. Maybe someone can help? I had a new water pump, leakless style, installed by a repair shop. It was mounted the old 2-blade fan. After about 100 miles, the fan was loose and the mount for the fan shaft key in the fan was defective. I then installed a new 2-blade fan and reinstalled the water pump itself with great care. After again about 100 miles the same problem. The cotter pin is still present in its position, but the fan again loose. What went wrong? Maybe the cone on the shaft does not fit? I am pretty sure that the fan shaft key did not slip during installation.

Re: several times the 2 blade fan loosens on the water pump
 

Re: several times the 2 blade fan loosens on the water pump
 

Re: several times the 2 blade fan loosens on the water pump
 

I would also caution you to make sure you are NOT using one of the original steel 2 blade fans. They are destined to fail and the results are catastrophic damage to radiators, hoods, and who knows what else! Get your hands on one of the cast aluminum 2 blade fans from one of the suppliers as you go about making your repairs on the water pump and you will be glad you did.

Re: several times the 2 blade fan loosens on the water pump
 

"I then installed a new 2-blade fan and reinstalled the water pump itself with great care."

Re: several times the 2 blade fan loosens on the water pump
 

After putting the new fan on the shaft did you whack it several times to seat the fan on the shaft. The instructions on the fan blade I got from Arizona A's had specific instructions about seating the fan to the shaft then installing the washer between the fan and castellated nut.

Re: several times the 2 blade fan loosens on the water pump
 

I've had a six-blade plastic fan for the past ten years. No troubles and no worries. But if the look is important to you, then I wouldn't go there.

Re: several times the 2 blade fan loosens on the water pump
 

I had the same problem very recently! I installed the two blade aluminum fan but I added a lock washer then placed the cotter pin. No trouble at all been on a handful of rides and all is well… Good luck friend

Re: several times the 2 blade fan loosens on the water pump
 

Have to make sure that the nut is actually tight against the blade and hasen't run out of threads? May need washer!

Re: several times the 2 blade fan loosens on the water pump
 

Do you happen to have one of the after market water pump shaft collars that keeps the shaft from moving forward and back? It mounts on the WP shaft between the WP housing and the fan. I had an experience where there was not enough room with the collar and the fan seated on the collar instead of the shaft. On a long trip, the impeller slipped on the shaft, that allowed the collar to slip and the fan cam loose. I removed the collar when I got home.

Re: several times the 2 blade fan loosens on the water pump
 

Your fan hub fits (or should) fit tightly on the tapered shaft.
The nut snugs it up on the taper but only when it can’t fit on any further. It sounds like you are pulling it up against an end float locking ring as mentioned.
Fit the fan only ,firm it up with the nut then check how much end play you have.
Your fan will always work loose unless it makes full contact on the taper.
You may also need a stepped washer behind the nut as said as you may be out of thread depth to enable the fan to fit far enough on the taper.