dugay

I trying to set up ultegra shifter ST-6510 for double crank set but the shifter has like 4 clicks and I dont know what positions I should use for double.

I disconnect cable and used shifter from low. 1 full sweep of the lever gave 3 clicks and next sweep 1 click
does this mean there are 2 trim positions between inner and middle ring and that is where I should use double pls help I dun wanna break it

dsbrantjr

When using a triple shifter for a double you have to choose locking out the high position or the low one. I prefer the latter since if you forget and shift into the low position the shift wire merely goes slack while the derailleur stays in the small ring. If you lock out the high position there is the possibility to jam the shifter ratchet by attempting to shift against the high limit stop; unjamming it may be difficult and there is the possibility of damaging the mechanism.

Retro Grouch 

I don't know for sure about that particular part number but I used to have a double chain ring road bike and a triple chain ring tandem which used identical Ultegra 9-speed brifters.

Like yours, my brifters had 4 shift positions. On the double, I had two shift positions for each chain ring. On the triple I had one shift position for the granny, two for the middle ring, and one for the big ring. I also had flight Deck computers, that indicated the chain ring in use, on both bikes. The Flight Decks always indicated the correct chain ring so I'm sure my shifters were set up the way that Shimano intended.

HillRider

+1 Shimano 105 and Ultegra 9-speed brifters were both double and triple compatible. Their 8-speed and later 10-speed models were double or triple specific. IIRC, the difference between the 6500 and 6510 brifters was the 6510 corrected an annoying faceplate rattle with most 6500s.

Jamminatrix

As HillRider said above, Shimano designed the shifter for both double and triple by putting a trim option on the middle ring position.

Install the derailleur as you normally would, ignoring the final two clicks. The derailleur limit screw will prevent you from accessing them.