AN MD88 CREW DEPARTING LGA RWY 31 TURNED TO HDG 360° INSTEAD OF HDG 340° BECAUSE OF A DISTRACTION WITH THE ACFT'S UNUSUAL TKOF CONFIGURATION.

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THE CAPT AND I WERE ON DAY 3 OF A 4 DAY TRIP ON THE MD88. IT WAS LEG 3 OF THE DAY FLT. WE WERE VERY HVY FOR TKOF AND THEREFORE WE WERE DOING A NORMAL PWR FLAPS 24 TKOF ON RWY 31 AT LGA. ON THE MD88 THIS CONFIGN IS NOT THE NORM. WE ARE USUALLY AT FLAPS 11. WE DISCUSSED SEVERAL TIMES THE TKOF AND CLEAN UP IN THE RESPECT OF WHICH IT WOULD BE DIFFERENT FROM WHAT WE NORMALLY DID. WHEN WE THEN RECEIVED OUR CLRNC FROM CLRNC DELIVERY WE WERE GIVEN A R TURN HDG 340 DEGS ON DEP. WE KEPT DISCUSSING THIS AND FOR SOME REASON WE WERE BOTH CONVINCED THIS WOULD BE A BIG TURN AND WE HAD TO REMEMBER TO TURN R. ON TAXI OUT WE DISCUSSED TKOF AGAIN AND SAID WE HAD TO BE CAREFUL TURNING ALL THE WAY TO 340 DEGS. ON THE MD88 IF YOU TURN THE HDG SELECTOR MORE THAN 180 DEGS IT WILL START THE TURN THE OTHER DIRECTION. SO THE CAPT STATED; WE DON'T WANT THE ACFT TO TURN L SO LETS PUT IN 090 DEGS IN THE HDG SELECTOR AND WE'LL TURN IT FROM THERE ONCE IN THE AIR. SO WE CHANGED IT FROM 340 TO 090 DEGS. FOR SOME REASON IT DID NOT OCCUR TO EITHER OF US THAT WAS NOT GOING TO WORK. WE ONLY NEEDED TO TURN FROM 310 DEGS TO 340 DEGS. SO WE GOT TO THE RWY AND TOOK OFF. AT 400 FT I ASKED FOR HDG SELECT AND I STARTED THE R TURN. AS THE CAPT CALLED DEP HE SAID WE WERE TURNING 340 DEGS. AS WE GOT TO APPROX 360 DEGS IT OCCURRED TO BOTH OF US WHAT WE HAD DONE WRONG. I TURNED BACK TO 340 DEGS AND WE CLEANED UP THE ACFT. ATC ASKED WHAT CAUSED US TO GO PAST 340 DEGS AND THE CAPT JUST STATED WE MADE AN ERROR. I FEEL WE WERE SO ENGROSSED IN THE ABNORMAL CONFIGN AND HVY ACFT ON THE SHORT RWY WE WEREN'T THINKING CLRLY ABOUT THE HDG. WE NEEDED TO HAVE MORE SITUATIONAL AWARENESS BUT GOT A BIT OF TUNNEL VISION.