GYRO INSTRUMENT FAILURE.

Narrative

UPON PREFLT I NOTICED THE AIRPLANE ON THE ARTIFICIAL HORIZON WAS VERY HARD TO ADJUST SO I FORCED IT AND IT ADJUSTED THE DG WAS ALSO DIFFICULT TO SET BUT IT WAS NOT AS BAD AS THE AI. I ALSO NOTICED THAT THE AI ADJUSTMENT KNOB WAS LOOSE BUT DIDN'T THINK IT WAS A PROBLEM. ON TKOF WHEN IN THE 500 FT OVC LAYER THE KNOB COMPLETE WITH THE STEM FROM THE AI ADJUSTMENT KNOB FELL OFF. THE ARTIFICIAL AIRPLANE DRIFTED INTO THE FULL PITCH UP POSITION AND I NOTICED A LARGE AMOUNT OF PRECESSION IN THE DG. I IMMEDIATELY COVERED BOTH INSTRUMENTS AND UPON CLIMBING TO VFR CONDITIONS CONTINUED TO MY FIRST STOP WHICH WAS VFR AFTER CANCELLING IFR. I DIDN'T FEEL LIKE PLUNGING BACK INTO IFR WHEN I COULD CONTINUE VFR AND BE A LITTLE SAFER. I AM SOMEWHAT CONFUSED AS TO THE PROBLEM WITH THE GYROS BUT WHEN I REPOSITIONED THE ACFT AFTER UNLOADING MY CARGO FOR THE REPLACEMENT PLANE. I NOTICED A LOUD HISSING NOISE AND FOUND IT TO BE AIRFLOW COMING FROM (INTO OR OUT OF) THE HOLE WHERE THE ADJUSTMENT KNOB CAME FROM. I REPORTED THE CONDITION TO DEPARTURE BECAUSE I WANDERED SOMEWHAT ON MY HEADING WHILE DISCOVERING MY GYRO PROBLEM. AFTER THE AI WAS REPLACED AND THE DG KNOB WAS REAFFIXED TO IT'S STEM (IT WAS LOOSE MAKING IT HARD TO ADJUST.) I FOUND NO FURTHER PROBLEM WITH THE ACFT. AS FOR THE SUGGESTIONS TO AVOID A FUTURE RECURRENCE SECTION. I'LL NEVER FORCE A KNOB AGAIN! CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: REPORTER STATES EXTREMELY OLD INSTRUMENT AND KNOB HAD BECOME VERY STIFF. WHEN TRYING TO CENTER ATTITUDE INDICATOR; AND FORCING IT; SHAFT APPARENTLY BROKE. IT CAME APART IN THE AIR. COULD NOT HEAR AIR BEING SUCKED INTO INSTRUMENT UNTIL ENGINE SHUT DOWN.