LOSS OF ACFT CTL; PVT PLT IN AN SMA.

Narrative

I FILED A VFR FLT PLAN AND DEPARTED RENO AT APPROX XA30 LCL TIME AFTER RECEIVING A WX BRIEFING FROM FSS; INITIAL ALT WAS 12500 FT. AFTER DEP I REQUESTED AND RECEIVED FLT FOLLOWING. TURB FORCED ME TO 14500 FT AND THEN 16500 FT; CTR WAS NOTIFIED ON THESE OCCASIONS. IN THE VICINITY OF LAKEVIEW I ENCOUNTERED AN E TO W CLOUD FRONTAL BUILDUP HIGHER THAN 16500 FT WHICH FORCED A CLB TO 17900 FT (IT WAS CLB OR REQUEST AN IFR CLRNC). AT NO TIME DID I GO ABOVE 17900 FT PRIOR TO ACCEPTING AN IFR CLRNC. THE CTLR CALLED ME TO STATE THAT I WAS AT 18000 FT WHICH WAS CLOSE TO THE TRUTH. I REQUESTED AN IFR CLRNC WHICH WAS GRANTED AND ASKED FOR 18000 FT WHICH WAS DENIED (I AM NOT SURE WHY; IT DIDN'T SEEM TO BE A TFC PROB). I WAS OFFERED 20000 FT OR 16000 FT AND OPTED FOR 16000 FT BECAUSE OF MANIFOLD PRESSURE FLUCTUATIONS. AT THE TIME I ACCEPTED 16000 FT I DID NOT KNOW WHAT CLOUD PENETRATION WAS GOING TO LITERALLY TURN THE ACFT UPSIDE DOWN. THE WX BRIEFING PRIOR TO DEPARTING RENO STATED THAT CONVECTIVE PROBS ALONG THE RTE OF MY FLT WERE NO LONGER NOTED AS A FLT WARNING. I WAS GIVEN A TEMPORARY CLRNC TO STAY AT 18000 FT. I MAINTAINED THIS LEVEL UNTIL PASSING THROUGH THE CLOUD FRONT AT WHICH POINT I THEN DSNDED TO 16000 FT IN CLR AIR. ANOTHER E TO W CLOUD FRONT FORCED ME INTO THE CLOUDS. MOMENTS AFTER CLOUD PENETRATION I EXPERIENCED SOME OF THE WORST TURB I CAN REMEMBER -- UP; DOWN AND UPSIDE DOWN. I RIGHTED THE PLANE OR PERHAPS MOTHER NATURE DID IT FOR ME; AT 17000 FT PLUS; IN CLR AIR. I DON'T KNOW HOW MUCH TIME ELAPSED FROM 16000 FT TO UPSIDE DOWN TO RIGHT SIDE UP AT 17000 FT PLUS. AT ANY RATE CTR INFORMED ME THAT I WAS ABOVE MY ASSIGNED ALT; AND AFTER SOME CONVERSATIONS AGREED TO LET ME STAY AT 17000 FT. I SPENT THE REST OF THE FLT PRIOR TO DSNDING FOR AN ILS APCH TO RWY 28R AT PORTLAND DUCKING AROUND THUNDERHEADS AND MASSIVE CLOUD SYS. WITH PORTLAND ARPT IN SIGHT I CANCELED AND INFORMED PORTLAND TWR OF MY INTENTION TO LAND AT PEARSON AIRPARK. BACKTRACKING; I SHOULD ADD THAT A SUPVR AT CTR ASKED ME TO CONTACT FLT STANDARDS; I DID. HENCE THIS LETTER. I BELIEVE THE PROB; WHICH CREATED THE NEED FOR THIS LETTER; WAS DUE TO NOT INFORMING THE CTLR AT CTR AS EARLY AS THEY MIGHT HAVE LIKED. HOWEVER; AT THE TIME I EXPERIENCED THE SEVERE TURB IN CLOUDS BOTH HANDS WERE BUSY CTLING THE ACFT. PICKING UP AND XMITTING INTO A MIKE WAS NOT APPROPRIATE TO MY IMMEDIATE PROB.