I have a 26 hp Briggs & Stratton engine on my Scag Tiger Cat mower. Engine has an on-going problem with the valve rods (aluminum & steel). I will adjust the valves @ 0.005 clearance with the piston @ 1/4" down from TDC. Engine will run great. Within 10 hrs of operation, a valve rod will bend and fall out of place. Of the three remaining valve rods, the clearance is way off (about 0.035 on one). A second rod is bent but still in place. The retaining nuts are tight to factory spec. I've had this problem since the engine was new and replaced all rods after only about 25 hours of operation on the new engine. Engine ran ok for a few years then bent another rod. Went ok for a while but now I can't keep the valves in proper adjustment for very long at all. Engine is old now and I'm thinking of replacing it with a 27 hp Briggs available at Amazon. Still, it sure would be nice to know what's happening. Thanks for the help.