Briggs Valve Rod Problem

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.

pull both the heads and have a look at the tops of the pistons.
Only 2 reasons why rods bend.
1 because it is trying to push the valve down and the valve can not move ( stuck in guide or hitting piston )
2 Because the rod itself is stuck can can not move.

The piston aircraft guys have shown that it is possible for valves to stick in their guides. One reason : coked (carbon buildup) valve guides. Momentary sticking can cause bending as the cam tries to open the valve. After the slight (or great) bending the pushrod usually drops out, and in the case of my Intek (and others') the pushrod can enter the sump for digestion. The only other things I can think of that would stop pushrod action is an improper guide ID (was not reamed?) or inadequate or no oil mist/splash over the guide tip. The warning sometimes includes excessively heated head combined with immediate engine shutdown which is a formula for carbon formation in the guides. None of this should apply to a newer machine, but...IMO

Thanks guys. The comments were very understandable. This has been an on-going problem since I bought the engine. I've gotten many good years of service from the Briggs and do not want to be overly critical of it but some of the things that were wrong just showed bad factory work. For example, the spark plug holes were not fully threaded on one side. Although its been a minor hassle, I've got over 1000 hrs of hard work out of it and its probably time to replace it. Amazon seems to have a good replacement Briggs (27 hp). Thanks again for all the help.