If I recall correctly, Great Plains says that they don't use oversize valves because it can/will cause cracking in the head between valves... and they don't use aluminum pushrods.
I have seen other engines use aluminum pushrods with steel caps with excellent results, but those were not made the same - instead the steel ball ends had a longish skirt which slipped over the tube, not sat on top. It appears that one tube end in the picture is ground or filed on a taper, to a smaller diameter to fit inside the ball cap, thus losing strength by reduced contact area. If the exhaust pushrod that failed was similar, then that could be the reason it failed so dramatically. That said, there seems to be a missing contact on the lower sparkplug, and a dent or impact mark on the upper head area. Does the piston show any sign of physical contact with the exhaust valve or a foreign object in the cylinder? Regardless, with little to no material remaining between the machined area and the stud position of the heads, it is little wonder that area failed. That is something which should have rung alarm bells when assembled, seems to me. For a company like Revmaster, it is hard to believe they would permit such work to leave the plant. Any chance the original engine had a rework after it came from the factory, and an original smaller displacement version was enlarged by persons unknown? George --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
