Hi, i'm having issues with the valves on my ztrak 920a. After replacing one of the cylinder heads because one of the valve guides had moved and one of the push rods was bent. I fitted new push rods and set the gap between valves and rocker guides at .05mm to .10mm. The machine ran well for about an hour and a half and then i heard a ping and the engine lost power. It was only firing on one cylinder. After taking off the rocker cover i noticed one push rod had come out of place. I noticed that the gap between valve and rocker was very big and measured at .15mm. All the bolts on the rockers were tight. I did notice that the rocker arms seem to have wear (refer attached photos). I assume that the rocker where the push rod came out could have had the same gap. I set everything up previously to the JD technical manual. Any ideas why i could habe got such big gaps after only an hour and half of running. PS i am going to check the gaps on the good cylinder to see if they are ok or are excessive the same as the bad side.