Power Major push rod woes

[Roadless63]

Sorry to be bombarding the message board today, Power Major began playing up by misfiring which quickly lead to blowing back through the air inlet, it was quickly stopped and investigations began. It soon became apparent that the push rods for inlet and exhaust on number 4 were both bent, also the rotating cap on top of the exhaust valve had jumped off, luckily not disappearing into the sump!
Two new (s/h) push rods later and all seems well again. Checked all tappet clearances, all were a bit on the tight side so were re-set, took tappet chest cover off to check cam followers were in place and undamaged, seemed OK.
Cant think how push rods ended up bent, tractor been running for a couple of days since with no probs! There was a slight witness mark on the pushrod where it looked to have been rubbing on the head although could have happened whilst running bent?
Any thoughts as baling season is around the corner and could do with it not playing up on a saturday morning in the middle of a packed Marlborough high street with baler and sledge in tow.....
Thanks, Ed

[Brian]

Bent pushrods can happen.

Has the engine got a de-compressor? If the tappets were too tight the pistons could have hit the valves. The valves be sticking. Are the special valve spring retainers and rotator caps on the exhaust valves only?

Just a few ideas.