bent pushrods what's the cause

3 bent pushrods in #2#3#6 cly's. all on intake valve. What would cause this before i just put new ones in and start it. I have checked the timeing chain ok it has 2 gears instead no play or slack there. help before i tear the whole head off this thing. is there some way of checking the valves or related lifters? this is a 4.9ltr strait 6 efi

3 bent pushrods in #2#3#6 cly's. all on intake valve. What would cause this before i just put new ones in and start it. I have checked the timeing chain ok it has 2 gears instead no play or slack there. help before i tear the whole head off this thing. is there some way of checking the valves or related lifters? this is a 4.9ltr strait 6 efi
There are several things that could be the possible cause. Overrevving, broken valve springs, collapsed lifters(due to low oil/oil pressure, or mechanical failure), worn rocker arms/pivots(allowing the end that meets the valve stem to run off to the side) , worn valve stem, spring bind due to a faulty pumped up lifter(unlikely with a stock cam though), and probably a few others im not thinking of right now, but a few things to check. You shouldnt have to pull the head, the lifters are accessible from the inspection cover on the drivers side of the block. Good luck.
Later
Ed

One thing you might want to check is the rocker arm to valve stem clearance. This must be done with the lifters collapsed(not pumped up). You also need to have the engine(ie camshaft lobe) in the correct position. If you watch the respective valve open and close, then turn the crankshaft 1/2 turn more, all in a clockwise rotation of the crankshaft that should be close enough. The clearance should be 0.125- 0.175 per Haynes manual. The trick may be to get the lifters COLLAPSED. If you have too little clearance, shorter pushrods can be purchased. Well, you probably have a manual, just a suggestion.

On a 4.9L I-6 (old 300-6) about the ONLY way to bend push tubes is to have the nylon cam gear come apart (strip).

Drain the oil and look for small pieces of plastic in the oil. If they are there, you have found the problem.

I have NEVER had to replace a head, piston, valve, or rocker arm, just the gears and push-tubes. Expect a 4-6 hour job.

A question for ModMech: How would the plastic pieces bend the pushtubes? I could see it if the cam gear stripped clear off and this was an interference engine(don't know) but then you would probably have more problems. Please understand, I am not doubting what you are saying, just curious. One more question, Do most 4.9l engines of this vintage(1988) have plastic cam gears? I have an '88 F-150 with this engine. ModMech, if you would, please answer these two questions.

The cam gears are made of a fiberous plastic, and when they strip, the cam stops turning, the pistons come up and slap the valves.

Fortunately, the valves and pistion tops meet flush, and typically the only damage is bent push tubes.

Ford "automotive" I-6 engines from the late '70s onward had these nylon gears. The Industrial versions did not, they had steel gears, and were MUCH noisier than the automotive engines (they wined).