Bent pushrods
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Logistics Pro
Joined: Jun 2004
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From: South Central Utah
Bent pushrods
I have a 1979 F-260 with a 400 engine, a T-18 trans and 3.54 gears. When pushing the engine pretty hard like pulling a load up a canyon in third gear at WOT 55 mph, the engine will bend a pushrod in cylinder #3. One time it bent one and just a very short time it bent the other pushrod in the same cylinder. Both pushrods within 1/2 mile. It has done it one other time.
It tends to do it after it has been run hard for a period of time. I can run the truck through the gears and not do a thing. I used to be able to hold 55 mph in 3rd gear over the same canyon with the same load, now it bends the pushrods in just cylinder #3.
The original engine lost compression (1/2 of it anyway) in cylinder #3 too. This is a reman engine and has roughly 60k miles on it and has run just fine until this happened.
Thanks for any advice. I have not replaced valve springs yet.
It tends to do it after it has been run hard for a period of time. I can run the truck through the gears and not do a thing. I used to be able to hold 55 mph in 3rd gear over the same canyon with the same load, now it bends the pushrods in just cylinder #3.
The original engine lost compression (1/2 of it anyway) in cylinder #3 too. This is a reman engine and has roughly 60k miles on it and has run just fine until this happened.
Thanks for any advice. I have not replaced valve springs yet.
Lead Driver
Joined: Nov 2000
Posts: 6,619
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From: Albuquerque
This problably should be in the 335 forum but..
Has the engine/heads been rebuilt before the problem started? The clearance might be too tight. Just on the one cylinder? Maybe valve seat recession took up all the slack? You might also have valve tip wear that causes a loose condition and spits the push rod out.
Has the engine/heads been rebuilt before the problem started? The clearance might be too tight. Just on the one cylinder? Maybe valve seat recession took up all the slack? You might also have valve tip wear that causes a loose condition and spits the push rod out.
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Logistics Pro
Joined: Jun 2004
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From: South Central Utah
I just joined, didn't realize there was a 335 series section, thanks.
They are reman heads. The reman claimed to drill out the guides and install all new valves, lifters, springs and so on.
I am sure things wear, I have changed the oil every 3k and the engine runs 99% of the time on propane. When I change the oil it is still clean so I am sure engine wear has been kept to a minimum.
Remans have always scared me. I have seen them with 3 different size pistons, hardened seats in jus one head and everything else you can imagine. I am actually happy I have gotten 60k out of this one. It uses no oil too!
The motor has been pushed in the same manner since it was broken in. I guess all I can do is to replace the valve springs (maybe a floating problem?) and if it does it again, punt.
Thanks
They are reman heads. The reman claimed to drill out the guides and install all new valves, lifters, springs and so on.
I am sure things wear, I have changed the oil every 3k and the engine runs 99% of the time on propane. When I change the oil it is still clean so I am sure engine wear has been kept to a minimum.
Remans have always scared me. I have seen them with 3 different size pistons, hardened seats in jus one head and everything else you can imagine. I am actually happy I have gotten 60k out of this one. It uses no oil too!
The motor has been pushed in the same manner since it was broken in. I guess all I can do is to replace the valve springs (maybe a floating problem?) and if it does it again, punt.
Thanks
Senior User
Joined: Mar 2004
Posts: 137
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From: Washington
An over-revved engine can cause a bent push rod. A sticking valve can also cause this. Basically, if a valve isn't closed when detonation occurs it will get closed very quickly and that's going to cause your damage (the push rod being the weakest part in the link).
Posting Legend
Joined: Sep 2001
Posts: 30,255
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When the engine is being pushed hard everything inside grows a little due to thermal expansion and if clearances are marginal then something gives. Check your lifter preload on that cylinder (and others) and shim the rockers if required.
Laughing Gas
Joined: Nov 2002
Posts: 1,198
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From: Fairmont
Are you always bending exhaust valves, or both intake and exhaust, or only intake?
Are you using an EGR valve?
Are you using an EGR valve?
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Logistics Pro
Joined: Jun 2004
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From: South Central Utah
Exhaust both times, and both pushrods the first time. No egr. I am just going to guess, but the engine is probably only running around 3500 rpm, 3:54 gears. Our Bronco has the same gearing (but shorter tires) and it runs around the same rpm give a couple hundred rpm.
What chances do you think I have of ruining the camshaft lobes to replace the lifters on #3? I am going to replace the springs.
The truck had a Holley 780 with vacuum secondaries and I could hold 55 mph. I replaced it with a Motorcraft 4300 (like new, excellent condition, almost a virgin, even has the small washer behind the spring on the accelerator pump lever) off a 1974 460 truck. Truck runs better but now because of the pushrods I pull the hill at 45 mph with the throttle held just before the secondaries open in 3rd gear. I do believe the truck could do better than before but very reluctant to find out.
Sorry for rambling.
What chances do you think I have of ruining the camshaft lobes to replace the lifters on #3? I am going to replace the springs.
The truck had a Holley 780 with vacuum secondaries and I could hold 55 mph. I replaced it with a Motorcraft 4300 (like new, excellent condition, almost a virgin, even has the small washer behind the spring on the accelerator pump lever) off a 1974 460 truck. Truck runs better but now because of the pushrods I pull the hill at 45 mph with the throttle held just before the secondaries open in 3rd gear. I do believe the truck could do better than before but very reluctant to find out.
Sorry for rambling.
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Posting Guru
Joined: Dec 2002
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From: chicago burbs
it's not the lifters and it's not the springs. neither will cause a bent pushrod. it could be a tight valve guide that is siezing when the engine sees a heavier than usual load and gets a little hotter than usual. only other thing i can think of is mechanical interference, which could also be happening if the valve sticks open. a stiffer valvespring might cover up a sticking valve problem, if that is what is happening. or the valve sticks open, the pushrod goes loose because the valve and rocker didn't follow it back down, and the next time the lifter comes up the pushrod is not in the cup of the rocker and it gets bent?
Last edited by grclark351; Jun 15, 2004 at 10:55 PM.
Laughing Gas
Joined: Nov 2002
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From: Fairmont
We had a customer that had problems bending pushrods and valves. They were also ex. He also did not have an egr valve. I can't tell you why this could be a 400 problem, but I've seen a few instances like this. I know that by not having an egr valve the combustion temps will run higher, which will also make the ex. valve hot also, hence this problem. What I don't know is why is the 400 "touchy" about this compared to other engines. Maybe the valve guides have poor coolant flow around them?
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Logistics Pro
Joined: Jun 2004
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From: South Central Utah
grclark351, thank you, but why is it that you never really want to hear the answer. After thinking about it for a minute, I do believe the head might be coming off. Hate remans.
The original 400 had 184,000 miles on it with no mechanical problems what so ever, until the crank broke right behind the #5 rod and pushed the crank out of the front of the block. I was hoping it would see 200,000 miles.
thanks again
The original 400 had 184,000 miles on it with no mechanical problems what so ever, until the crank broke right behind the #5 rod and pushed the crank out of the front of the block. I was hoping it would see 200,000 miles.
thanks again
Senior User
Joined: Oct 2003
Posts: 347
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From: in a house
you really should check your valve train with a dial indicator before pulling the head. I've had this happen to me and it turned out it to be insufficient preload on the lifter. it poped out and bent itself on the rocker.
Posting Guru
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From: chicago burbs
you can do a lot of diagnosis without pulling the head, once you take it apart it's pretty hard to see how the parts are interacting. it could be insufficient preload as ranchero77 suggests? you can remove the valve spring and feel the valve in the guide as it travels up and own. check for how loose or tight it is in the guide by wiggling it side to side in all directions. maybe pull another spring to compare to. and inspect the rocker cups for wear or damage that could be allowing the pushrod to come out of the cup. just go in and see what you can see.
