77 f250 460ci bent pushrods ?

[alwaysFlOoReD]

I was pulling my loaded work trailer up a slight hill and heard two pops, then the truck lost power and ran rough. Got home and pulled plugs. Two had signs of gas and all were due replacement. Good, hopefully bad plugs is the problem. Decided to do a compression test, started on the drivers side. Getting between 125 and 140 psi all the way until the second last cylinder [#3] I got 0, nada, no compression...damn. Pulled the valve cover and found one intake pushrod bent like a hockey stick. Looked a little closer and the exhaust pushrod is missing...crap. And its not laying up top, so that means its under the intake. Went fishing with a magnet and got lucky, got the pushrod out the first try. It's also bent like a hockey stick.

So....what would cause both pushrods on the same cylinder to bend? The rad fluid is where it should be. The sparkplug on that cylinder had no signs of gas or any other fluid.

This motors head casting indicates it is a 73 build. The block is 1977. I have a spare 1970 460 that I'm going to rob a couple rods from, hopefully they are the same length.

Any input appreciated.
Thanks.

 

[adsm08]

I think you have a problem.

I would spin it over by hand with that plug out once, make darn sure the cylinder is clear, and then make sure you can compress those valves. If you can get a scope in there and inspect the piston too that would be good. The only thing I can think of is those valves didn't open because something stronger than the rods was in the way.

 

[alwaysFlOoReD]

Everything was spinning over fine with no sparkplugs. I'll go grab a scope now. Compress the valves by hitting with a brass hammer?

 

[alwaysFlOoReD]

Bought a scope, Autel MV400. The battery has to charge for 5-6 hours so it will be morning before I can use it.
The pushrods from my other motor are ~1/8" longer than the 77 motor. Would the lifters absorb that much difference? The 70 motor has an adjustment on the rocker pedestal, the 77 does not.

 

[alwaysFlOoReD]

I used the scope this morning. No signs of any damage inside the cylinder. Valvaes looked untouched as did the piston. Took a look at the lifters and they look normal to me. Bought two new pushrods and installed and spun the motor. Nothing happened out of ordinary. Put in the compression tester and got 120psi after cranking for awhile. Pressure held so valves are OK as is piston.

I'm stumped! Could a lean mixture cause this? I was going uphill under load full throttle when this happened.

 

[alwaysFlOoReD]

Installed new sparkplugs and buttoned everything up. It fired right up, smooth idle [I have manual choke] after warm-up just like normal.

I will drive it and see if it happens again.

 

[adsm08]

Man, I have no idea what happened there. I doubt running lean would have done that, especially on just a single cylinder.

 

[alwaysFlOoReD]

Yeah, it's both pushrods going at the same time that's nuts. Another mechanic theorized if some shit got into the lifters and they hydro-locked, but again, both at the same time seems unlikely.

 

[85_Ranger4x4]

Might be an oiling issue.

Dad had a '80 400 that had a habit of bending pushrods, it only did one at a time though. Those things were kind of noted for not having great top end oiling though and 300k miles later it had to have 15w-40 oil to have much for oil pressure.

 

[alwaysFlOoReD]

I have a buddy that remembers talk of 70's and 80's motors bent pushrods being common. I don't remember that.

 

[85_Ranger4x4]

Dad only had GM back then but I don't remember it either.

His much lower mile and all around healthier '80 350 has never bent a one.