Bought a 1980 Ford F150 with a 351M engine
I belive that the truck was at one time used by a excavation dude and he has modified the truck to pull a heavy trailer
By modified i mean that he has added a heavy spring pack to the rear sup. and changed the ratio to 4:56
The truck sat for 3 years in a field before I bought it at a church auction (the former owners son in laws son donated the truck to the church)
anyway $200 bucks was too cheap to pass up
With new gas, carb gaskets, vaccum hoses and battery the old gal fired right up
I then tuned the engine, rebuilt the carb and redid the entire brake system
the truck has a fair idle and seems to really power up at 1800 rpm plus
I have not dial indicatored the lift on the cam, but i suspect that it also has been changed.
The church told me they where told that the truck had a rebuilt engine and only had 40,000 miles on it/
I can drive the truck for a short while like 15 miles, say into town to a store, shop for a bit and drive it home with no problems
However, I took my son fishing and the trip one way was like 50 miles, we fished for about 4 hours and returned home.
on the way home, within 5 miles the truck started to miss and then blow back thru both the carb and exhust
pulled the valve covers and there was 3 bent pushrods
I replaced them with the std pushrod and put it back together.
the parts store had .060 undersize and .060 oversize pushrods also available
again the truck ran fine on the short haul
a few weeks later I made anothe trip of over 50 miles and again it bend pushrods.
4 this time and 2 where on the same cylinder that I had previously replaced.
I wonder if they had gound the valves and didn't replace the pushrods or when/if they changed the cam did they change the pushrods.
should i go with the .060 undersize?
I am thinking that when it warms up enough over like a 50 mile trip that it expands enough to remove all lash from the valve train and the pushrods bottom the valve springs out with enough left over to start bending the pushrods
any advice would be helpful,
Thanks Rand

 

You might have a vavle guide problem, maybe the vavles are galled from lack of oil. Are thay int. or ex. that keep bending, or both?? The longer ore shorter pushrods are for lifter preload.

 

I had a customer that we did a Valve Job on his heads (351M/400)
He bend push rods and a valve a few times. We open up the valve guide clearences to the high side of spec.
Little later bent a push rod again. Come to find out he was not running an EGR valve. Combustion temt. can get higher without the EGR valve. The valves can swell alittle more and stick or get "lazy"
and then the piston can tag it.

I hate to say it being a Ford man, but the only problems Iv'e had with valves were both a 351M/400 engine. Possible the chambers are not very efficient with no squish area.

Good luck

 

Had a similar problem with a 351M a few years ago. happened right after I bought a Ford car with this engine. Turned out that there was so much shellac formed in the engine that the lifters pumped up at heat and higher rpm's and froze in the pumped up position. Valves kissed the pistons and the result was bent pushrods and valves until I cleaned it all up and replaced the lifters.

 

I think the sticking valveguide theory is probably the first thing to look at, but before you pull the heads take a good look at valvesprings and valvetrain geometry in general. If you pull the plugs and look as best as you can at the top of the pistons you might be able to tell if there has been contact. The only other explanation that I can think of for bending pushrods besides guides and actual physical interference is resistance from compression. When a cam lobe goes and your valve timing goes haywire it can bend the companion pushrod from the strain. Get a bright light and take a close look at everything.

 

...I vote for sticking valves. I'd run a quart of marvel mystery oil if you have room in the oil pan for a short duration then change oil. Also pour it down the carb while running at fast idle. I seen it work wonders before.

 

There is a tech article about this. Look it up and see if it applies to you or not. Just a thought, better to get more info then to shoot in the dark.