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. Bent pushrods on both exhust and intake
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

 

Rand,

Only one thing bends push rods; that's a valve spring going solid with coil bind before the cam stops pushing up on the lifter.

Stock M-block (351M/400) valve springs go solid at 0.490" lift (0.480" on the multi-groove exhaust valves used in '78-'79).

Even a cam as mild as the Edlebrock Performer #2172 will bend push rods with stock valve springs.

 

Could be many things but I think the springs are most likely.


Strange, Very Strange........

 

I dont know a whole lot about cams and springs , but when I put a crane cam in my 351M I was considering not changing the valve springs, crane told me that even a mild cam would probaly cause problems if you didnt change the springs. I am running a Crane Cam with a .524 lift and I changed the springs to the recommended spring and so far have not had a single problem and I driven as far as 80 miles round trip for car shows.

The valve spring may be good place to start:-X12

 

 

It is alwasy best to use the springs recommended by the cam manufacturer, but even then you have to check for proper bind height. You will need to indicate your cam to find out how much lift it has then multiply by 1.73 to figure the spring travel you need plus an allowance for bind specified by the manufacturer. But then you don't know who that is... Maybe use 1/16"???

Thanks for using FTE!




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