Same engine as before, 302 in 74" f100. Bought from a friend of a friend who said he rebuilt it all the way through. After the initial start up cylinder #7 broke the rocker arm & push rod. Took it to mech who worked on older trucks & he replaced both rocker arm & push rod. Drove it home & it seemed to be running poorly. It started backfiring through card & shacking real bad. Took off valve cover & sure enough cylinder #7 had a broken rocker arm & bent push rod again. I wouldn't think that two different rocker arms & push rods would be bad & especially on the same cylinder without some major issues going on inside the engine. Tearing down I can do, but rebuilding is out of my ability.

So the question is, should I just call it a loss & replace the engine with a crate one. I ahve about 1200 in it as is. If I changed out the engine I would wnat one that had a lot more horsepower than this one.

So lets hear from you guys.

thanks
Garry

 

That is a tough deal Gary. In a previous post I had suggested some internal problems based upon the symptoms described. Pushrods only bend when the valve makes contact with the piston. (Ok once in a while there are other reasons but in this case me thinks there is a piston clearance issue)
Thats being said, pistons do not like to strike things especially valves. Pistons usually crack, or deform. This can also be very hard on the cam shaft and lifter(s).
Connecting rods, wrist pins, and bearings are all subject to damage when this happens.

Only one way to verify if there is a problem. Pull the head and look to see where the problem exists.

In the same previous post I recall a connecting rod being changed? I questioned the mechanic that might change one rod. Maybe I am thinking of a different post and completely different situation. If I am mistaken,,,,,,well it happens, but I do recall that the mechanic was a guy that worked on older engines.
Older push rod engines and brand new ones are identical by design, so any competent mechaninc would be qualified to work on this 302.

regardless, something is missing. I mean there is a variable that is not onthe surface here. As I am typing I am thinking about how a rocker could become damaged, and a few things come to mind. I stated that the piston has to strike the valve but his is not true. I remeber using the wrong spring and retainer combo on a set of Pontia heads back when I was running GTO and I had a probelm with the retainer hitting the top pf a guide. The cam had more lift than the guide would allow, and I broke an ARP screw in stud.
Another issue might be a broken valve spring or bound up damper. Since the valve springs rotate and move around during normal usage a broken spring can stack or bind up and prevent the valve from opening up all of the way. If this is the case, you can bend a push rod, and if it is strong enough the rocker might just give way.

Do you have any parts other than stock in this engine? Larger cam or anything that might contribute to the problem here?

 

Well first of all, don't take it back to that mechanic that just replaced the parts without finding the root cause of the problem.

Second, don't give up on that engine just yet.

Like Ed said, somethings stopping where it shouldn't. Why there's only one is weird.

Buy a cheap lever type valve spring compressor and check the action of that cylinder. Sucks that it's #7, the second hardest one to get to but it can be done. Just make sure the piston is away from TDC.

Another possibility is a stuck lifter plunger. You need another pushrod and rocker to see if it's stuck. Put it together (just make it look like the other cylinders for now) and turn the motor over by hand with the valve cover off and note when the offending valve opens and closes, BUT IF IT STARTS TO BIND, STOP!!! If it doesn't look like the valve is closed, bad lifter.

Or the pushrod is too long.

By the way, which one is breaking, the exhaust or intake? If it's the exhaust, it's less likely that the valve is hitting the piston.

Anyway, the best thing you can do is get another rocker and pushrod and SLOWLY turn the motor over by hand, after pullling ALL the spark plugs, see what's going on, and compare it to the other valves. It may be subtle.

Look for damage on the #7 plug too.

 

ok sounds good, thanks

 

I should have typed valve here.

 

10-4 will do

 

It could also be the valve is seized in the guide. An easy way to check is to remove the rocker arms from that cylinder, take a ball pien hammer and tap the valve stem. If the valve is siezed you will notice a definite "solid" feel and sound. If its not seized, the feel and sound will be more "hollow". I doubt both valves would be seized so you will probably see a difference between the two valves.