I broke a valve, why?

I just had this 1990 SBC 350 engine rebuilt back in June. The rebuilder has been in business for a long time.

The engine ran fine and had good vacuum, but it would pop if you got on it hard going into passing gear.

I only had around 2,500 miles on it and it blew on the freeway. The number 4 exhaust valve broke, demolishing the piston and going through the side of the cylinder. Water in all cylinders and bits found in the intake below the carb.

The block and head are ruined.

I have never seen this before. Anyone have any idea what likely went wrong?

Thanks,
Tony

 

Did you get new valves with the rebuild, stock replacement or stainless? Or did they just regrind the old ones? Stock type valves can and do fail over time; usually the head snaps off where it's fused to the stem. If you dropped the whole valve, the keeper/retainer failed. Improper installation/parts or valve float would be the culprit here. Other than that, a valve sticking in a too-tight guide will cause the same mess. Sorry to hear about it....

 

Thanks for the reply. The head snapped off at the stem.

The thought of valve float makes me wonder about correct adjustment of the valve lash. How far off would the it have to be to get float and cause the valve to break?

 

float is not caused by adjustment, it's caused by weak springs or over revving.

Used springs will often break a few thousand miles after a rebuild, do you know if the old ones were reused? I always put in new springs...having learned that lesson...

 

That sucks... Hopefully the builder will work with you to get another one together. When you go and talk to him, try not to get into a big rant and such. At the shop I worked at, the owner was more inclined to work with a person who didn't come in yelling and screaming. We don't like to see stuff like this happen either, and it's not the first time it's been seen. We had a bad valve in a brand new set, with nothing but new parts... Dropped a valve going into turn 4... $60k boom!!

Good luck getting it sorted out...

 

I got an old ford 200-6 head from my uncle it was off a65-66 falcon or mustang. I started cleaning it up and a few valves were stuck. I oiled them and tapped on them with a hammer to get them working. the head of one just popped right off. Looking at it It had been cracked and just let go from light tapping. i am glad i didn't use it, i didn't know it till then but They can break, not your fault.

 

I had a big name shop do the machine work on my block (454) and heads. The heads were ported and assembled. While breaking the cam in, there was a loud bang and the engine stopped. I thought it threw a rod. Looked underneath and found nothing.

Pulled the plugs and cranked the engine. Water squirted out from #4 cylinder. Pulled the head and discover the valve guide was too tight. The valve stuck open and smacked the piston. Took the head back to the machine shop and it was fixed for free.

I actually consider myself lucky it only bent a valve. The engine was at 2500 rpm when it occurred.

 

Pulled the plugs and cranked the engine. Water squirted out from #4 cylinder. Pulled the head and discover the valve guide was too tight. The valve stuck open and smacked the piston. Took the head back to the machine shop and it was fixed for free.


You bent a valve, And water came out of the cylinder ?

Did i mis~read, All that happen was a bent valve & a tight guide?

Where did the water come from ?

 

My dad was doing this same thing with a 440 we had.
He was going around the engine lightly tapping the valves.
The head of one popped off and sent the stem, retainer and spring flying across the garage.
It was fun.

 

Valves will also stick if the stems undercut goes into the guide. This traps crap in the recess eventually sticking the stem in the guide.

It's real easy to miss this

Cure is to shorten the guide in the port

 

I am not sure, but I think they normally reuse the old unless they spot a problem. Not that I had a problem with that, but I had never heard of this happening before.

I am not mad at the rebuilder because he gave me a break when he rebuilt this motor. When it blew, I took the engine straight to him and said look at this and tell me what went wrong. I did not remove anything from the engine. He comes back and says that I must have messed up the timing or was red-lining the motor. Neither of which was the case.

The engine does run at around 3000 rpm at 70 mph due to turbo 350 (1:1 3rd gear) and 3:90 rearend gear ratio. Many of my cars growing up ran at 3000 at 70 with no ill effect.

I was not even going to have him rebuild it right now, I just wanted to know what went wrong so I did not make the same mistake in the future. Then he finally called me yesterday and says, ok I have an engine ready for you and I would like $400 for the block and head. If you see fit to pay anything on labor that is up to you, but I won't warrant this one.

Well, I tell him I am not prepared to pickup at this time due to other circumstances. I said I thought you going to call me and discuss what went wrong. At that point he thinks I am mad at him. He says just come get it, pay me later. OR, come get and don't pay me. OR, come get and tell me how much cash you want. I may die broke, but my customers are going to be happy. I really like the guy, but by now he is making me feel like shit.

He said by the sound of your voice i can tell your mad. I kept telling him your mis-interpreting he sound in my voice. The sound is me low on cash. I don't want charity, especially if it is because if it is my fault. But I do want to know why all my work went down the drain.

The original 283 was pulled due to excessive blow-by, but it had lasted 40 years. It still sits in the garage. This motor only went two months. Bummer!

 

If he gave you a good deal on the first one, maybe the $400 for this one will put you right about even. Go pick it up and tell him you're short, but will pay as you can. How can he refuse that, especially since he even offered it for free?

 

New timing set with the rebuild?

Here's a good compromise: Tow the car to his shop and see if he'll help you along as you install. Pay him the $400 as you can. He'll be able to see that you are doing everything correctly and you'll be able to ask him any questions you may have. It will also show that you aren't dissatisfied, but more interested in his knowledge.

I work with a lot of really talented racers and builders and sometimes just asking a question I already know the answer to helps them understand I'm no criticizing, just interested.

 

Thanks for all the replies.

I am leaning towards a faulty, probably reused, valve.

I do plan on paying him. I may give him some for labor, because he help me out before.

One question on timing. Although I know I was running the stock setting, if it were too advanced, wouldn't there have been evidence in the other cylinders? (Or maybe not, after so much damage from #4)

Tony

 

Too much timing won't break a valve. It'll make it hard to start, rattle under acceleration, and will eventually pound the bearings out of it and damage the piston tops. That didn't happen to you.

 

Sorry man, I was asking about the timing set as in the timing chain or gears. Was a new one installed with the head job?

 

I was thinking about the popping sound you first described. Was wondering if the valve didn't seal for some reason. In a rebuild the fellow would have noticed if it was bent, so must have some other explanation.

 

pull the heads and look for shiny spots on the pistons....you may have more that are sticking.... brandon

 

I agree with "Squirrel's" assessment. If the engine pops under hard acceleration, the valve is floating or more accurately bouncing off of the valve seat due to a bad spring or improper installation of the spring.

BTW, $400 for the repalcement is a pretty good deal.

 

I should have caught that. I believe the answer is yes, but a good point. I assume you were thinking it might have jumped time or broke the chain.

My son would drive the truck and say that after it got warm he would hear a "fluttering sound" and the motor would seem to lose power. I honestly could not hear it. He probably drives it a bit harder that I do. I thought it was maybe a compression leak, then this happened.

It may be too late, they already took it apart and I may not get to see whats left.

If the spring broke, could the float be so bad as to get into the piston? Why the popping sound? Combustion occurring while valve open and sound going out the tailpipe or just unfired mixture hitting hot gases in the exhaust?

I agree $400 is excellent, but he sure is trying to make me feel like it is my fault.


BTW - I just want to say, that I know a little about engines, having played with them since 1974 or so, but I am always amazed at the amount of knowledge on the Hamb. Also, I am so aware now, that the guys with some racing behind them, know a heck of a lot more then a guy like me. I guess they just see and experience more stuff in a shorter period of time. I wish I had done a little (other than the street kind).

Tony

 

Actually on a street engine with flat tops and any thing short of a radical race cam, I doubt the valve hit the piston prior to the head of the valve breaking off. Any cam that someone would normally use on the street just does not have enough overlap to cause piston interference problems.

Also, the valve can break without the spring breaking. A weak spring allows the head of the valve to bounce off the valve seat (a very bad thing) and really stresses the valvetrain. When the valve bounces, obviously the intake and exhaust charges mix and will result in the pop you described under acceleration. If the bounce is not fixed the head of the valve will break off and the stem drops into the cylinder wreaking havoc.

If you are ever at the drags, goto the pits and check out the big dollar racers. They will check valve seat pressure after every pass (a real pain in the ass on a motor with 3 stages of nitrous) to keep exactly what happened to you from happening to their motor.

 

I agree it would be tough to hit a piston with a street setup in a sbc, but thought a loose or broken chain could cause some pretty devastating valve sequence issues. Good thought on the bounce.

Hey EL Graco, watch out for this btmatt guy. He sounds like one of those deep Texas Outlaw 10.5 guys.... That means he knows EVERYTHING but will only tell you 75% to keep an edge!

 

Aha, you figured me out. No need to worry though, as long as we are not racing, I am always a straight shooter with my HAMB buds.