Duke, Jack, Joe, Dale -- This engine's for you....

[Pat Bush #35083]

Guys --

I give up. After six months of agitation, I fired the 73 up with no trouble. Adjusted the valves, burned in the cam, had her aligned, she ran great and I took her (proudly) to my local Corvette club meeting last night feeling pretty good about the work I had done....

It may be helpful to know what caused the engine work in the first place.... Broke a rocker stud that was pinned, and the top end always had a bit of a miss so I decided to pull the heads and redo them. The cam had a lift of 510/510 but a stock valve train so I figured too much stress on the rockers. This I thought was the problem so I replaced with Dukes suggestion of a GM stock L82 350 cam and a new timing chain and both gears. Did a valve job, replaced ALL the valve train components (rockers, lifters, push rods)with STOCK Sealed Power components, installed screw in studs, and replaced the wanton seat and guide causing the miss. The thought being "I don't want to go here again".

So I get home last night and thing is running like pooky. Started it tonight, checked all the obvious stuff (vacuum, timing, dwell), and still running like pooky. Pull the valve cover and what do I find but the exact valve that had the problem before now has a pretzel for a pushrod and the rocker is cocked and completely off the valve stem. And this is the one I replaced the seat and guide in.... And it is straight!!!! Put another pushrod in and it drops at least a 1/2" lower than any of the others. Haven't pulled the valve cover on the other side yet to see what there is to see there. Rotated the engine, there is still plenty of lift on the pushrod just low.

But I am perplexed..... I can only think the cam is wiped or I have a collapsed lifter (defective????). I find it terribly scary that it is the same valve that started this whole mess in October.

What am I missing? I am going to pull the top end apart again and replace the lifter but if ANYBODY has any ideas on this I would welcome them. I am about at the point of finding a core and completely rebuilding the bugger and dropping it in. Hate to do that with original heads etc.... Do you guys have any ideas?????

Thanks -

Pat

[Ed Jennings]

Re: Duke, Jack, Joe, Dale -- This engine's for you

Had a very similar problem a few months back. It was indeed a collapsed lifter. Repalced the lifter and have had no further trouble. Lots of trouble to replace a $4 part, but.........

[Duke W.]

Re: Duke, Jack, Joe, Dale -- This engine's for you

I'm scratchin' my head. I suspect it's not a coincidence that the same valve is acting up. Just a WAG, but could the pushrod port in the head be out of alignment? Does it look like the pushrod has been contacting the head?

I'm assuming you don't have pushrod guide plates. I know those require hardened push rods. I don't thing Clem is back from his vacation, but he's also someone you want to check with.

You might want to e-mail him.

Duke

[Pat Bush #35083]

Re: Duke, Jack, Joe, Dale -- This engine's for you

Duke -

Everything is STOCK on the top end. No guide plates, nothing fancy, just GM original. I see plenty of clearance for the push rod through the head and I examined the bent one - no major wear I can see except where it bound against the head once bent. Its not really a pretzel but it is bent about 10 degrees. I suspect that happened when the rocker jumped off the valve and jammed against the side of the head.

My head is shaking as write this. I've re-built hundreds of small blocks and I just can't figure this one out unless the rocker jumped, wacked the lifter and broke its spring and then bent the pushrod from pressure of the rocker....

One thing I didn't check was the bore clearance in the block for the lifter -- honed too big maybe?

At least you validated I'm not crazy!

Thanks -

Pat

[Jeff Grablick]

Re: Duke, Jack, Joe, Dale -- This engine's for you

Pat,
A frind of mine had something similar happen, it turned out to be valve spring coil bind. Just a thought!
Jeff

[Pat Bush #35083]

Re: Duke, Jack, Joe, Dale -- This engine's for you

Jeff -

Oh, yeah, replaced all 16 springs too and put the rotators back on.... Go figure....

[Clem Z.]

Re: Duke, Jack, Joe, Dale -- This engine's for you

check to see if the valve has stuck in the guide. anytime you replace a guide you should replace that valve because the finish on the stem is most likely damaged even if you can not see it with your naked eye.

[Duke W.]

Next time get a mechanical lifter cam

The collapsed lifter theory has credence. It could be a case of "infant mortality", but if that's what it turns out to be, what a coincidence that it was the same location as the prior problem! The only hydraulic lifter cars I have ever owned are two Mercedes 190s. Both the '84 four-cylinder and '88 six, which I still own, have small hydraulic lifters built into the rocker arms of the SOHC hemi heads.

At about 20K miles one of the lifters on the '84 collapsed, and it was repaired on warranty. My current fleet includes three sixteen valve four-cylinder enginess, all with shim over bucket lash adjustment. I have special tools to change the shims on all, plus a shim collection for each. Compared to that the Corvette is a piece of cake - no shims or special tools required to adjust the valves, and the "585" edge orifice mechanical lifters can't collapse!

Duke

[Jack H.]

Re: Duke, Jack, Joe, Dale -- This engine's for you

There are different style rocker arms out there. If someone mixed up or reused worn original, it's possible for the rocker to twist/spin and do colleterol damage....

[Joe L.]

Re: Duke, Jack, Joe, Dale -- This engine's for you

Pat------

The fact that the problem occurred on the same valve as the previous problem is WAY too much of a coincidence for me to accept as purely a coincidence. From a purely statistical perspective, there's only about a 6% chance that this would occur coincidentally.

I think that your suspicions regarding that lifter bore warrant further investigation. Excessive clearance might cause the lifter to fail to operate properly. Also, there may be some reason that pressurized oil is not getting to that lifter bore. I just have a feeling that there is some block problem associated with that lifter bore that's at the bottom of all this.

Although I don't think that it could be related to this problem, there is something else that you mentioned that you might want to check. You said that you replaced the valve springs and re-installed the rotators. On certain Chevrolet cylinder heads, when rotators are used on the exhaust valves, special, shorter springs have to also be used on the exhaust valves. To identify which heads that you have you'll need to measure from the top of the valve guide boss to the valve spring seat. If the measurement is 0.75", then you need to use the shorter valve springs which are still available under GM #6263796. If the measurement is 0.84", then you need to use the "standard" length valve spring.

[Alan J.]

A (minor) correction

Actually, Joe, I believe the probability is closer to 0.004 -- assuming each valve is equally likely to fail, then the first failure happens with probability 1 / 16 = .06, but the second failure would also occur with probability 0.06. If the two events are independent, then the probability that the same valve fails twice is (.06)^2 = 0.0036.

(It's unlikely!)

Alan Johnson

[Mark Ring]

Re: Duke, Jack, Joe, Dale -- This engine's for you

Your lifter is probably shot and maybe the cam lobe too. Like Joe says, I vote for lifter bore issues, or oil supply issues with that lifter bore. Adding back the rotators is less of a worry to me as the problem would show up in more than that one place. If you installed screw in studs, you should check to see that the machinist did the job right. Milled the tops of the bosses, etc. Look for stud alignment too. I like to put guideplates on when the screw in studs are added as it allows you to improve the valve train geometry if the rocker tip is a bit off center on the valve stem. The pushrod holes need to be opened up for this as a matter of course. Make sure you get oil at that rocker when you pre-lube the engine before startup. I might like to watch that pushrod with the engine running to verify the lifter is rotating properly, spinning the pushrod. Worn down lifter/lobes could cause a clearance in the valve train large enough for the pushrod to come out of its seat and/or get in a funny position and then the pushrod could end up bent like it is. Time for a tear down. Hope you find the problem soon.

-Mark.