Finally got my car together and on the street for some tuning. The setup is a darton sleeved LS1 block 428ci, prc 247 heads running stock rockers with CHE bushing upgrade and 7.475" pushrods, cam specs: 243/248 .627 .596 115+5, morel linkbar lifters at .070" preload, twin pt6766's, ls6 intake, holley dominator efi on e85.

Back story: I got the car to idle and cruise pretty good so I started doing a few short pulls. The holley self learn was working well and each pull was getting cleaner and smoother. Finally I did one near WOT pull to 6200 rpm. Car felt great during acceleration but upon letting off the throttle, it sounded like the engine nearly shut off. I gave it a little gas and the car continued to run but definitely sounded like it had a miss. I thought maybe another intake rocker had come loose as I had this issue a few weeks ago. I limped the car home and pulled the valve covers.

To my surprise, the pushrod for #1 cylinder exhaust valve was sitting between the upper rail of the head and the rocker arm. The rocker was still torqued to spec. I removed the rocker and pushrod and found the pushrod is bent pretty badly. The valve spring was still compressed even with the rocker removed indicating that piston to valve contact had occurred and the valve is bent.

I haven't had a chance to pull the head and inspect all components but I'm just trying to get some ideas on what could have caused this. All other pushrods are still in position. I tried wiggling all the rockers to see if any other pushrods were slightly bent but any movement in the rockers were on the cylinders that had the valves closed and the movement was just a slight "click click" side to side, which is normal from my understanding. I thought maybe the lifter seized in the bore, but when the barring the engine over, the lifter moves up and down as it should. I verified this by adjusting my pushrod length checker to the same 7.475" and installing it, then tightening the rocker and rotating the engine. The rocker travels as it should until it hits the bent exhaust valve and then stops obviously. The valve is seized at what appears to be near wide open.

I thought maybe I made an error measuring pushrod length but all my measurements came out the same for all cylinders and valves. The prc 247s have thicker decks and I'm running .051" head gaskets so I thought the pushrods would need to be longer than 7.475" especially setting the lifter at a higher preload number, but again all measurements came out the same so I went with it. And again, this is the only cylinder/valve that had an issue. The cup of the rocker arm where the pushrod rides seems fine as well.

If there was ptv clearance issues it should've shown itself long before this point especially since the valve was hit when nearly wide open. If it was that far off, I would think it would bind long before running it at 6200rpm.

At this point I'm kind of at a loss. Assuming no foreign debris somehow made its way into the engine, I'm leaning toward the lifter collapsed at high rpm, allowing the pushrod to come loose and wedge itself between the head and rocker arm which held the valve open and the piston hit it. I did purchase the lifters used and they sat for a while before install. That's all I can come up with currently.

Any help/advice is appreciated. I really don't want to have the head repaired, buy a new pushrod and lifter set and have the same thing happen. Hopefully the piston isn't hurt too badly Thanks in advance guys

James

 

My first thought was those PR's seem pretty long, just based off the stock 7.4's-I would have thought the lifters might be higher than stock.
How were the heads set up, based off the cam specs?
Was the VP clearance checked-I am guessing the pistons have valve reliefs
How did you ck the preload
I guess its possible the valve seized in the guide, maybe piston hit it
Hopefully pulling the head will reveal something

 

I bought the heads brand new but it is possible the valve seized in the guide for whatever reason.

The pistons are wiseco -20cc dish and yes they have valve reliefs. I didn't measure ptv clearance (I know, I should have) but I would think if there was clearance issues it would have revealed itself before now especially considering the valve is currently seized pretty far open.

The PRC heads have thicker decks than stock casting heads. That coupled with a higher range preload and stock thickness head gaskets made me think that 7.475" was about right.

I measured for pushrod length and lifter preload using the EO/IC method posted by thunder racing years ago on this site. It has been debated if this is the best method but many, many people have used it with no issues. Also the problem only happened on #1 cylinder on the exhaust side. I'd think of the pushrod length was off too far it would have happened on multiple cylinders/valves.

Thanks for the reply. Hopefully, as you said, pulling the head will shed some light on the situation.

 

Another thing I didn't think of is I'm running .080 hardened pushrods. Maybe I need to step up to the 11/32 pushrods but still it's weird that only one would fail. Unless they're all on the brink of bending and that one was weaker than the rest

 

Let's think about this. The pushrod is bent because it got jammed somewhere it didn't belong AFTER the valve bent. I'd have my doubts the pushrod bent and then bent the valve. UNLESS it bent, fell toward the rocker trunion therefore increasing the rocker ratio (and lift) and pushed the valve into the piston. Pull the head, inspect the lifter, valve spring, recheck the p/v clearance and put it back together.

 

When I pulled the valve cover, the pushrod was between the back edge of the rocker arm (where the pushrod seats) and the head. I'm unsure if maybe it wedged there, holding the valve open and maybe the lifter traveled up on the cam and caught the pushrod with the edge of the lifter housing causing the pushrod to bend. This is why I thought maybe the lifter collapsed initially, allowing the pushrod to come loose.

I will check ptv clearance just so I'm not assuming anything when I reassemble but wouldn't it make more sense that more valves would be bent if there was a ptv clearance issue?

You very well may be correct and the bent valve could have been the first event in the chain. It's all speculation at this point and I just wanted to get opinions on possible causes that I may have been overlooking. Hopefully I can pull the head this weekend and come to a conclusion on the cause.

 

I agree, a ptv issue would likely cause damage to more than one valve. Collapsed lifted is a possibility as well. You sound mechanically inclined. That being said, I'm sure you'll come to a conclusion once you tear into it.

 

Currently, I'm thinking it was either the lifter or the valve seized in the guide. If I have the time this weekend, I'll yank the head and see what I can find. Thanks for the responses

 

I'll vote for valve guide too tight.
Have that checked when you get the head off.

 

I will have the heads thoroughly inspected and repaired once removed. I'm going to call around to some vendors and see if one of them will sell me one pair of the morel lifters for that cylinder as well just for peace of mind.

I'm holding out hope that the piston isn't beat to crap and I can slap it back together once repairs are done and send it

 

Yep!!

 

A similar thing just happened to a friend's car, but the pushrod actually broke in half.

Same thing though. Bent valve. Just barely a kiss mark on the piston. Only one cylinder.

We are leaning towards a collapsed lifter at this point, but are thoroughly looking things over.

I hope you find a definitive answer. Good luck!

 

Postmortem is always difficult unless you find the obvious smoking gun.

Seems like a collapsed lifter would result in much less valve lift and hence no piston valve kissing......

 

It will be very difficult to pinpoint the culprit.

I guess the thinking is the pushrod came out of the cup and pushed the rocker/valve into the piston.

Not too sure.

There was plenty of PtV.

 

Ok. Car is back together and running strong again...

New theory as well. He's using a tq cut box that cuts spark during the shift. He had it set pretty high at 70ms. That's a lot of unburned fuel at 6500 rpm. We're thinking something must have happened in the chamber to affect both valves. Still not sure, but the exhaust valve was bent and the guide was cracked. The retainers were knocked out. The pushrods were bent. There was a slight mark in the piston from the exhaust valve.

So, end result is no smoking gun, but he's turned down the timer on the TQ cut box.

OP, hope your deal gets fixed quickly.