Last night, I'm heading up the highway at around 60 mph, 2300 rpm. I hear this nasty clacking noise, and the car starts running on 7. I get it off onto a side street, and try to figure out what happened. To make a long story short, the number 8 exhaust pushrod was bent. I'm thinking the lifter froze and bent it against the rocker. I tow it to a friends garage, and tear it down. I replace the lifters with new Crane hydraulic rollers, and I replace the pushrod. This afternoon I get everything re-assembled. When I go to start the car, I get the same noise, and bend the pushrod again. When I called it a day, I was thinking that the cam might be screwed up, but now I'm not so sure. I think the only way that the cam could cause this is if the lobe had spun, and I'm not sure if that's possible. I'm wondering if the rod cap might've popped and the piston might be hitting the valve. I'm planning on tearing it down again tomorrow, and possibly pulling the head to see what the hell is going on. If anyone can think of things to look for, I'd appreciate it.

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91 Firebird Formula. Converted to carb. Holley 750. Forged Flat top pistons. 10.5 to 1 compression. ZZ-9 Cam. Ported L98 Heads.
175 shot NOS, 700r4 w/ 3000 rpm converter and 3.73s in the back. Headers w/ full dual exhaust. 27.5 inch McCrearys.
12.98 @ 106 Natural,
11.83 @ 113 NOS.

 

Bent valve, seized spring, worn lobe, improper valve lash, incorrect valvetrain geometry. All I could think of off the top. If I had to pick one, I'd go with the worn lobe.

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"To provoke dreams of terror in the slumber of prosperity has become the moral duty of literature"

-Ernst Fischer

 

I don't see how a worn lobe could bend a pushrod.
If it was the piston coming up in the bore enough to cause this, it would have a VERY bad knock down deep in the engine.

I would bet it is something in the heads, like a bent valve, or something like that.

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Working on:
'84 Z28 LG4 305 with 200,000 original miles!
Added dual elec fans.
145 MPH IROC Speedo
Building 430 HP 350 (ZZ430)
using primarily GMPP parts.
Short block sitting on a stand. (Man, those Fast-Burn heads sitting on it look good!)

Starting to look like the Kicker poster child!

ASE Certified Master Tech

 

Im gonna bet on bent valvestem.

The question Yopu need to ask, is what bent the vavle ?



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60 Ranchero - Project ( Money Hole )
85 Sport Coupe LG4 - Daily Driver
Reader's ride -> My Ride

Just another Hot Rod kid, or thats what they all tell me.
Livin' the Stereotype

 

That is the question. Especially since these heads have only been on the car for three weeks. Brand new out of the box. Maybe 50 miles on them. I'm using the stock pushrods w/ 1.5 roller rockers, so the geometry should be on. The springs are 1.250 and the exhaust lift is only .510, so that shouldn't be the problem. There was no give in the number 8 exhaust lifter, would that cause a valve stem to bend? I'm gonna tear it down again today, and hopefully have a better idea this afternoon. Thanks for the ideas.

 

Hmmmm. Did you have the guides machined down for clearance? Either the retainer or locks are hitting the guide, or the spring is binding.

 

1¼" springs at 0.510" lifts? Are the coils binding? What was the spring installed height?

I'm thinging the Devil was right - you might have a tolerancing problem with the valve train design.

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Later,
Vader
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"Fear is how I fall, Confusing what is real..."
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