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The Lounge Its 5:00 somewhere... This is the area where, after a hard days work, you sit back, relax, and unwind...

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Old 02-23-2010, 01:00 PM
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Bent pushrod in my small block Chevy

Can anybody give me some insight on how this happened and what to do next?

I pulled the valvecover on my SBC to find one exhaust rocker arm off it's valve (rotated out of position) and the same cylinder's intake valve pushrod bent. After pulling off the rocker arms, everything except the bent pushrod appears to be fine. I tried actuating the valves using a valve spring compressor and they both moved fine. Thinking I may have wiped a lobe out on the cam, I put a straight pushrod on the lifters and rotated the crank by hand to check lifter movement - they seem to move as much as the adjacent cylinders, but I haven't put a dial indicator on it yet.

What else should I check before I put a new pushrod in and readjust the valves? Any theory on how the pushrod may have bent?

Here's my theory - tell me what you think. The exhaust rocker arm loosened up for whatever reason and rotated off the valve. With the exhaust valve closed, the cylinder intaked and fired normally, but couldn't exhaust. So when the intake valve tried to open, it was opening against tremendous pressure in the cylinder, causing it to bend.

The symptoms I had from this was sudden backfiring through the carburetor and significant lost of power. The backfiring slowly went away and the engine ran poorly afterwards. This seems consistent with the exhaust rocker rotating off the valve suddenly and the intake valve bending over a number of engine cycles.
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Last edited by TheBandit; 02-23-2010 at 01:04 PM.
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Old 02-23-2010, 01:12 PM
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Some ,if not all, of what you detailed could be the problem. Have you checked the valve springs on those two valves? If broken or weak they could possibly allow the valve(s) to hang up and be impacted by the piston. Also if the guides are worn that could lead to a valve slipping off a rocker.
HTH
John
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Old 02-23-2010, 01:23 PM
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Hydraulic or mechanical lifters?
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Old 02-23-2010, 01:28 PM
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What kind of RPM are you running? What valve springs and where was the installed height set?
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Old 02-23-2010, 01:33 PM
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Visual inspection of the springs looks good and they felt normal when using the valve spring compressor, but I haven't pulled them to check seat pressures (don't have the right tool for that anyway). Not sure if that would be worth the time?

The lifters are hydraulic. Normally I adjust them cold by taking them to zero lash and going 1/2 turn tighter.

I did not check the installed height on the springs. The heads came assembled from Edelbrock with their springs. I am using a Comp Cams XE274 hydraulic cam. When I purchased the heads and cam I talked to Comp about the Edelbrock spring specs and they said it would be okay with the cam. I used +.100 length pushrods which seemed to center the roller travel on the valve pretty well. I have not taken this engine above 6,000 rpm. Just prior to this problem I was hotrodding around and shifting at about 5,500rpm, but the backfiring didn't start until a few minutes later while I was at a normal cruise RPM (maybe 2,500 or so).
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Last edited by TheBandit; 02-23-2010 at 01:44 PM.
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Old 02-23-2010, 01:48 PM
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*edit*

the rockers you are running are heavier than stockers. reving it up to 6k will stress the valvetrain. it probably bent the push rod then...once bent..... while it was running at lower RPMs it bent further.
but this is my GUESS.

Last edited by ANTI BLING; 02-23-2010 at 01:52 PM.
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Old 02-23-2010, 02:19 PM
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I'm gonna throw out the theory that the Exhaust rocker loosened up and the rocker arm spun towards the intake rocker and some how bound up the intake rocker's movement and then the intake push rod bent. I might look for a mark on the exhaust rocker that would prove this theory.

Jaysin
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Old 02-23-2010, 03:53 PM
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I would suggest that you check those two rocker arm studs for straightness before reassembling as well as checking for possible other bent pushrods. Are you running a good oil with enough ZDDP to protect that flat tappet cam?
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Old 02-23-2010, 04:09 PM
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Jaysin - I looked at the rockers carefully and I don't see any markings from contact with the rest of the valvetrain.

Captain, how would you recommend checking the studs for straightness? I can put a straightedge against them and look that way, but is there a better method? The rocker adjusting nuts seem to spin freely on the threads so they can't be too far off. I have been running Valvoline senthetic blend 10W-30. I don't know what ZDDP is, so I'll look it up.

Thanks everyone for the continued ideas. I'll do some more snooping tonight before I reassemble.
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Old 02-23-2010, 04:09 PM
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All Good thoughts above.

So what brand are the P-rods? and bought approx when?
I ask because it looks like a P-rod failure and I would glass roll all the rest of them just for drill.
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Old 02-23-2010, 04:19 PM
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The pushrods are Elgin brand, which I believe is a US-based OEM. I bought them from a local speed shop / engine builder. They are a welded design.

I'm not sure if I want to pull all of them until I verify that things are okay with this cylinder after reassembly. If the loosened rocker theory I posted above is correct, there's no reason the rest of them should be damaged.
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Old 02-23-2010, 04:35 PM
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Here are a couple places for you to learn about current day motor oils and their lack of ZDDP which is a wear additive necessary for longevity of flat tappet cams.

http://www.bobistheoilguy.com/forums/ubbthreads.php

http://www.67-72chevytrucks.com/vboa...d.php?t=381780

and I think this is the right thread.....I didn't have time to go thru all of it.

http://www.chevelles.com/forums/showthread.php?t=128203
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Old 02-23-2010, 04:54 PM
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Quote:
Originally Posted by TheBandit View Post
The pushrods are Elgin brand
The pushrod may have simply bent.
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Old 02-23-2010, 05:42 PM
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Quote:
Originally Posted by entropy View Post
The pushrod may have simply bent.
Yeah that's a possibility too. It doesn't really explain the exhaust rocker being off though - seems like the two must be related (exhaust rocker rotated out of position and intake pushrod bent on the same cylinder).
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Old 02-23-2010, 05:52 PM
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I'm wondering if you floated the valves and/or a guide is tight and was sticking. I've seen cases where if the cylinder head temp started to get hot the valve guide would close up enough to gently bind the valve. That coupled with the heavy rockers may have floated it. If you were hot rodding it a bit with high rpm shifts for a few minutes, then the head starts to heat soak, and all this comes into play. Of course it is damn hard to check for this condition because the head has to be well heat soaked to see if it is the culprit.
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Old 02-23-2010, 06:17 PM
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^^^ I'm thinking along the lines of what he said about a valve possibly sticking/binding. I run those same rocker arms and Elgin push rods in my race engine (my rules are pretty limiting on the engine side) and haven't had a bit of trouble with my normal set of 441 heads. However, last year I bought a set of 487X heads cheap from a guy, first night out bent two pushrods, second night bent two valves and the same pushrods (looked like they stuck and piston hit them). I took them off and sold them to a drag racer for more than I paid for them.
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Old 02-23-2010, 06:36 PM
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Do you think if I floated the valve(s) it would cause both the intake pushrod to bend and the exhaust rocker to go out of position on the same cylinder?
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Old 02-23-2010, 07:01 PM
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Yup. In that situation, you got lucky. the exhaust side floated, the valve hung open and the rocker happened to turn and get out of the way. The intake side stayed open, allowed the pushrod to get away from it then came slamming down. The slam would cause the bend....

I would check ALL the pushrods for straightness as a first step. To be honest, this situation is fraught with disaster. I know I would have a tough time deciding what to do. If my scenario is what happened, you could have bent more pushrods, and maybe even cracked a piston if the valve reliefs are not perfectly aligned with the valves. Then what caused it? If the valve guides are too tight, the heads have to come off to be reworked. Of course, the only way to actually check is to take the heads off and apart. UGHH...

sorry that I am giving you a worst case scenario. Having been there before, this whole thing sucks.
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Old 02-23-2010, 07:20 PM
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Do Comp cams still go flat? not that this applies to this thread. I know in the 80's they had some issues with the flat tapet cams in bbc's
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Old 02-23-2010, 07:21 PM
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I had a small block one go away in about 1993. That is the ONLY cam I have ever seen fail unless there were outside forces that made it fail.
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