Bent push rods….calling engine builders

bullsnot

bullsnot

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Last fall I lost an impeller and boat overheated. Engine is a 2000 5.7L Mag MPI. After compression test and leak down tests I decided to replace the heads. Swapped stock ones for brand new stock heads. I reused the roller lifters and push rods. I followed the manual. Started at cylinder 1 TDC. Tightened until lash gone, then another 1 full turn on each valve listed for that position in the manual. Then moved the engine to cylinder 6 TDC and tightened the appropriate cylinders in the same fashion.

I fired it up and it ran rough and had valve train noise. Never could fix it. I pulled the valve covers off an discovered 3 bent push rods. Other than DIY mechanic error is there anything else that could cause this?
 
Lou C

Lou C

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If there were stuck valves from the overheat, that could have bent the original pushrods. How'd they look when you took em off? Some say to roll them on a piece of glass to see if any are bent.
Also when doing the hydraulic lifter preload adjustment it is easy to make a mistake with the intakes and exhausts, I did the same job you are doing on my '88 4.3 V6 about 8 years ago, I marked all the rocker arms with a cyl # and I or E to avoid that, lol. First time I did it and turned out fine.
 
Scott Danforth

Scott Danforth

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are you sure you have adjustable valve train? GM went to non-adjustable with the vortec heads.

so if you have stock heads, you tighten down each rocker retainer to 20 ft-lb
 
bullsnot

bullsnot

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Scott Danforth said:
are you sure you have adjustable valve train? GM went to non-adjustable with the vortec heads.

so if you have stock heads, you tighten down each rocker retainer to 20 ft-lb
Click to expand...
I Definitely have hydraulic roller lifters. But the manual (Clymer) says tighten to “just” zero lash (zero lash meaning you tighten rocker nut to the point that you can no liger twist the push rods under the rocker with your fingers). Then tighten rocker nut one more full turn. In theory this collapses the lifter just a little and will allow it to expand over time as your valves wear and keep it tight.

What is the source of the 20 ft lbs?
 
Scott Danforth

Scott Danforth

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yes, all 1996 and later SBC motors have roller lifters.
The other change GM made with the Vortec heads is non-adjustable self-aligning rockers mounted to pressed in studs

Unless you are running aftermarket adjustable valve train, you have non adjustable rockers.

You should have a step in the rocker stud

The clymer manual has information from the prior vintage of motor. You should get a factory manual
 
bullsnot

bullsnot

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Scott Danforth said:
yes, all 1996 and later SBC motors have roller lifters.
The other change GM made with the Vortec heads is non-adjustable self-aligning rockers mounted to pressed in studs

Unless you are running aftermarket adjustable valve train, you have non adjustable rockers.

You should have a step in the rocker stud

The clymer manual has information from the prior vintage of motor. You should get a factory manual
Click to expand...
Very interesting. Honestly this seems like the correct answer based on my frustration with this valve train. The one turn past seemed very excessive as I was doing it and literally watching the valve spring compress. I will get a factory manual.

One question though. I noticed in a prior thread (I did a search on the forum) you mentioned 30 ft lbs. Is it 20 or 30?
 
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Lpgc

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If he only tightened to one full turn after lash and the minimum suggested is one one full turn after lash the question still remains what bent the pushrods?
 
Lou C

Lou C

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The fact that the heads were replaced from what the originals were, makes it very important to verify just what valvetrain you have. There are many variations in Chevy small block heads, and it's possible you had a net lash (not adjustable) set up on the originals, but if you didn't get the same casting # for the new ones, it might be the same or different.
Post up a pic of your valvetrain with a rocker arm removed, it will be pretty obvious what you have then....
PS
I was looking at reman engines at Michigan Motorz's site and for the 5.7 Vortec, listed for 1996-2000, in the pix they look like they have the adjustable valve train.
The ones that appear to have the net lash or non-adjustable valve train are the 6.0 liter motors, and the Vortec 4.3s.
 
Last edited:
Bondo

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But the manual (Clymer) says tighten to “just” zero lash (zero lash meaning you tighten rocker nut to the point that you can no liger twist the push rods under the rocker with your fingers). Then tighten rocker nut one more full turn. In theory
Click to expand...
Ayuh,....... I Hate that explanation, as the pushrods will twist, long after 0 lash,........
Instead of a twisting motion, it's the up / down, push / pull motion yer looking for,.....
0 is easy to find that way,.....
 
Scott06

Scott06

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Bondo said:
Ayuh,....... I Hate that explanation, as the pushrods will twist, long after 0 lash,........
Instead of a twisting motion, it's the up / down, push / pull motion yer looking for,.....
0 is easy to find that way,.....
Click to expand...
100 % up and down slack is gone at zero lash. twisting for drag will lead to over tightened rockers.
 
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Mc Tool

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Ask any Ducati mechanic how to set the closing rocker ( desmodromic valve actuation )clearance ,0 clearance is Found by rotating the valve stem or closing shim . You can do it with dti's and feeler gauges but it takes 10 times as long and is no better . In fairness there is a required "knack" .😁
 
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Lpgc

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The OP tightened to zero lash then tightened another full turn, no mention of tightening to 20ft lbs etc so like I implied above the OP tightened them to a spec we wouldn't expect to bend pushrods... they were either set correctly or too slack but not too tight.

Wouldn't expect zero lash then another full turn to bend pushrods with hydraulic lifters if the lifters were pumped up but might expect the full turn to cause bent pushrods if the lifters were not pumped up when the full turn was done. The OP said he reused existing lifters but were they removed / bled down / did they bleed down themselves before the adjustment was done .
 
C

cyclops222

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Hydraulic lifters and standard production valve parts ? Were designed so NO NO adjustments were needed or possible . By D I Y goof balls.
 
Lou C

Lou C

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GMs not the only one with non-adjustable valve trains, many V8 engines had non adjustable valve trains even back in the old days. Mopar for one, I'm sure there were others.
When I did the heads on mine, I used the up and down play method and found it much easier to feel play.
 
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