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Old 09-18-2011, 04:07 PM
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Pontiac 400 Valve Lash Problem

It's a Pontiac 400, hydraulic tappet cam, ARP 7/16" rocker studs, poly locks and factory stamped steel rocker arms & pivot balls. Cam is rotated so valve being adjusted is on base circle. Lifters are new and full of oil. No interference between poly lock and rocker arm side.

Things were going great adusting zero lash until #3 cylinder exhaust valve. I am finger rotating the push rod until resistance is felt. Here I had to use a wrench on the poly lock to get zero lash then I gave it half a turn to set preload and I had to use more than normal torque on this poly lock; much more than any of the other poly locks. Actually I gave it 1/4 turn with a wrench and it stopped then I put a cheater bar on and gave it another 1/4 turn.

Almost seems the poly lock and rocker stud ran out of threads? This is not a problem with locking screw. Locking screw is backed way, way out.

I suppose I could place a flat washer between the poly lock and rocker arm pivot ball? This would raise the poly lock slightly so it has a few more rocker stud threads to engage? What do you think?

Looking forward to your input so I don't have to redo this job.

BT

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Old 09-18-2011, 04:26 PM
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Does your heads have screw-in or press-in rocker studs? If they are pressed in, you might have one that has began to pull. If you have screw in studs, check for a worn cam lobe.
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Old 09-18-2011, 05:05 PM
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Studs are screw in and cam is new in engine. I haven't started the engine since new cam, lifters, poly locks and screw in rocker studs were installed. The heads had a valve job performed and it looked like the valve stems were ground even.

The screw in studs are all uniformly screwed down even with the head. No gaps or any thing like that I wonder if I have a defective stud? Maybe the fine threads on stud top are not cut to the same length as the other studs?

Since I did set the valve in question to zero lash I am wondering if I should leave well enough alone. I had to cheater bar the poly lock nut 1/2 turn.

BT
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Old 09-18-2011, 05:50 PM
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You sure the center set screw wasn't screwed in further than the other polylocs were, causing it to bottom out? The threads munged on that stud or lock?
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Old 09-18-2011, 06:29 PM
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Nope, the first thing I did was back out the set screw so it was held in place by 1 rotation of threads. After I cheater barred the poly lock 1/2 turn I spun the set screw down and it locked in place.

I spun the engine over and watched the valve train and every thing moves as expected; no problems I could see.

bt
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Old 09-18-2011, 07:20 PM
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Prime with crank rotation by hand. The valves could have been adjusted wrong. Did you do it correct like this ? Not just turning to base circle.

Go in 90 degree turns , engine primed.
At TDC position , Adjust intake valve on cyl , Adjust exhaust vlave.

1 , 2 , 8
8 , 1 , 4
4 , 8 , 3
3 , 4 , 6
6 , 3 , 5
5 , 6 , 7
7 , 5 , 2
2 , 7 , 1

Last edited by spinn; 09-18-2011 at 07:47 PM.
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Old 09-18-2011, 08:24 PM
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Does the poly lock in question sit at the same height as the other poly locks?
Can you see the lifter?

The lifter in question may have bled down and collapsed while you were trying to adjust it.
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