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andymarkv · 04-29-2018, 08:30 PM

Question about diagnosing a worn cam....

A buddy of mine inherited a square body C10 with 454 & th400, 4.10 gears. (swapped in 14 bolt full floating rear, truck was used to tow) His uncle had rebuilt the engine 30 years ago, but we do not know any specs except the visible; holley vacuum secondary carb, no name single plane intake, stock HEI dist, generic long tube headers, aftermarket chrome engine tin, stock rocker arms, maybe aftermarket valve springs & cam (there are manley push rod guides, so there may be other aftermarket parts in it)

It runs and idles OK, but has no power at all.

60psi oil pressure on a mechanical gauge, no overheating issues, oil looks good, new plugs wires cap & rotor
WOT is achieved when the gas pedal is pushed to the floor, the secondarys are not stuck shut
Compression check, all cylinders about 150psi
Vacuum at idle, 15

I used a piston stop to determine “true zero” on #1. True zero was at 3-4 degrees advanced on the aftermarket chrome timing tab
We set the timing to a true 16 degrees advanced, 35 degrees total, all in at 3000rpm

We heard some lifter clatter so we pulled the valve covers to look for a worn lobe
All valves moved about the same amount, no visible difference detected, BUT all rockers were loose when on the base circle.

We put #2 on the base circle (intake open, intake closed and a little further) and adjusted to zero lash, spinning the pushrod while tightening until initial bind, then another 1/2 turn on intake and exhaust valve.
Performed the same procedure on #4.
Did a compression test again on 2 & 4, zero compression. Checked the rest of the nonadjusted loose cylinders, still 150psi. Loosened the rockers, compression came back but the rockers were loose

At this point we figured we it was possible that the cam was worn, even on the base circle. Most likely this engine has aftermarket valve springs and has not had ZDDP/zinc additive it its oil since it was removed from all “off the shelf” oils.

We removed the cam and lifters….and they look PERFECT! No detectable wear whatsoever. The cam bearings look beautiful as well.(engine is clean inside as well)
What did I miss? It seems slightly hard to believe this cam is worn 100% even on all surfaces?
I don’t want to stab a new cam in this thing if I have totally missed the diagnosis and there is some other problem.

04-29-2018, 08:30 PM	#1
andymarkv Registered User Join Date: Jun 2010 Location: Upstate NY Posts: 66	454 Worn Cam Diagnosis, what did I miss? Question about diagnosing a worn cam.... A buddy of mine inherited a square body C10 with 454 & th400, 4.10 gears. (swapped in 14 bolt full floating rear, truck was used to tow) His uncle had rebuilt the engine 30 years ago, but we do not know any specs except the visible; holley vacuum secondary carb, no name single plane intake, stock HEI dist, generic long tube headers, aftermarket chrome engine tin, stock rocker arms, maybe aftermarket valve springs & cam (there are manley push rod guides, so there may be other aftermarket parts in it) It runs and idles OK, but has no power at all. 60psi oil pressure on a mechanical gauge, no overheating issues, oil looks good, new plugs wires cap & rotor WOT is achieved when the gas pedal is pushed to the floor, the secondarys are not stuck shut Compression check, all cylinders about 150psi Vacuum at idle, 15 I used a piston stop to determine “true zero” on #1. True zero was at 3-4 degrees advanced on the aftermarket chrome timing tab We set the timing to a true 16 degrees advanced, 35 degrees total, all in at 3000rpm We heard some lifter clatter so we pulled the valve covers to look for a worn lobe All valves moved about the same amount, no visible difference detected, BUT all rockers were loose when on the base circle. We put #2 on the base circle (intake open, intake closed and a little further) and adjusted to zero lash, spinning the pushrod while tightening until initial bind, then another 1/2 turn on intake and exhaust valve. Performed the same procedure on #4. Did a compression test again on 2 & 4, zero compression. Checked the rest of the nonadjusted loose cylinders, still 150psi. Loosened the rockers, compression came back but the rockers were loose At this point we figured we it was possible that the cam was worn, even on the base circle. Most likely this engine has aftermarket valve springs and has not had ZDDP/zinc additive it its oil since it was removed from all “off the shelf” oils. We removed the cam and lifters….and they look PERFECT! No detectable wear whatsoever. The cam bearings look beautiful as well.(engine is clean inside as well) What did I miss? It seems slightly hard to believe this cam is worn 100% even on all surfaces? I don’t want to stab a new cam in this thing if I have totally missed the diagnosis and there is some other problem.