1968 chevy 250 I6 camshaft and lifters

[dennislbrooks]

Does anyone have experience with any cam lobe wear on a GM 250 6 cylinder engine. The engine upon starting seems to run fine but when warmed up it sounds like a burned valve(s) popping.

Prior I had cleaned and inspected the head and there was not any indication of a burned valve. All valves seemed to have good seat contact and I replaced the oil seals.

I just pulled the valve cover and 3 of the rockers were dry on top with all the rest of the rockers full of oil. 1 and 2 intake and 5 exhaust.

I removed the lifter on 5 exhaust and it may/may not be pumping up with no oil going up thru pushrod. Push rod is not plugged. The bottom of the lifter shows no wear.

I will pull the other 2 lifters and also check all for comparative lift. Worst case scenario is I pull the cam. Engine is on a stand.

At this point I may just have 3 bad lifters. Engine does not burn any oil and has been rebuilt in its life. It has HEI.

[geezer#99]

Reset all the valves to zero lash.
Then give each one 1/8 turn.
See if that eliminates the popping.

['68OrangeSunshine]

What oil were you using?

[dennislbrooks]

Geezer I will remember that when I get back to that.


68Orange --- Oil is 5w30. It was probably straight 30w in 68. I dont have 30 miles on this engine. I cannot say before.

Too cold to work on today.

['68OrangeSunshine]

It may be too late now, but modern oils have no, or very little ZDDP. [?Zinc Di-something, di-something-else Phosphate] The zinc was corrosive to catalytic converters, so the petroleum industry had been gradually decreasing it in mainstream motor oils over the last 15 or 20 years. Since 1968 trucks have no Cat, you don't need zinc-free motor oil. Zinc was included on old formula oils because it provided better lubrication in tight areas, like cam/lifter/pushrod/rocker arm/valve trains. More modern builds with roller rockers don't need zinc or zinc additives.
If you've been using post-millennial oil formulas in a 1960s engine, you've been slowly wiping out your cam lobes and lifter surfaces. [Or the PO was.]
You could either use regular oil but with a ZDDP additive or use Valvoline SAE 30 VR1, which has the proper zinc levels for this vintage engine. I used to run VR1 50-W20 in the winter. Thin oils might be easy to find, but these older engines used thicker viscosities.
Try an oil change with the right viscosity and zinc additives, when the weather permits.

[vince1]

I removed a well running 250 and there had not been any indication of valve problems. When the head was removed the lobes showed that they were still doing their job but possibly not have much life left on them because of indications that the surface hardening was gone. If the engine was original to the truck it would have had just over 100k miles on it. Being over fifty years old the seals were gone and oil rings had seized in the pistons.

So is this engine a used one that you just acquired?

[dennislbrooks]

Yes, I just acquired it. The valve lift appears consistent on all lobes with the lifter faces pretty square. I will check the plugs for any differences.