"Let me tell you - it is not rocker noise that we here; it is somewhere in between rod knock (loud) and rocker noise (soft and clicky). With the motor running, if I back off the #5 exhaust rocker until the valve is not being depressed at all, the noise almost completely goes away."
Rod knock starts off "soft and clicky" but it's hard to hear. When it gets loud there goes the oil pressure/flow.
If the crank is "ok", bearings and a high volume oil pump should work.
wws
I pulled the head last night. Found that the top of the #5 piston has been making contact with the head - the top of the piston was pretty beat up from the piece of exhaust valve that came off, but I thought it would be ok. Now I see that some of the raised bumps are making contact at the flat area of the head - not the dome where the valves are but the squish area at the back. I'm going to replace the piston, rings and bearings and check to make sure the rod is ok. I don't understand why baking the exhaust rocker would make the problem less - maybe the pressure in the cylnder from not letting the gasses out is keeping it from touching as hard?
The bumps are small - maybe the size of #2 pencil lead, but there are several and they are shiny from hitting.
Will post a followup when it's all back together.
Thanks again for all the help - lot of smart people here.
What I want to know is what genius (not) thought it was a good idea to stick the distributor right in the middle of an oil passage anyway. Another reason I rarely touch anything Chevrolet. Shims on starters, distributors in oil passages, connecting rods that are too short for a model airplane engine, oiling through pushrods, skinny lifters that limit the cam profiles that work... GAH!!!
I wonder if in the process of pounding that piece of valve around, the #5 piston hurt itself. Cracked maybe? Or egged out the wrist-pin hole. Bent the #5 connecting rod slightly?
When you back off the valve on #5, you're basically shutting down that piston so its not getting combustion loading. See if the sound goes away when you just pull the #5 spark plug wire. I'll bet it does....
My problem is fixed. Once I got the piston out and examined it on the bench in good light, I saw that the bumps on the piston were actually pieces of the exhaust valve that were embedded in the top. The biggest one was probably sticking up 1/16 of an inch, overall there were 2 or 3 pieces. Just enough to make noise when running.
Thanks for the advice on the piston; I replaced it with a new one and new wrist pin, with new bearings and rings. The bore was actually in good shape, a few passes with a hone cleaned it up.
Put it back together - runs like it should, no noise, oil everyplace it belongs.
Thank you to everyone who helped me get through this - the truck was done today and is needed for corn harvest, which we started today as well.
replying to jmweb, Gerald Pate wrote: first off is this a early model Chevy motor.if so it could be that the wrong cam berrings were installed.also check the distributer.if after market it could be it needs O rings to create a seal so oil is forsed through the galleys.or at the start of the motor a betting could have spun blocking the oil hole.
this is a long time ago, but i just ran into this and it sounds similar, so maybe it'll help someone. rebuilt one of these motors out of a 95 topkick. the distributor was bad, so I bought napa direct replacement PN# NNDST1830. no knowing, it cut off oil flow in the main galley, but shows oil pressure. after rebuilding the motor again, i machined the new dist. to stop the oil passage blockage.
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