I am having a problem with my 1981 z28. Halfway from driving it down from Louisiana to Florida the problem occurred-I have replaced the fuel filter-put in a thermostat adtrer it over heated and checked oil pressure and got that fixed. I took it to a mechanic and he says that the sound is also come from the lower engine and i should replace the "Wrist pin's". I also have taken the valve covers off and discovered the 3rd and 4th rockers on the right were very loose and on the right the 3rd rocker was at 45 degrees crooked...i tightened them and straitened them after setting the damper/torsional to 0 and the problem knocking has lessened but is still there after 2500rpms..CAN SOMEONE PLEASE HELP ME DETERMINE WHAT I NEED TO FIX THIS PROBLEM. Everything i have on this car is original and i would hate to rebuild/replace and do not trust anyone working on my car cause they just wanna take my parts... THANX!!!

 

Setting the harmonic balancer to zero and adjusting the rocker arms only gets the rockers on #1 and #6 adjusted. You need to take the engine through it's firing order and check the other rockers when the lifters are at bottom, and see if there are still others that need adjusting.
And if you had a rocker that was so loose it was found at 45 degrees, you've got possible big problems! That loose tells me at the least the adjusting nut isn't staying tight, and worse case, the cam and lifters are well worn, or the stud is pulling out of the head!
Wrist pins could be a problem, but if they are you'll need a complete engine rebuild, not just a "wrist pin replacement". Some mechanic is trying to get your engine apart, and then the price will go up quickly once he comes back with a total rebuild. I'd steer clear of anyone who tells you it only needs wrist pins.

 

Humidity+ a bad batch of fuel? Detonation?

 

Big difference between a "ping" caused by detonation, and a knock in an engine! They don't sound anything alike. Even a lifter rattle doesn't sound like engine knocking. Nothing easy or cheap about an engine knock.

 

Undoubtedly. Well the damage has already occured when it overheated. I imagine that the stress it underwent loosened the attachment points between the pistons and rods as your mechanic suggests. When an engine overheats one thing it produces are the same as detonation conditions. The results of overheating are severe even overheating once, New oil viscosity can= zer0 even if still gold or new looking, friction, massive forces, extreme heat, extraneous expansion that doesn't return to normal shape but to a new modded shape or size, bearing death where heat may have visited, . All enemies to a healthy engine, yes possible all these events could occur. It sounds like a nightmare, but your engine runs with a 'knock' so maybe the pins, and extra piston movement may be the knocking. I would recommend a rebuild to rescue your engine and parts. Race engines are rebuilt all the time in minutes and parts are kept like new as only the wearing parts are worn and replaced primarily i.e. rings, bearings and component parts only if needed secondary. But I would do it myself as long as I knew how to identify things that need to be changed or machined etc. The only way to tell what is wrong is if you pull the entire engine apart and inspect or notice all the damage. You can buy a cheap GM crate to stay running and build you a performance piece in the meantime. Just ideas not telling you what to do by no means. Just my 2cents. Hope anything helps. P.S. your cam could have even been damaged but without opeining it up there is no way to tell.

 

Even a "cheap" GM crate engine will cost more than a rebuild at a good engine builder! I just got my 350 back from my local performance builder and it was under $1500. That was boring, turning the crank, align boring. squaring up the bores to crankshaft/cam centerline, decking the block, opening up oil passages, polishing the crank, sizing rods, balancing the rotating assembly, blueprinting the engine, a new Crower cam degreed in, and assembling the short block. Didn't need head work, as my heads were new, so just a quick check, and measuring the combustion chambers to determine what piston to use to get to 10:1 c.r.
If a person is in a big hurry, then it's probably best to get a running cheap used donor engine to drop in while building the old engine. I paid $100 for the 350 4 bolt main donor engine that I had rebuilt. 350's are often found as good running old engines for less than $500.

 

Hopefully you didn't develop a vacuum leak!

 

No one (who is a professional) wants to take you parts. Original 81 Z28 parts are not really in demand unless They are restoring a 81 Z28. If you really think they are just after your parts then just require them to return all replaced parts. Here in PA if you request the old part they are required by law to return them.

I disagree with 1971BB427. A base 4 bolt main GM crate engine can be bought for around $1600 to your door and is complete long block with a GM warranty. By the time you do a valve job and pay to have the motor assembled you are going to have a easy $2500. The new crate engine with your original parts will only output about 250 HP but if you upgrade your exhaust and intake you can hit 280hp. For $2100 you can get almost 300 factory reliable HP with a GM warranty. Even the 250hp is a nice upgrade from your stock 350 which I think was 165 hp? These are old school Gen1 engine built with modern tolerances and ready for 200,000 miles of service.

If you want to keep the car all original go to a car show and ask around for engine builder. The car show guys will tell you the good and the bad and they will tell you who you can trust.

Unless you know what you are doing you should stop adjusting things. When you pull a valve cover it is normal to have some rockers sitting at and angle and with the motor not running a loose rocker can be normal. There is a set procedure to adjust valves and when you do it you really want to do all of them with the same procedure.