Nocking at high RPMs

[surfamtn]

Hey gang, rookie home taught floundering mechanic here again.
Had Rusty out for a drive today, noticed a Knocking sound at high RPMs, more so under load (ie. up hill or quicker acceleration). Seems to be left side, under the valve cover, but its a guess.

I was a dummy and neglected to check the oil level recently, it was lower than I care to admit but still registering on the dip stick.
Added oil and the sound improved but has not resolved.

I plan to look under the valve covers this weekend. What am I looking for?

I realize this is a vague and rudimentary description, but I made a disclaimer in the first line of this post. I have never been inside an engine before! Lol.

-thanks

[TJ's GMC]

Is it really a Knocking, or is it the normal clatter from the rocker arms? If the rockers are looser than they are already the tap tends to be much louder. Have two I need to tighten a bit on my 305.

[Jim A]

It is hard to locate a knock at high speed and under load very accurately.
I would look first at ignition timing to make sure it is not too advanced. Fuel problems are a possibility as are spark plugs.
I sure wouldn't take too many things apart without some guidance from a friend or relative with mechanical experience.

[jimjaz]

Possibly engine timing like Jim A describes. You will need a timing light to check this. The distributor most likely has a vacuum line coming off the back side of the manifold and goes to the distributor- this provides for more vacuum advance (less throttle/gas) or less vacuum (more throttle/gas). The distributor also has springs under the cap and are "calibrated" to have a specific mechanical advance at a given RPM to advance or retard mechanical timing, or advance. If you don't have a timing light, then;
1- take off the valve cover on one side and check for anything in the valve train that may be loose, like a rocker stud or the nut that holds the rocker in place which would cause the rocker to make a lot more noise than the others. I would say that given your admitted lack of experience, you would be wise to elicit the support and knowledge of either someone you know, or a shop in your area that is capable of diagnosing a big 'ole V6 that is definitely old school.
Good luck! - hope that it is something that can be identified and resolved without emptying your wallet.

[TJ's GMC]

If you don't have a light you can always retard the timing a bit by ear. Retarding the timing Lowers engine idle speed and advancing Increases. Not gonna hurt anything to play with the timing a bit. But I would retard it a little and see what happens. You can always put it back.

[surfamtn]

So after some valve adjustment (a few needed small adjustment), learning how to use a timing light (it was at 10deg, turned back to 6ish) and new plugs, wires, cap, rotor and points, it starts quicker and runs smother at idle .... Buuuut no change in the nock at high RPM under load.

I'm resigned to the diagnosis that I've burned a rod bearing.

Stay tuned for panic posts about pulling a Big block V6 in the driveway!
And the subsequent need for gaskets and bearings!


Sent from my iPhone using Tapatalk

[TJ's GMC]

A gasket kit can be purchased from O'Riellys that works for the 305's. Rod bearings can be purchased from them as well. As for Mains you will have to get them NOS on Ebay. Good luck, and ask away.

[timmy]

I have one in SW WA that purrs like a kitten. My project wont need it and it could use a good home. Besides, I'd like to have the indoor storage room. Make me an offer.

[surfamtn]

Thanks Timmy, I'll keep your offer in mind.

In the mean time, I pulled the motor last week and today got around to taking the heads off, valley cover, and generally started cleaning the gunk off the top half.
I got my first look at the bottom half after I removed the oil pan....low and behold...

The number #4 piston rod was loose!

I removed the nuts that hold the rod on to the crank shaft (what's the term for that?) they sure seemed torqued down tight, and found the bearing/bushing was loose and pretty worn looking. I suspect his was the cause of the noise?!?

So, now... how do I go about replacing this worn bushing/bearing? I would like to just replace the one that is shot, am I foolish in not doing them all?
How do I go about sizing a replacement with out pulling the crank shaft and taking to to a machinist? Fwiw the crankshaft looks and feels fine in this spot.

As a reminder this is the first motor I've ever pulled and the first crankshaft I've ever laid eyes on. Lol

[TJ's GMC]

If the bearing is just really worn then a set of standards should be ok. I did that with mine but it didn't have a knock. The unaided eye can't see the small damage that could have been done to that crank so I would have it checked and ground if needed. I am not even sure if .010 over r Normally if you do one you may as well do all. I ended up doing rods and mains in mine! Can you post some pics so we can see?