Nocking at high RPMs

[WE7X]

Since you already have the pan off, it would be foolish to not pull all the rod bearings and have a look at the crank journals.
Beg, borrow, or buy a micrometer that will handle that diameter crank journal, and measure them all.
That will give you a chance to see if any others are scored or badly worn, and you will likely have to buy a whole set of bearings anyway.
The manuals show the dimensions of a standard crank. You can gauge how much the crank journal is worn, and compare it to the others.

CAUTION:
Since this is your first 'rodeo', so to speak in this, realm:
Understand that connecting rod caps ONLY go on one way.
They are bored out of a casting, but the hole is not exactly centered between the rod bolts, so if the cap is put on 'backwards' the circumference of the bored hole will not necessarily be exactly round.
Nearly all rod bearing caps have some sort of marking, grooves, numbers or notches, that will indicate the correct alignment.
If you get them installed wrong, you Will do damage to the bearing and/or the crankshaft.
If installed incorrectly, you probably will not be able to turn the engine over with a standard ratchet. If the engine does turn over, but seem to crank very slowly, I would also suspect an incorrect bearing installation, even if it is only one rod bearing.
Do not attempt to tow the truck down the road to try to get it started.

If you are already aware of this possibility, then GREAT and I have wasted a few words, but if not it might save you a whole lot of frustration nd expense.
Good luck,
Rod Johnson
Issaquah WA

401M in a RM7500 chassis

[John Mann]

Rod,

I, too, am an amateur at all this. Your words were not wasted on me. Thanks for the detailed info.

John

[surfamtn]

WE7X--- not wasted at all! I'm always weary of Pitfalls and things an experienced guy might take for granted. I'm proceeding with caution but keep the wise words coming-thanks!


I'm trying to load pics from y iPad..... grrrr

[surfamtn]

Here's the pics I just took. The one with the screwdriver is pointing out a noticeable gap but I guess that gap was previously filled in by the shiny silvery residue in my oil pan.

[WE7X]

I cannot be sure, from the pictures, but it looks like that bearing has spun in the connecting rod. I don't see any trace of the tabs that should have fit into the notches in the connecting rod and the rod cap. It also looks like there are scrape marks on the outer surface of the bearing shell.
I would strongly suggest pulling the others, to have something to compare. BE SURE to mark the caps, so they go back on the same connecting rod, and in the same orientation, when you finally assemble it.
Rod J
Issaquah, WA

[surfamtn]

Yea... my thinking is that it spun as well. What I have now are two semi circles that are just a piece of metal, I was thinking they would actually be a bearing?
I'll pull another so I can compare.
Assuming the that it spun: would I expect significant damage to the rod/rod cap or crank journal? To recount: this noise started abruptly after driving w low oil level


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[TJ's GMC]

Judging by the wear on the notch of the bearing I'd say it spun. It is pretty scarred up. Luckily you caught it quickly. Pull All of the caps off and check them over. Thing is if ran with low oil that can cause major problems. 1 quart low isn't bad, but much more and you can had issues. Just cause one was knocking doesn't many any others aren't going to or are far behind. I'd be checking the whole bottom end over. Pulling the mains and rod caps and checking all bearings. You have the motor Out and Apart so why not? Bearings are "cheap" insurance. I'd also have the crank checked over, mainly that one journal where the bearing went south. You may have to have over size bearings installed on that one. My 305 didn't have a knock but the bearings where worn down. So a standard set was fine, but slight damage needs to be ground out and may not be visible to the eye.

[TJ's GMC]

Looking at the pics again I noticed a groove in the bearing that runs straight along both bearings?

[surfamtn]

Great, thanks for the advice guys. I'll pull them all and take the crank in to a machine shop.

Now... how do I get the crank shaft out? Lol


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[TJ's GMC]

Originally Posted by surfamtn

Great, thanks for the advice guys. I'll pull them all and take the crank in to a machine shop.

Now... how do I get the crank shaft out? Lol


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It's gonna be a pitb. But doable. I did this in August. haha Pull all the main caps off. The rear main is a large block and will take some tapping with a hammer to remove. Like wise the rest of the mains. You'll have to remove the water pump/timing cover housing off so you can remove the crank. Not as bad as it looks. Then remove all the caps. As stated, Mark EVERYTHING!!! All the main caps Must go back on as they came Off. And same with all the rod caps. I just number them and make a mark on the left side reminding me which side they way they go on.