98' Vortec 350, just built. Stock rotating assembly except replacement flex plate and one rod. Has 25 miles on it. Sounds like a bottom end knock right off the bat. Sounds heavier like the bottom end, like a wrist pin or something. Frequency is about every crank rotation. First description that came to my head is a rattle, like a diesel motor but not as fast of frequency. Most noticeable at idle, can only hear it with a stethoscope at higher RPMs(maybe just because the engine drowns it out). With stethoscope to valve covers, sounds louder on driver's side. Not coming from accessories or pulleys. Retorqued flexplate bolts.

Pulled the engine back out and stripped it down. The oil pump pickup was at the bottom of the oil pan. Just put in new gauge cluster so not sure how accurate the oil pressure gauge was however no signs of oil starvation besides some premature piston wall wear marks, identical on all cylinders. Otherwise bearings don't look oil starved at all, even the cam bearings look great, crank endplay is good, wrist pins are not wobbly, everything torqued, rockers look fine, no signs of other premature wear.

I'm going crazy. I don't want to put this back together without finding something.

Sorry for the chunk of text. Ever hear of something like this before? Thanks.

 

Couple of things come to mind:

It's not a rod knock. You would have nailed that down pretty quick and it sounds like you know your way around and engine so I wouldn't insult you with such a comment.

Main bearing clearance (one or more). If it's wide you can get a knock, but it'll be deeper than a rod knock and half the frequency. Will often fade away to almost nothing as you rev it up.

Wrist pin..... ruled out.

Last one that you haven't thought about...... and I'm keying off this comment:

Too much piston-to-wall clearance. Piston slap, basically. Happens like a banshee on motors drilled out 40-over but fitted with 30-over pistons. Not saying this is your problem, but we had one just a couple months ago here and that was the problem.

 

Thanks a bunch for the reply. I'll re-plastigauge the main bearings and measure piston to wall clearance this Sunday. I'll keep you posted.

 

Main bearing oil clearance is perfectly fine. Re-measured bore, highest measurement was 4.002". Here's what I got for piston to wall clearance;

Cyl 1=0.004"
Cyl 2=0.004"
Cyl 3=0.0037"
Cyl 4=0.0054"
Cyl 5=0.005"
Cyl 6=0.004"
Cyl 7=0.0045"
Cyl 8=0.006"

According to my shop's internet database for a 98 Vortec 350, you want no more than 0.002" piston to wall clearance--seems awfully small to me. From what I've been reading 0.001" of clearance per inch of cyl bore is normal.

In you guy's experience what is normal piston to wall clearance?

 

That's a little wide, but it's not likely your problem. You need more than a few extra thou to hear piston slap as bad as the sound you described. You need like 10-12 thou, unless the pison skirts are collapsed (bent inward). Time to look elsewhere for the noise, I think.

 

I agree, I don't think the noise has to do with piston to wall clearance. Gah.. its just so strange... The whole bottom end passes inspection...

 

I had a BBC 495HP 454, one of a pair in a Bayliner boat, that made a bottom end noise that I thought might be rod knock. It made the noise from the very first start up and didn't get better with a 25 minute run in. My machinist looked at it with me and we were both puzzled. The block had been completely checked and prepped. The rotating assembly had been balanced and clearanced. I primed the oil system before start up as I always do.

I pulled the motor and went back through it with my machinist. Ended up being the brand new Melling oil pump. Melling paid parts and labor for the redo. That's the only issue I've ever had with one of their pumps.

 

Very interesting. Its a brand new Melling oil pump I was using as well. I'll check it out.

 

Well, I ended up getting the block bored 0.030" over and getting new hypereutectic aluminum pistons. The good news is, so far no more knock. I have about 700 miles on the motor now.

This seems to suggest that 0.006" of piston to wall clearance can indeed cause a knock. I didn't suspect it.

HOWEVER, there is one other possible cause. I replaced a rod during my initial build and had attempted to press the wrist pin in instead of heating the rod. Half way in I realized that the pin wasn't going in the rod straight and that it was impossible to press the pin in straight because the inside ends of the wrist pin bore (where the rod small end sits) are slanted. Thus, the rod couldn't be supported flat as the pin was pressed. It didn't look like I damaged anything but somehow I might have damaged the piston.

So what I learned from this was; When installing pistons its safer to heat the rod instead of pressing the pin and to check piston to wall clearance and piston diameter if reusing pistons.

 

Probably a combination of both.
002 isnt really tight there are forged pistons out today that take .002-003
Hypers have a very tight fit. that diesel noise is slap

When you said the pickup is on the bottom of the pan Im guessing you know its not supposed to fit right up against it needs to be some clearance there so the pump doesnt cavitate at rpm

 

Did you or anyone prime the engine oil system before starting it? Just curious.

 

Im not suggesting that superstitious people don't do it, but I dont think priming accomplishes anything but washing out assembly lube that will protect parts at startup.