Hey guys, would love to get your opinion on this.
I rebuilt my 350 out of my 1975 L48. New everything except the block and the rods. Everything else is new, new crank, pistons, heads, fuel pump, accessories(water and fuel pump). Only thing reused were the rods and the block itself.
Started up great, broke in the engine, changed the oil after break in. Oil looked good, when I turned the filter upside and drained it, it was metallic, as I would expect.
It has 40 miles on it total. I’ve noticed a knock at cold start up that goes away after 2-3 minutes. Tracks with idle and sounds like a rod, near the front of the engine.
Dropped the oil pan and checked the torque spec on every bolt, main and rod. Everything was torqued to spec, no loose cap.
I started shaking the rods to see any movement. I know rods are supposed to slide back and forth, parallel to the crank A LITTLE BIT. But rods 3 and 4 slide back and forth so much that you when you shake it, the rods will hit the crank and make a loud knocking noise. When you grab and shake the other pairs of rods there is very little movement and def not enough to generate an audible noise.
Has anyone encountered this problem before? My assumption is my brand new crank from scat was not machined correctly.
Can I run the engine with this issue? The knock goes away when warm.
This is assuming that play is the source of my knock.
thanks

 

An engine needs to be more than spotless before assembly. Unless YOU scrubbed each piece several times, it is never clean enough. Never assume any part is clean enough. This is my first thought.

Second would be how you verified the crankshaft bearing clearances. What were they when you checked them?

There are obviously other possible issues but from here, these are what I would worry about.

 

I know this is a hot topic that many people disagree with, but I use Plastigage to verify the clearances. I trusted that the crank and bearing were manufactured to spec, and Plastigage verified clearances to be within band.
But when you jiggle the rods 3&4 there is no movement up and down, only move along crank, front to back, it’s as if that section of the journal is too long.
I pulled the rod cap off rod #3 and the bearing looks fine. There are some marks, (engine must not have been clean enough) but there is plenty of babbitt present.

 

I also wanted to add that oil pressure is 75 pounds at start up and will drop to 20-25ish hot idle, as soon as you touch the throttle it’ll jump up to 60. Also, when I dropped the pan there was zero evidence of bearing damage(no flakes or silvery sludge)
I also checked the rear main and it looked fantastic.

 

that back & forth is "side play" ... depending on which reference, spec is about 0.008" - 0.014" ...
... (w/ rod-pair spread tight & feeler gage between rods) even if it's as wide as 0.030" it's probably Not an issue.

dozens of possible causes: e.g. new cam lobe(s)-lifter(s) wiping, crank end play (spec 0.002" - 0.006" total fore-aft) ...
... timing set tapping against timing cover/block (some wider/double-row sets or dented timing cover), fuel pump/rod etc

-addendum-
? maybe you installed "Backwards" one or both rods on that "loose" journal ... each rod has a "chamfer" on one side that must mate ...
... with corresponding radius aka fillet where journal adjoins cheek. Such an error typically wrecks rods & crank.

Also, both OE L48 & most replacement type pistons have an offset wrist pin to help minimize piston "slap" in bores ...
... If piston installed backwards in bore ... it will typically make noticeable "slap"

 

The gap is .0015, which isn’t that wild judging against the specs you provided. All the other rod caps are less than .0010.
Is there a way to find piston slap or a wrist pin knock from under the car?
It has hypereuretic aluminum pistons which I had heard are less susceptible to piston slap.
Im going to pull the valve covers tomorrow to rule out a flat cam lobe, but the noise seemed lower in the block than the cam. I also need to try and see if it’s the fuel pump. Any ideas on ruling that one out?

also, the pistons had 2 valve reliefs, so it was easy to install the right way. I double checked the rods and they are paired next to each other correctly.

 

? What is manufacturer and part number of your new hyper pistons ?

With that info, I can probably find piston specs and if they have offset pins.

From below motor, It MAY be possible to look up into undersides of pistons ... there MAY be a casting pattern differing from right-to-left ...
BUT, it really matters to First learn if the pistons have an offset pin ... or not.

 

I have never seen it, nor heard it, but people swear by it, a broken fuel-pump pushrod. Sounds just like a knock. Tip wears off the cam end of the rod due to inferior metals of one or the other parts surface.
Its pretty rare but does happen I guess.

 

video of the knock.
As for the piston wrist pins, I had a local shop install the pins that came with the pistons. I’m not sure if they are offset style or not. I’ll try to find the part number for you. I know they were speed pro, flat top two valve relief 30 over

 

Hard to say but if it goes away after it warms up that is a pretty good indication of piston slap.

 

Check for an exhaust leak at the manifold or header gasket.

 

If you did not check your bearing clearances for your crank/rods when installing, sounds like it is time to do that.
Yes a mic is much more precise, however when doing this under the engine with oil dripping on you, plastigage gets the job done. If it was mine, I would double check the clearances with plastigage so you can rule a bearing mix up out. If you find you had a bearing that is the incorrect size, then you can likely just replace the bearing now. If you run it much more, you run the risk of needing to machine the crank.

 

Just to update everyone. I have checked the rest of the mains today. I still need to check rod caps 5 and 6 but I doubt that I will find anything. All the bearings so far have looked great or good. No evidence of wiping. I checked the clearances with Plastigage before install. But, if a knock is from too loose a clearance, wouldn’t the associated bearing show damage?
The worst looking bearings were from 3 and 4, the rods with the side play. They still looked good, shiny babbitt, but they had a score mark in the middle.

For a potential exhaust leak. I checked all the header bolts and they are tight. I guess potentially the gasket could have blown out? I will pull the passenger header off tonight and inspect the gasket, since the knock is louder on passenger side.

Before I took the oil pan off. Using the screwdriver to the ear test I noticed the knock was loudest around the passenger motor mount. That general area. I did not hold it directly to the fuel pump, because honestly that as an issue didn’t even cross my mind.

When I heard the knock, I immediately thought rod. I am really confused that I have not found any issues on the bottom end yet(unless it’s rod 5 or 6)

 

This is rod cap #3 all other bearing looked better than this one. Looks fine to me though.

 

Not great but as you said it should be fine. That sound actually sounds like it’s a valvetrain noise to me. If you pull plug wires does the knock go away?

 

Just to update anyone that is interested.
after checking every single bearing and finding no issues, put everything back together and started it this morning. Only knocked for like 10 seconds. Not long enough to do any spark plug testing or anything. I listened with a engine stethoscope on the fuel pump and it’s fine. This leaves me to believe that it’s a piston that is slapping or it’s a wrist pin. Drove it 10 miles today and let it up to 3500rpm, pulls hard and not a single issue.
Annoyed that I spent many hours dissembling and investigating to not find anything conclusive, but I’m also happy that I don’t have a wiped bearing.
Thanks everyone

 

It does seem like a piston slap but a hypereutectic piston is more commonly fitted with a fairly tight piston to bore clearance. Any chance you measured that at assembly? Those type of pistons are not known to expand very much when raised to operating temperature. Could they actually be forged pistons???

 

if that is your worst rod, it wasnt a rod knock. if main caps look good, leave upper mains alone. they are good too. crank is being pushed into the main caps, away from the main saddles. i would replace the rod bearings if they were all pulled..check every bearing shell. i once bought a set of 8 .010 under bearings from manny, moe and co. girl gave me 4 std and 4 10 under. i didn't check em. wrecked a 400 small block crank.

 

i agree 100% that’s one of the reasons I went with hypereutectic aluminum. Less susceptible to thermal expansion, (15% less expansion than cast) plus it’s a stronger metal. I personally did not check the machine shops work(bored it out 30 over). And since it’s only on one piston(so it sounds) I wonder if one bore is just a touch too big and I happened to get a piston that is on the small side of their manufacturing tolerance.

 

I really appreciate you following back up and reporting the bearings looked good.

please let us know if you find anything else more conclusive. I have a similar sounding noise, although mine is making noise from around 1500rpm to 2k rpm, so I would really like to know if yours ends up being an exhaust leak somewhere.
Are you using shorty headers or long tube?