L31 knock after rebuild

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TBMG270

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I have a 96 sierra 5.7l I blew up a while ago that got put on the back burner for a couple months. Recently finished the rebuild and got the truck running today. Motor has reground crank, new connecting rods and piston rings, new main, rod, and cam bearings, and new oil pump. Crank journals and rod bored mic'd out perfect, 2 thou clearance on all 8 rods. Mains were .0018 on #1 .002-.0022 on #2,3, and 4 and #5 was .003 (on the high side but from what I've read is within spec). All double checked and then checked a third time with plastigage. Everything was thoroughly cleaned/oil holes deburred/torqued to spec. Used plenty of assembly lube and primed the oil pump prior to start up. Flywheel and pressure plate bolts torqued and loctite was used. Started it up for the first time today and ran good except for light-ish tap/knock. I say that because it's not all that loud but it's deep. The sound is most noticable at idle but always there (hard to tell how loud it gets. The truck is fairly obnoxious but from what I can tell volume stays basically the same) and fluctuates with engine speed. Underneath the truck it sounds like its in the oil pan, especially towards the rear/oil pump area. You can feel the knock when touching the pan. I tried several different harmonic balancers with no change. Unplugged spark plugs wires one by one and nothing. Sound remains unchanged whether engine is hot or cold. 40psi oil pressure warm. 60 cold. Stumbled across this thread here, this guy posted a video of a knock his was making but never posted what he found out (if he ever did) and my knock sounds identical to this one. https://www.chevytalk.org/fusionbb/showtopic.php?tid/303314/tp/1/ anyone have anymore ideas? I ran the truck for 15 minutes or so initially and then drained the oil. Looked perfectly fine, put another new filter on and filled out back up with new oil. Shell rotella 10w-30. I'm thinking maybe wrist pin knock? I did reuse the pistons, new wrist pins tho. I cut the old rods with a dremel to release the press fit and I had a set of 30 over pistons laying around that I snagged the wrist pins from. Really hoping this is something stupid. Thanks everybody
 

Hipster

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Not liking what you came up with for clearance on the crank. .0018-.003 . Were the line bores checked? Either the mains are off or the crank work was sloppy. .003 is ok for a race engine but not what I like to see for a nice long living street build.. I didn't see a linked video and I'm not doing a wild goose chase through 2 different threads to find it but if you followed the advice of hammering on the balancer in those other threads you could have damaged the thrust bearing. Using wrist pins from one set of pistons in another is not something I would have done. Plastigauge can be hit or miss if dealing with an egged out bore and it takes some quality measuring equipment to find issues.

What was blown up the first time, what was found upon disassembly, and what was done to correct it? Unless you can provide some more information is sounds like a half-assed build that achieved half-assed results.

Cracked flexplate or the crank tapping on the oil dipstick also come to mind.
 
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GoToGuy

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When you say blew up. Thats too vague. Is this the same engine? I have a paperwieght connecting rod, broken in half and bent over 90 degrees. It being #4 and its mate #5 were all that held the crank together as it was broken at a 45 degree angle. Piston pieces went ballistic through the oil pan. Rod beams flopping in rotation knocked chunks off the cylinder bottom bores. It was so bad it was hilarious as hell funny. So you can imagine how short the rebuildable overhaul list was.
So what was dremel for again? Have you a mechanic and or engine guy like someone not involved in build for a fresh unbiased non anxiety opinion idea. Fresh eyeballs fresh ideas when you have a real big problem. Just a thought.
 

TBMG270

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Not liking what you came up with for clearance on the crank. .0018-.003 . Were the line bores checked? Either the mains are off or the crank work was sloppy. .003 is ok for a race engine but not what I like to see for a nice long living street build.. I didn't see a linked video and I'm not doing a wild goose chase through 2 different threads to find it but if you followed the advice of hammering on the balancer in those other threads you could have damaged the thrust bearing. Using wrist pins from one set of pistons in another is not something I would have done. Plastigauge can be hit or miss if dealing with an egged out bore and it takes some quality measuring equipment to find issues.

What was blown up the first time, what was found upon disassembly, and what was done to correct it? Unless you can provide some more information is sounds like a half-assed build that achieved half-assed results.

Cracked flexplate or the crank tapping on the oil dipstick also come to mind.
I get what you're saying. What I found when doing my research was that 1.5 to 2.5 thousandths was acceptable on the mains and that #5 was supposed to have a little extra because it's where the oil pump is, which made sense to me. I'm certainly no expert. I measured everything myself and had a machinist buddy double check my measurements. I just checked everything for out of round and taper. Don't have the money to spend at a machine shop honestly, this was never supposed to get as expensive as it did. The original motor spun 2 rod bearings after an oil cooler line blew out. The motor in it currently is not the same block. I used the same heads but the block is from another truck. That engine had no issues, truck was junk and motor was torn down and started as a project for a previous daily driver that ended up getting totalled and then the block just sat. All the cylinders were in great shape and within spec for taper and out of round. Can't remember exact numbers off the top of my head. Same with main bores. I did use a bore gauge that reads .0001 and not an inside mic. It. The crank was damn near perfect. All journals were spot on or .0001 out of round/tapered. Again, measured with proper tools. No cheap measuring equipment was used. The video I'll go back and find the link for the youtube video itself. I don't trust plastigage myself but used it anyways after I measured everything I needed to. And the balancer went on fairly easily, started a little bit by hand and then I just used the bolt and a ratchet to push it on. I tried 3 total, none of them fought me or went on difficult. Went fairly easily without much effort until pulleys lined up and they got snug and then torqued to 70ft lbs. And care to elaborate on why you wouldn't mis match pistons and wrist pins? Genuinely curious, I didn't think anything of it
 

TBMG270

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When you say blew up. Thats too vague. Is this the same engine? I have a paperwieght connecting rod, broken in half and bent over 90 degrees. It being #4 and its mate #5 were all that held the crank together as it was broken at a 45 degree angle. Piston pieces went ballistic through the oil pan. Rod beams flopping in rotation knocked chunks off the cylinder bottom bores. It was so bad it was hilarious as hell funny. So you can imagine how short the rebuildable overhaul list was.
So what was dremel for again? Have you a mechanic and or engine guy like someone not involved in build for a fresh unbiased non anxiety opinion idea. Fresh eyeballs fresh ideas when you have a real big problem. Just a thought.
I used a dremel to cut through the small end on the old connecting rods. Wrist pins slid right out and I didn't deform or break any pictons that way. Bought new rods. They were out of round and that's the one thing I took to a machinist, to get resized. He ended up having my parts for almost 2 months and never touched them so I went and got them back and bought new s***. And I just had an oil cooler line blow out and it spun a couple bearing, nothing to that extent. Block currently in use is not the same blocki "blew up"
 

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here's the video. Also, my sound isn't quite as loud as this one seems but sounds exactly the same. Especially from below the engine
 

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For future reference, machinists can resize a big end with the piston still hanging on it. So unless there a problem rod, destroying a set of rods was not necessary, and a shop can press the pistons on and off without destroying a set of pistons if they just have to do it that way. A shop can also order a stock re conditioned rod if it necessary to have just a couple. As you found out and you didn't say what or who's set of rods(aftermarket?) you bought but they usually need work also.

Pistons pins and piston pin bores are designed around an oil film thickness/clearance. new pin/old worn bore sometimes you're better off leaving what's working alone. Like anything machined there's tolerances and it's better to have a matched set. Not sure what the spec is on the piston pin bore off hand but on floating pins the clearance is less than .0005. They also need to be installed correctly. Some homebrew methods of trying to press them on can bend the piston to the point of locking it up on the pin.

I also run, not walk away from those "it runs great but it's sitting in the corner needing a rebuild" type of deals. Too many variables as to whether the block is any good.

Near perfect can chew up a bearing. Near perfect should have been polished.
 
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TBMG270

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For future reference, machinists can resize a big end with the piston still hanging on it. So unless there a problem rod, destroying a set of rods was not necessary, and a shop can press the pistons on and off without destroying a set of pistons if they just have to do it that way. A shop can also order a stock re conditioned rod if it necessary to have just a couple. As you found out and you didn't say what or who's set of rods(aftermarket?) you bought but they usually need work also.

Pistons pins and piston pin bores are designed around an oil film thickness/clearance. new pin/old worn bore sometimes you're better off leaving what's working alone. Like anything machined there's tolerances and it's better to have a matched set. Not sure what the spec is on the piston pin bore off hand but on floating pins the clearance is less than .0005. They also need to be installed correctly. Some homebrew methods of trying to press them on can bend the piston to the point of locking it up on the pin.

I also run, not walk away from those "it runs great but it's sitting in the corner needing a rebuild" type of deals. Too many variables as to whether the block is any good.

Near perfect can chew up a bearing. Near perfect should have been polished.
Yeah you see, the machinist in my area that I went to told me the opposite. He said the piston had to be removed in order for the rod to be resized (3 rods actually) and he told me that he had no way of removing the pistons without breaking or deforming them. I had no reason to doubt him, I'm not a machinist. He also held onto the parts for a little less than 2 months, I gave him a call and asked about it and his response was "I'll call you then, i didn't get to em yet." I understand this is his busy season but it was pretty unprofessional and frankly the guy was a dick so I went and picked them up, in the same box they were dropped off in. Only machinist in my general area (way out in the sticks and I can't drive anywhere without a truck) so I wasn't about to go back to him with the crank for work as well. From everything I read and what others told me on forums I didn't think one ten thousandths would've presented an issue anyways.

As far as pressing the pins into the rods I froze the wrist pins And I put the rods in the oven. That part went seemingly well. Nothing bound up on any of them, all were centered. Seemed ok but theirs a reason I'm asking questions on here. Not an expert. The rods I bought were scat by the way, again they all mic'd out good and we're round. I have a rod vise and torqued everything when I checked it. I've heard good things about them along with eagle products. I've also heard both brands are junk. At the end of the day it came down to cost so I took my chances. All 8 bearings I got .002 clearance. I don't think the noise is a rod anyways , it doesn't act like a rod knock nor does it quite sound like one. It does but its got a different note to it, more of dull sound. Not quite as high pitched. It's too constant and the sound doesn't change at all when unplugging spark plugs wires, when the engine is under load, when load is taken off, or between hot and cold. Theirs no difference any time. It is the same noise under any conditions. Just gets faster as rpm rises up until maybe 1800-2000? Not sure exactly, I'm pretty sure the exhaust is just drowning out the noise at that point.

And this block I'm using didn't "need" a rebuild. The truck had a rotted frame and no title. It did in fact run great before it was torn down. Was just a project for "***** and giggles". At the time I had a 4.3 truck I wanted to swap a v8 into, I was in no rush and I was 17 at the time and wanted to "build" it, as a teenager i was only concerned with making power, loud exhaust and doing burnouts. I just had to ****** it. That was the original plan for it until I was rear ended by a kid I went to highschool with. I was at a dead stop and he was driving 60-65 no brakes on his phone. Driving an f150 too, go figure. So much for that engine swap. This block was just in better shape and so I decided to use it. Far less miles on it. Cylinders weren't as far out and in better shape, bearing bores we're all still round. Plus it had been hot tanked already and sat covered with a bag and wrapped in plastic wrap under a moving blanket so it was already clean. Even if it needed work I was still ahead of using the other one. My logic anyways
 

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Also, I came across something awhile ago and now I'm thinking about it, it was a TSB for 96-99 5.0 and 5.7s I believe. I'll try and find it again, but it said that GM had left extra oil clearance on the #5 main cap and in some cases the crank was supported by only 1-4 mains. And under load the crank would flex and pound against the main cap on 5.that wouldn't happen constantly though? You wouldn't hear that at idle I wouldn't think. I'm gonna drop the pan today and see what I can find out and go from there. Just thinking of possibilities.
 
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