I've got a strange one to ask about here. Truck that belongs to a buddy of mine blew up a few weeks ago. Death knocking sounds. I went and got it on the trailer (of course it was 50 miles away when it went) and brought it back to my shop. Here's what it looked like inside:
Yep, that one's done for.
Motor was a 6.0 LQ4, and he sourced an LQ9 as a replacement. We pulled the old one and put the new one in; I put gaskets on it but didn't disassemble it further. Start it up... knock knock noises. Not a knock knock joke, unfortunately. I had him unbolt the flexplate just in case it was cracked. Noises continue. Pull accessory belts, noises continue. It will idle fine and rev, but it's just clanking. Compression is good on all cylinders.
I pull it out and take it apart, and everything looks fine. Bearings are factory standard and look perfect. Pistons, bores, rods look ok. Cam, lifters, pushrods, rockers, all look good. Where did that clank come from?
The flexplate wasn't visibly cracked. I went back and looked at the converter, and the pilot on it was chewed up like it was contacting the flexplate after he unbolted it. I didn't get down there and look at it myself then, unfortunately. I need to bolt the old flexplate back on to see if it was bent and contacting the converter but that's my theory right now.
Took the block in to get bored as it had some scratches in the cylinder walls and at this point might as well...
I'm still a bit nervous about putting it back together and putting it back in without finding a clear cause for the noise.
Ideas?
Measure the piston to wall clearance. I seem to recall that early motors had problems and with a bit of wear, it gets worse.
LQ9 is 0.6-0.7 higher compression, fyi.
Did you transfer any covers or intake manifold, balancer, etc? Sounds like a clearance issue. I don't know all the differences, but I'm sure there are a few.
That being said, it's always the converter/flexplate.
In reply to Masher_Mfg :
I haven't put a dial bore gauge in them but there was no wear ridge at the tops of the bores and the piston skirts looked good. It did not sound like piston slap to me and didn't get better with warm up. It's getting bored 0.030 and new pistons put in it so we should be good if it was something like that.
In reply to llysgennad :
Yes, the owner was happy to get the flat top piston motor for a replacement. We used the intake and a few other parts from the old motor as it was drive by cable and the LQ9 was drive by wire. The accessory brackets, water pump, balancer, etc are the same. At this point I'm going to go over everything with a fine tooth comb when it goes back together, it'll get a new flexplate and converter, and hopefully it'll run and make it out of here!
The post above mine made me think.
What years are we dealing with? 2000 era motors had a SBC bell face to flex plate distance where as later motors had a shorter distance.
Meaning, the crank flange on the 2000 era motors stuck out farther than the newer motors. How much clearance did you have between converter pads and flex plate when you unbolted the converter and pushed it back?
In reply to Masher_Mfg :
I believe those were 98-99 engines with the different crank flange. These were 2004ish and had the same crank flange and the flexplate was the original from the truck. I even marked it for "Out" when I pulled it as I've been known to screw up and put one on backwards in the past. 😀
Edit: as I mentioned above, I didn't actually get under it to see the space between the converter pads and the flexplate. I'm trying to make the owner do the crawling underneath bit. I should've but oh well..🤷♂️
Kendall Frederick said:
In reply to Masher_Mfg :
I haven't put a dial bore gauge in them but there was no wear ridge at the tops of the bores and the piston skirts looked good. It did not sound like piston slap to me and didn't get better with warm up. It's getting bored 0.030 and new pistons put in it so we should be good if it was something like that.
One issue is piston skirts collapse from heat but can still look good.
In reply to Kendall Frederick :
Did you inspect the block for stray woodpeckers?
Seriously, though- that's odd. Is the harmonic balancer good? Those can make some weird noises when they fail...
Mr_Asa
MegaDork
10/31/25 10:11 a.m.
Recon1342 said:
In reply to Kendall Frederick :
Did you inspect the block for stray woodpeckers?
Seriously, though- that's odd. Is the harmonic balancer good? Those can make some weird noises when they fail...
I had one in a 4.3L V6 fail just enough that the outer ring shifted and eventually wore a semi-circle 1/4" deep into the timing chain cover. Give it a little go-pedal and it would shift a little bit further. It was like I was machining it
Recon1342 said:
In reply to Kendall Frederick :
Did you inspect the block for stray woodpeckers?
Seriously, though- that's odd. Is the harmonic balancer good? Those can make some weird noises when they fail...
I do live out in a pine forest...
I think the balancer is OK. The LS truck balancers have this cast 3 spoke center, then I think there might be a thin elastomeric layer in there, then the pulley. It has no visible cracks and you can't wiggle the pulley by hand, anyway.
Mr_Asa said:
Recon1342 said:
In reply to Kendall Frederick :
Did you inspect the block for stray woodpeckers?
Seriously, though- that's odd. Is the harmonic balancer good? Those can make some weird noises when they fail...
I had one in a 4.3L V6 fail just enough that the outer ring shifted and eventually wore a semi-circle 1/4" deep into the timing chain cover. Give it a little go-pedal and it would shift a little bit further. It was like I was machining it
I had one on a 2JZ where the outer serpentine pulley portion came all the way off and somehow the belt was still on there and spinning all of the accessories. No idea how the belt could've stayed on when the pulley walked off.
