Good thinking! I'll check into that.

 

I’ve been giving this noise thing a lot of thought, and here are the latest ones.
I had the truck in for steering issues, and while on the pit the mechanics and I did some more analyzing. A stethoscope doesn’t seem to help much because the noise just seems to telegraph everywhere. Rocker covers or oil pan, not much difference. But while on the pit a length of 3/8” fuel hose seemed to localize it towards the back of the pan, no particular side, which might indicate a rod bearing, or a broken flex plate.
I did some experimenting, thinking of the flex plate thing. When first fired up, there is no noise for about a second or so, The noise then appears, is worst at idle, and gets worse as the engine warms up. Again, no difference in noise when any particular plug wire is pulled. Oil pressure is good. An increase in no load RPM decreases the noise. If I put it in gear with my foot on the brake the noise diminishes greatly with increased load, but the revs have risen as well. The noise is not always constant at the same RPM. Sometimes, especially at idle, it is quite loud. Sometimes, just audible.
I put a timing light on it to see for sure if it is at crank or cam frequency. Definitely crank frequency as the noise is at twice the timing light frequency. This would indicate to me that it is not a valve train related noise.
I suppose the next step is to pull the starter and the inspection plate in front of the flywheel housing and see if I can find anything suspicious in there. Perhaps un bolt the torque converter and slide it back as far as I can and see if a mirror will reveal anything on the flex plate itself.

 

That's gonna be fun....... But, I think you are heading in the right direction.

 

Well, it wasn’t raining today, for a change, so I decided to investigate the possibility of my knocking noise being a cracked flex plate.

One nice thing about the 2500 4x4 is that there is enough room to work under the truck that for a lot of things, so jacking and blocking is not usually necessary.

So, off came the two braces from the block to the bottom of the transmission torque converter housing. Off came the starter, and off came the inspection plate in front of the housing.

First glance revealed nothing suspicious. No marks anywhere, no metal bits, no rusty areas that might reveal a crack.

I pried the engine over in the direction of rotation about a 6th of a turn at a time. All torque converter bolts were tight. No sign of cracks anywhere. I tried to pry the flex plate toward the front of the engine. No perceptible movement. I tried prying it backward. You could see a bit of relative movement between the flex plate and the torque converter as the flex plate did what I suppose it is supposed to do. Flex a bit. If I let it spring back quickly I could here a slight “clunk” like the torque converter was coming up solid on the crank, or crank bolts. There was no sign of rust or filings to indicate it was the hitting the bolts, so I assume the pilot protrusion is coming up against the crank.

I thought I might as well put the starter back on, less the inspection plate, fire it up and have another listen. So I did. I brought out my trusty length of 3/8” fuel hose, plugged my bad ear and used my good ear to see if I could pin point the noise.

Definitely not coming from the flex plate area. It was loudest close to where the oil pan meets the block, and loudest towards the back of the engine. No noticeable difference between left and right side.

Off comes the starter and back on goes the inspection plate starter and braces.

So, unless there is a serious crack in the plate around the hub, which I see no signs of, I am reasonably sure that the flex plate is not my problem, but what the hell is?

It’s kind of looking like a rod bearing, in spite of the good oil pressure, or perhaps a cracked or collapsed piston, or wrist pin, most likely on # 7 or # 8.

I had a look at the possibilities of dropping the pan in situ. It’s a bit tight to the front cross member, but there should be enough room to angle it down from the back and clear the front axle.

It wasn’t raining yet, and god hates a coward, so I drained the oil, un-bolted the pan and tried to wiggle it out. No go. It was hitting on the inspection plate. So, off come the braces, starter and inspection plate. Again. Now the pan comes off quite easily. Nothing ugly in the pan. None of the rod big ends wiggle in directions where they shouldn’t wiggle.

I pulled the caps off 7 and 8. So far so good. Bearing wear looks normal. Crank looks like new, but I’m running out of daylight. Time to quit, put the pan back out to keep the nasties out and try again some other day.

I’ll have to do each rod big end, check both shells, and mike the crank journals.

I’ll post an update when I get that done.

 

Thorough write-up indeed. Thanks.

 

Agreed. Appreciate the report and the thought process for analyzing it.

 

It didn’t rain today! I exercised my right, as being self employed and played hooky to do some more investigating as to my engine noise.

I checked every con rod bearing and crank journal. My factory service manual tells me that the journal diameter should be between 2.124” and 2.125”. I’m not a machinist and it took me most of the day, checking and re-checking, but as I measure, they were all ~ 2.125” +_ 0.0005”. All crank journals looked like new.

All con rod bearing shells looked very similar. None of the lowers showed any copper or score marks. All of the uppers showed copper, some worse than others, but not much. No scoring or other bad things. All clearances measured between 0.004” and 0.005”. The manual states that clearances should be between 0.0005” and 0.0022”.

Because I am only hearing one particular knock, I fully expected to find one rod bearing with seriously excessive clearance. Didn’t happen. Obviously, I have some bearing wear, but they are all pretty much the same, which doesn’t explain a singular knock.

However, I did see something that could probably explain that knock.

I am going to install a new set of con rod bearings. The cost is relatively small, and seeing as I am into it this far anyway, it will be worth it to me if it helps to prove the point.

To be continued.

 

The suspense is killing me...
What did you see that looked suspicious?

 

The cause of the mystery noise has revealed itself, or at least I am 99% sure it has.

My connecting rod bearings arrived yesterday, the weather was nice, but I had work commitments. I really had work commitments today as well, but the weather was supposed to be good in the morning and start to rain again in the afternoon, so I blew it off and played hooky again.

I checked the first new bearing for size. I measured it at about 0.004” clearance, but in all fairness, because it is difficult to torque the cap off the journal to the correct torque there may be a crush factor here. Anyway partly because I didn’t have a lot of time to spend on it today, I decided to take the slight gamble that all bearings were the same and installed them all, torqued them down and buttoned things up.

If you recall, I mentioned that I saw something that would probably explain the noise when I did the bearing inspection.

As I was working myself along the inspection, I just happened to notice that #8 had some vertical scuffmarks on the cylinder, right down to the bottom. It didn’t look like it was scored, just scuffed. None of the others had that. On further investigation, when the piston was at or near BDC, I could see scuff marks on the piston skirt. “Aha!” sez I to me. “I’ll bet I have a collapse piston skirt on #8”. Then I decided to see if I could move it from side to side at all. Shor ‘nuff. Couldn’t get any noticeable movement on any of the others.

So, now I have a new timing set, a new set of bearings, and a fresh change of oil, even went heavier (20W50) to see if that might quiet the piston down a bit, or at least help preserve it’s life somewhat.

I fired the old boy up, and as expected, no noticeable change in noise. I let it warm up to operating temperature, and it might have gotten a bit better, but not much. The noise is worse at idle but by about 2000 RPM no load, it is all but gone. It changes a bit on accel and decel, which kind of makes sense to me for excessive skirt clearance.

A collapse piston skirt was one of the first things I suspected, but discounted any piston or rod noise early on because the noise didn’t change when I pulled the plug wire on that cylinder. So, I’ve learned something new.

I retrospect, the plug on #8 was quite oily, none of the others were. Compression on that cylinder was only 100 PSI, but compression for the engine was all over the map anyway. The low compression would be consistent with excessive piston to wall clearance or ring damage. I just didn’t make the connection between noise and an oily plug or the low compression.

Other than take it easy on the engine, I’m not going to do anything about it as the engine and transmission are scheduled for change out within the next 6 months or so anyway.

Years ago I had an early 60’s Ford cab over 5 ton truck. It had a British 6 cyl Daganham diesel engine. It had a collapsed piston skirt when I bought it, and it had had that same collapsed piston skirt for many years service before. It sounded like something was going to come through the side of the block, and not just diesel noise either. I transplanted the engine and transmission into a Boyerton step van I was using for work. When I finally had to tear it down because I was getting antifreeze in the crank case (just liner O rings) I had a good look at the offending piston. Yup, it was scuffed all right, as was the liner, but not scored at all. I installed a new piston and liner and got several more years service out of it before the rust bugs did a number on my poor old van.

So, that is the story. I’m curious to see if the engine dies because of this piston, or it will just carry on, rattling merrily away.

 

It will probably last as long as you keep feeding it oil. Of course, it's appetite for oil will continue to increase.