Failed conrod, no knock?

[Mongrel]

Hi all, I've been lurking here for a few years soaking up useful information. Now, I have a question for you all;
Engine - 3L Toyota diesel. Lost oil pressure and chewed up the conrod bearing on no.4. Bearing remnants were still in the bigend, thickest part was down to 0.5 mm. Total clearance in the joint was measured roughly using a feeler gauge at 2mm, with the top of the joint resting on the crankshaft. So about 1mm clearance all round the bearing area.
Now, the question - this engine had no rod knock whatsoever. Just a light tapping sound which exactly mirrored diesel rap. Compression was even on all cylinders at 250 psi, +/- 10 psi. The top of no.4 piston was clean of soot, showing signs it had been touching the head and intake valve, although there was no damage.
Has anyone encountered this before, or could tell me why this might happen? I've been in the game a relatively short time at 15 years, and have never seen or heard of this before.
Appreciate any replies!

 

[TugboatEng]

We had an Electro-Motive engine start smoking. A compression check was done and the front two cylinders showed low compression. A crankcase inspection revealed a spun rod bearing. Further inspection also showed severe damage to a main bearing. The engine never made an unusual sound from the reports I have heard, it just smoked a bit. Ours was caused by a clogged lube oil passage due to a rag being left in a pipe and the y-strainer being gutted.

How did you get a feeler gauge in there to measure rod bearing clearance?

 

[djhurayt]

Once a bearing spins, the time between spin and destruction can be quite short. As once the bearing spins, the passage for oil to the bearing no longer aligns with the oil port in the crank and it is starved of oil.

 

[LadaTrouble]

You wouldn't be getting rod knock, but the knock of the piston hitting the head. I've heard that before, and loud enough to be worried about. 3L problems are usually a cracked head.

 

[Mongrel]

Appreciate the replies.
TugboatEng - pulling the bigend down against the crank allows enough room to get the feeler in. Just. Note, I did use the word "roughly"!
Lada - yes, I believe the noise was indeed the piston tapping against the head. There seems to be no damage, but we will be replacing it anyway, along with the conrod, which is badly scarred inside the bigend. The cylinder head is in good condition, it was replaced with a new one roughly 150,000 km ago.

I was just surprised about the lack of knock. Oil pressure on the gauge had remained at a steady 50 psi; I realise that this has little relation to what's happening in the bearings, but I was wondering if it was possible for there to still have been sufficient oil supply to muffle any knock somehow??
Oh, one possibly related piece of information - the engine is turbocharged, max boost is around 10 psi above atmospheric. I doubt this would have any effect on whether or not it knocks at low RPM, but... just throwing it out there.

 

[LadaTrouble]

Pistons hitting heads seems to be the symptom of a worn bigend these days rather than a bearing knock...much tighter squish areas ? And the piston is much closer to the head on a diesel of course. On the Mitsubishi 4D56 we would measure piston height above the block and machine to suit.

 

[Mongrel]

I'm also used to needing to be aware of piston protrusion when setting up these older diesels. Fortunately there are often a good range of metal gaskets available in different thicknesses these days!
I guess I am just not understanding the process that allows this kind of bigend wear, and still making decent compression, without creating a knock. I suppose it may pass the knock point while the piston dwells at a higher TDC than normal, allowing clearance due to inertia, maybe. Oh well, I learnt something from the experience and so did the owner!