Rod bearings, failure mode

[cranetruck]

Hate to bring up my little engine problem again, but I would like to know how close I am to a catastrophic failure.

The engine is knocking, apparently on one cylinder only, number six. It has been doing this to some extent since before the California trip, 7,000 miles ago. Before and during the trip I asked a number of people about it and the problem was more or less shrugged off.
Perhaps I'm paranoid now and hear it so much more clearly, but it sounds louder than it used to. Loud at idle, not so bad when driving.

Question: What is the sequence of events for a failure caused by a worn rod bearing, not overrevving?

The engine is never (at least not to my knowledge) overrevved and I only run it at about 2,000 rpm max these days.

Engine oil pressure is low, even with fresh oil, about 20-25 on the road and as low as 10 at idle after it gets warmed up.

Thanks for any insight.

 

[gimpyrobb]

I have not done any work on these motors. That being said, if a missing retaining clip can make a diesel injector line explode, what can low oil pressure and that knock do?

 

[clinto]

Eventually, it will chew up the crank journal where the rod rides.

That's best case.

In my humble opinion, eventually, the rod cap fasteners will be weakened, allowing the cap to loosen and at that point, the rod and piston combo will attempt to beat a hasty retreat from inside the block.

 

[No.2Diesel]

Just delaying the inevitable. If you can't use the truck to its full potential because of low oil pressure and you can't get the sound of the knocking out of your kopf, I would tear the whole thing apart and rebuild everything not just for piece of mind but to restore the trucks usefulness. Whats the total hours/mileage on that engine?

 

[cranetruck]

I'm certain that the knock is not from the combustion process, it's a mechanical sound more pronounced at the lower rear portion of the engine. All injectors have been replaced without any change in the knocking sound.

Miles: 24,310
Hours: 1,460

 

[73m819]

if the bearing is bad, it will eat the crank, which in turn will eat more of the bearing, at some point the hammering will get bad enough that first the bearing will spin, then a broken rod or cap, if the bearing does not spin then a good chance of a SEAZED bearing, then a broken rod and a busted block. if the knock if from a main then a spun bearing that will require another crank and a line bored bolck, a seized main can bust the crank and take the block with it, every hit of the hammering (knock) is doing something not good, with the low oil pressure, i would more think a main then a rod,( like a rear main after you lost the rear main seal),SURE HOPE IM WRONG

 

[Rattlehead]

I saw a small block chevy with a spun rod bearing that really didn't sound that bad to me, hard to get it to occur. It had a light knocking on a decel after free reving the motor. It will also distribute metal through the rest of the motor. Best to drop the pan and find out for sure before possibly causing more damage.

Of course, that is assuming its a bearing problem. Did you cancel out the cylinder to isolate it to a cylinder? Could it be an injection knock?

 

[cranetruck]

Sobering, no way around it.
Thanks

 

[rdixiemiller]

Bjorn
Considering how many #6 rods come through the blocks......
The knocking means the clearance is too great for the oil film to maintain. You will start battering the bearing, which will increase clearances even more. Now you are putting a shock load on the rod bolts, and they will stretch or fail. Rod comes loose, opens up window in block.
My experiences with spun rods is usually on tight motors that suffer catastrophic oil pump failures (old Ford 289's would twist off the oil pump shaft). The oil film goes, the rod bearing galls to the crank, and the bearing spins in the journal.
I would pull the motor and do the bottom end. You may only need to polish the crank and install fresh bearings.
Since you recently did a clutch job, have you considered a loose flywheel or a bolt backing out of the pressure plate? You can get some nice knocking noises out of that as well. Your low oil pressure seems to indicate wear, or a loose oil pump, but don't discount mechanical issues not related to engine oiling.

 

[cranetruck]

Wonder if a thousand hours at relatively low rpm has anything to do with worn rod bearings.
Over the years, this engine has spent a lot of hours running the crane at 1,000 to 1,200 rpm only.
Trucks in the military service may also run for long periods of time at idle.

My 8x8 only has some 3,000 miles on it, but close to 1,000 hrs. One of the drivers (pershingmissile forum) said that the trucks ran at idle most of the time, which agrees with my readings....
In addition, the hour meter records fewer hours at low rpm, which makes the record even worse.

 

[WillWagner]

Bjorn, I've seen mains spin and cause knocks too and have even run them on the dyno and the only indication of the failure was white smoke out the breather under a load only and a knock similar to yours, down low. Usually, if a rod bearing fails on a diesel, the engine won't last too long. There's alot of pressure..3X as much as a gas engine...on the bearing. They usually let go in a catastrophic way, very soon after the overlay is degraded. Also, rod failures, upper bushings or lower bearings, will have a double knock, one as the throw is going from down to up and one up to down. Yours sounded like a single knock. Try shorting the cylinder by opening the intake or exhaust valve, or just bite the bullet and drop the pan. That's the only to tell what it is for sure. Oh yeah, let it sit overnight before you drop it, if not, it'll drip all over you and the floor!

 

[jwaller]

if you continue to operate the engine it will only get worse. IE more you drive the more damage you do and it might only last another trip around the yard or it might last another 7k miles.

do you want to risk a complete and total loss on the entire engine or just a single bearing problem?


best case is to open the pan and replace bad bearing and go on.

 

[OPCOM]

I think 1000RPM should not harm it if you had decent pressure of 30PSI or so. You have reported low oil pressure at idle and on road for a long time, could it be that the bearing is just gone and now there is nothing to do but fix the engine before something worse happens? Think of how much time and $ you have put it the truck to bring it to its valuable condition, now if the engine blows, the value is damaged very severely and it has to be fixed where it died or be towed.., versus having to only park it somewhere convenient and replace the bearings, or drive it to a diesel shop, whichever you prefer or have to do for reasons. Maybe it will be 'good' enough that you can just put in new bearings and be done with it.

 

[m139h2otruck]

For the cost of a pan gasket, I would strongly suggest that you pull the pan down and remove #6 rod bearing for inspection. I have experienced a number of rod bearing failures, and it almost always causes some amount of damage to the crank by the time you catch it (this in gas motors). Are the cranks in the multis hardened or chrome plated? If so there may be some hope for less damage. Have seen heavy equipment motors with really bad bearings, but the cranks were reusable due to the surface treatment.

I don't think operating the engine for extended periods at 1-1,200 rpm would hurt anything, if the oil pressure is OK. Heavy machinery operates all day at governed speed, and some of this stuff has 10,000's of hours.

 

[HeadWizard]

Bjorn-

Noise is never good. Low oil pressure is never good. Noise = damage. Also, while running at high rpm's can certainly be worse for potential rotational failures, lugging the engine at lower rpm's has it's downside also. The keep it at 1200 and it will be OK doesn't seem realistic.

When we see main bearing failures (or pre-failures) it's a louder, deeper sounding "thud" kind of noise - usually more pronounced with some load on the engine. The rod bearing noises tend to be more constant and not quite so deep tone sounding.

If a bearing is coming apart, or wearing thin, the metal has to go somewhere. That somewhere is probably where additional damage is occurring.

I know I'm sounding like one of the choir now, but stop driving it, pull the pan and check the bearings - all of them. It would be a shame to replace one, put it back together and have the problem still there. It wouldn't hurt to locate the bearing clearance specs and plasti-gauge them to check tolerances also.

You certainly don't want to open the hood and see the picture below.......

My $.02

Don

 

[Boatcarpenter]

I'll move my question to here that I asked before in another thread. " Speaking of rod noise etc; I can find oil clearance #s for the main bearing journals and bearings, but not for the rod bearing journals and bearings. The TM gives the acceptable journal measurements for the various crank sizes available (standard, undersized etc.,) but I can't find the oil clearance #'s. Any help? Maybe they only go by journal diameter and whatever set of bearings is needed for the particular crank size. Would like to drop my pan and Plastigage everything for starters.
Thanks,

Thanks,
BC

 

[WillWagner]

An easy way to spot a failure, if the oil isn't crude oil black, is to look for the darker colored rod or main cap. Same with an upper bushing failure. The top of the rod will be very dark or blue. Also, bearing material likes to extrude itself from the failure. Look for little jaged edges poking their heads out from the side of the cap.

 

[cranetruck]

Steve Turchet has an article on in-frame rebuilds in MVM issue 113 (Feb 2006), I'll be studying it in prep for this project.

 

[rdixiemiller]

Bjorn
Hook up an electric motor to the driveshaft on the crane pump. Change the motor with the K-Boom!

 

[cranetruck]

The possibilities....

Hopefully the bearings will be all I need to change. Of course, I have made space on the 8x8 for the crane and it will get one sooner or later.