I see on some you tubes, the test for play or failure in a rod bearing, is to tap on the piston with a mallet and screwdriver on the down stroke after TDC. There is supposed to be some slight downward movement, or at least not a solid metal sound if the bearing is bad. I am skeptical if this is a positive test or not. What are opinions on this?
Spun or worn rod bearing test
- Thread starter Fishfarmer
- Start date
I don’t see it as valid for a “failed” bearing. A “failed” bearing will display loss of material, smearing of babbit coatings, increased operating temperatures (darkened/overheated), or corrosion, etc. This requires physical examination. By the time sufficient material is lost in failure modes to detect looseness in a post-TDC test ….that failure likely would already be indicated by oil analysis, noise, and filter-debris.
Checking for “play” in a downstroke would not isolate a failure to a rod bearing ether. It might also me a wrist-pin/piston bushing or even a main crank bearing or cracked support.
That’ my opinion on it anyway.
Checking for “play” in a downstroke would not isolate a failure to a rod bearing ether. It might also me a wrist-pin/piston bushing or even a main crank bearing or cracked support.
That’ my opinion on it anyway.
Thanks GeoHorn, I am just trying to narrow down what the engine knock could be which you answered in my other threads. I see your reasoning and agree it can be quite a number of things causing it. Bottom line is i will get a few mechanics to give an independent diagnosis and then strip it down if it isn't a simple fix and share the results for others that are having similar problems.
Interesting article about diagnosing engine noises, with references to effect of temperature, load and speed, probably the most thoughtful I've found:
https://cat-engines.blogspot.com/2017/10/basic-engines-evaluating-engine-noise.html
If no stethoscope, suggests long thin screwdriver or piece of hose.
It doesn't mention the the mallet/screwdriver tapping method.
Trying to use a few facts about how realistic the mallet/screwdriver tapping method is:
The service limit for the big and little end bearings clearance combined 0.35 mm... but only one of those bearings might be worn... so we are looking at judging around 0.2mm. Assuming that a bit beyond 0.2mm knock will start to become evident?
So it's probably like judging spark plug electrode gaps: if they do it a lot, some people can get quite accurate by eye alone. But a novice has to use a feeler gauge. Trying to measure piston movement to tenths of a mm at an angle through the PC chamber port would be tricky.
If not all conrod bearings are grossly worn, it might help identify which cylinder/s are OK, if we can determine the difference in tenths of a millimetre.
Rings worn limits:
Service limit for rings in the grooves is 0.2mm, so if the piston bobs up and down more than 0.2mm without ring movement (hard to tell), then there's a ring groove clearance problem. Try measuring that by sticking something through the PC chamber at an angle... impossible - and the crank would have to be rotated a few degrees while doing it to prevent the journal clearances - if they are OK - confounding the measurement.
Conclusion: tapping method impractical for all but (maybe) regular users of the method who have a lot of skill, and who over years have stripped engines down and confirmed that their tapping diagnosis was right.
Thanks JohnDB. Very interesting article which seems to confirm the symptoms I am having. The article states it is important to locate the knock before stripping the engine, which makes sense in case it is something else. I am inexperienced using the stethoscope method and I am having a problem locating exactly where the knock is loudest. Rather than just getting a mechanic who says " yeah that's what it is", i will be none the wiser. So the answer is to call a mechanic who is experienced in stethoscope diagnosis, who can describe to me and I can listen for myself what is going on and where the knock is coming from.
I've been following this issue for the past month. You have established at least two major problems as I understand it:
1) compression on #6 is down
2) you have an audible knock
Either one of these is cause for concern. One way to see if the knock is caused by a bearing problem or a piston/ring assembly coming apart without taking the engine head and pan off is to take an oil sample, list your concerns on the form and have an oil analysis conducted, The analysis can tell you if materials in the sample are consistent with bearing material, piston material, cylinder wall material, etc. These tests can tell whether the material is normal "wear and tear" or accelerated wear.
If you just want to fix the engine to get it running, you can rebuild the #6 cylinder, the bearing that may or may not be failing and put the engine back together and use it as is. That's the minimum short term cost solution. If you want to keep this machine long term you can rebuild the engine. You can also continue to analyze the situation ad nauseum and get nothing done. Your $'s, your choice.
1) compression on #6 is down
2) you have an audible knock
Either one of these is cause for concern. One way to see if the knock is caused by a bearing problem or a piston/ring assembly coming apart without taking the engine head and pan off is to take an oil sample, list your concerns on the form and have an oil analysis conducted, The analysis can tell you if materials in the sample are consistent with bearing material, piston material, cylinder wall material, etc. These tests can tell whether the material is normal "wear and tear" or accelerated wear.
If you just want to fix the engine to get it running, you can rebuild the #6 cylinder, the bearing that may or may not be failing and put the engine back together and use it as is. That's the minimum short term cost solution. If you want to keep this machine long term you can rebuild the engine. You can also continue to analyze the situation ad nauseum and get nothing done. Your $'s, your choice.
Thanks for following JerryMT. When I first got the tractor I dumped the old black oil and didn't check it.
I filled up with a Stou farm oil 15w-40 and hadn't realized there were two sump plugs to drain, there may have been some old oil still in it because it went black pretty quick so I drained it again. I only checked it for metallic discoloration and put in a high quality oil we get here Penrite 15w-40. Its only run for three hours with the brand new oil. Its very clean, hard to see on the stick. Will this have run for long enough for the oil samples to return a positive? Does it matter where I take the sample from the beginning or the end of the change of the sump oil, and how much of a sample? Also you have a M4500, do you think the pan could be dropped after removing the front end only ? Then I could take off the head and replace the one piston without pulling the whole motor, providing the sleeve isn't ruined. From what I can see with the borescope there is some scuffing on the cylinder but wont be able to really tell until the head is off. I would take the head off before the pan but would like to know if it could be done this way and easier than pulling the engine out? I would go for that idea to replace the one piston, plus I could check all the other shells and journals.
I would direct your oil sample question to the testing company. I would suspect that the longer the engine is run, the more "stuff" will be found in the sample and the better answer you'd get but that's conjecture on my part. Generally the oil sample is take right before the factory recommend change interval and the test results often give you an extended change interval based on what the test results are.
I have an M4500/M4500DT Kubota WSM but unfortunately in in a storage container and I can't get access to it. If I recall correctly, the engine can be removed and or worked on by removing the front end. JohnDB, a person who posts on this forum, might be able to confirm that as he has an M4500DT and I believe he has a WSM. If the engine is sleeved than the sleeve can be replaced and a new piston and new rings fitted. Hopefully you won't find damage to the crank journals.
I just found out last week that the M4500 is a Fiat designed tractor
(and engine) built by Kubota under license from Fiat. So there might be an alternate source of parts. Dave-Eng another contributor informed me of this in another post in this forum.
Good luck in whatever you decide to do.
Thanks for your help. JohnDB does have a M4500DT and we communicate frequently. He has been very helpful and shared information with me from the WSM. The engine is sleeved but it appears it has to be re-bored and honed after fitting. We are not quite sure if that is the case but if it does I imagine the block would need to be stripped bare as opposed to just fitting a new sleeve and piston.
That is interesting news about the Kubota being built under licence of Fiat. I am sure JohnDB will be happy to hear that.
A kid that grew up on a gold mine in Alaska could detect any engine problem just by ear. Wish I had that ability.
The oil sampling suggestions are best ONLY if a regular sample has been taken over a sufficient period of time to establish a “Trend”…a “norm”for that engine….using that particular brand and grade of oil. (In fact, the method of collecting the sample is also important so that each sampling is taken consistent with how previous samples were collected. I.E., after a similar number of hours of operation …. and (for example) the sample taken in the “mid-point” of the drain-stream, not the beginning or end. Consistency of sampling is key.)
A single oil-sample does not easily diagnose a trending problem such as a bearing undergoing slow failure.
By the time the bearing is at the point of catastrophic failure… then it’s just as likely that a filter will have material in it to make that determination.…and it’s too late to begin oil sampling.
Does your filter media have bearing particles in it? (IE, copper, brass, or tin?)
Does it have ferrous material? (IE, steel or iron from crank or cam shafts, cylinder-walls, or piston rings? check with a magnet to see.)
You will likely always find a certain amount of small, black, hard grains in a filter. Most are likely carbon. (To check if a black chip is metal or carbon… lay it on an anvil and strike it with a hammer. If carbon..it will shatter. If metal…it will flatten.)
Hope this helps.
A single oil-sample does not easily diagnose a trending problem such as a bearing undergoing slow failure.
By the time the bearing is at the point of catastrophic failure… then it’s just as likely that a filter will have material in it to make that determination.…and it’s too late to begin oil sampling.
Does your filter media have bearing particles in it? (IE, copper, brass, or tin?)
Does it have ferrous material? (IE, steel or iron from crank or cam shafts, cylinder-walls, or piston rings? check with a magnet to see.)
You will likely always find a certain amount of small, black, hard grains in a filter. Most are likely carbon. (To check if a black chip is metal or carbon… lay it on an anvil and strike it with a hammer. If carbon..it will shatter. If metal…it will flatten.)
Hope this helps.
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I'm certainly no expert at this but if a bearing is failing, the sheer quantity of bearing materials per cc of oil would be well above any normal wear and tear and provide an indication that this in fact is occurring. Last time I checked, oil sampling tests were relatively cheap (<$50) but I can't see any other way for the OP to get any usable info that would allow him to make a decision on what to do next. The lack of compression on #6 is pretty conclusive that there is at least one problem in the engine.
He can pull the pan and check the bearing clearances but it is a bit of a chore on this engine so he wants some assurance that the bearing is failing.
Personally, I think that there is enough evidence with #6 compression to warrant a decision to either part the machine out, sell it as is, or fix it. However, it's not my decision to make.
I agree with you that long term monitoring of oil samples would have been better and provide more certainty but the OP is beyond that now.
Talk to a machine shop that works on engines to see what they say about boring and honing. I believe that these processes can be done right on the engine without removal from the tractor. The pan will need to be removed to remove the piston and rod assembly so you can't escape that part of the job and you can check the bearings. None of this will be cheap and you need to consider a replacement engine as a possibility. The alternatives are selling the machine "as is", fixing #6 and checking the bearings with a potential replacement of one or more bearings, parting it out, or running it until it dies and then parting it out and\ or selling it.
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Thanks Gentlemen. The original oil is now in a 44 gallon drum mixed with the transmission oil change which had some metal paste on its magnet so all of that is contaminated. It has had two oil changes in succession so it has not had enough running time for metal to accumulate. I checked the oil for material after the first oil change which I did again because I didn't know the sump had a second drain. This would have had a bare minimal amount of original oil. I did check this oil ( after the first change) and filter for material but could not see any flecks or coloring in the oil or on the filter. It was quite black after 3 or 4 hours of running, probably some of the original oil discoloration. Some tiny specks of sand only enough to see with a magnifying glass and rub between thumb and finger. IMO this could be from someone using a dirty filling oil funnel. The oil now ( second change) is very clear and from advice GoeHorn and JerryMT there is no point sending in an oil sample. It would need too much run time and by then close to catastrophic failure. I guess sure evidence of material in the oil by that time. Just the same I will run a magnet through when I drop the oil again.
This knock is not loud (yet). I have heard engine knocks a number of times in cars and it is in the block somewhere. Its not valve related. It may not be the rod bearing but its staring me in the face that #6 low compression is the culprit whatever it is that is wrong. JerryMT supports the inescapable fact that something is seriously wrong here with #6 anyway. Others suggest that you can get years running tractors that rattle/ bang/ smoke/ blow-by. Do i run it and see. NO ! because I know it wont last long like this and I will ruin a block that might be salvageable ATM, but would lose a few thousand selling a tractor that's not running. The tractor is an old beast but will keep running if the engine doesn't fail so in that respect is worth saving. They are built like Sherman tanks.
My down and dirty way is to drop the sump leaving the engine on the tractor. The WSM describes to remove the front end and subsequently remove the engine. The front end on these underlay the sump and prevent me from dropping it. If I can remove it and still support the tractor I can drop it then take off the head. JohnDB's helpful info suggests the sump may be able to be taken off this way. Then if I push the bad piston out and can replace it rod/ bearings and rings with a new Kubota original piston. Hopefully the journals aren't too bad and the sleeve would have to be real bad for me not to do this. I am not sure how thick the sleeves are in these to do a ridge removal so the new rings don't break. I have replaced all pistons in a car once with the block in the engine. I will never do it again because of poor access to the block from the under car cross-members, The tractor has much easier access. I will make an attempt at this over the next month, first i need to be sure I can source parts. Thanks for everyone's time to comment and your support. Open to further comment on this procedure.
rbargeron
Well-known member
Lifetime Member
Equipment
L5450, L48, L3250, L345 never enough attachments
If your rig is made like others of similar size, the engine can stay attached to the clutch housing with the front axle taken down for oil pan removal - see this thread. Good luck with the job - Dick B.
Do not forget that on many engines, the oil-pan can be removed and the bearings removed/inspected/replaced. (crank and big-end rod bearings) WHILE ENGINE IS INSTALLED IN MACHINE.
This is perhaps the best way to know for certain.
This is perhaps the best way to know for certain.
I don’t think Kubotas are sleeved engines. I also believe that if your bearings are so bad that they are producing a “knock” that the journals will probably be damaged and you’ll need to have the crank ground.
Pau7220
Well-known member
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L3650 GST, Landpride TL250 FEL w/ Piranha, 6' King Kutter, GM1084R Finish
If you decide to go with the hone, rings, and rod bearing job... and during disassembly you find the culprit (a loose worn rod bearing). Do NOT put a new bearing in without a machine shop measuring the rod ID. Most times with a rod knock it will stretch the crank end into an egg shape. If reassembled like this the bearing will spin and wipe out the crank within minutes.
the youtube test is why i don't trust youtube mechanics
it is inconclusive
in other words, one can have a failed bearing and the piston won't move. Why? stacked bearing (I see it somewhat often). This is when the bearing shells are so worn that they can actually spin and then stack on top of each other, which takes the play out that it would normally have with spun or severely worn bearing(s).
if you have a worn bearing chances are also pretty good that the crank is damaged too negating removal of the engine from the tractor. I know folks who have stuffed bearings onto a worn crank but it's never a permanent fix; only temporary. Temporary meaning it might last 5 minutes, or it might last 5 months or years who knows. It's always a gamble. And for that, I like to do things right once.
it is inconclusive
in other words, one can have a failed bearing and the piston won't move. Why? stacked bearing (I see it somewhat often). This is when the bearing shells are so worn that they can actually spin and then stack on top of each other, which takes the play out that it would normally have with spun or severely worn bearing(s).
if you have a worn bearing chances are also pretty good that the crank is damaged too negating removal of the engine from the tractor. I know folks who have stuffed bearings onto a worn crank but it's never a permanent fix; only temporary. Temporary meaning it might last 5 minutes, or it might last 5 months or years who knows. It's always a gamble. And for that, I like to do things right once.