Polishing crank rod journal while in vehicle

Messages
363
Location
Va
I have a light knock that started recently in my 4200 Vortec. Fairly confident it's a rod, going to pull the pan and take a look. I have new STD bearings from GM, if the journal checks out with a mic, could I polish it while in the motor and if so, what is a prescribed method for doing this. Granted it depends on condition of journal but hoping for best-case scenario.
 
Messages
25,805
Location
MA, Mittelfranken.de
If there is no major damage from say a spun bearing or bad scores you can clean it up with crocus cloth with a rope around it so you can pull it back and forth and spin the cloth, start with 400 grit and finish with 600. Don't get crazy just enough to clean it up and polish it a bit, clean it off with brake cleaner and use some moly paste or break in lube on the shells to prevent a dry start. I didn't look but there will be some sort of you tube video for sure. Doing it this way is far from ideal but I get what you need to do and this should be okay for what you need. Edit: I just looked..
 
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Kestas

Staff member
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13,946
Location
The Motor City
If it's a ductile iron crankshaft, it is very important to know which direction to polish the journal. Do it the wrong way and you can easily wipe out a new set of bearings.
 
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22,336
Location
Apple Valley, California
Don't forget to clean the oil passage in the journal when your done. Personally I have never polished a crank. I have rolled new bearings in and gotten years more service out of the engine.
 
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511
Location
USA
I worked in a shop where the Manager tried this years ago. The unfortunate mechanic spend two days with crocus cloth and re-assembled it per instructions. It let go on the road test. If it's bad enough to knock, you have no way to keep it within spec as you remove material.
 
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1,774
Location
RI
Use some 1000 grit sand paper with engine oil it works very well. If this is in a trailblazer chassis with 4x4 it's not an easy job to remove the oil pan.
 
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Messages
9,098
Location
Marshfield , MA
If it's a ductile iron crankshaft, it is very important to know which direction to polish the journal. Do it the wrong way and you can easily wipe out a new set of bearings.]/i] Kestas, Would the proper direction be the way the crank rotates? I've heard of polishing a rod journal. never tried it. I got the impression that early car engines were assembled by machinists working with hand tools to final fit the rotating assembly.
 
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i6pwr

Thread starter
Messages
363
Location
Va
Yes, this is a GMT360 platform with 4WD, Envoy. I agree it's a pita to get the pan off but I'm spending a few hours each day and slowly picking at it. Definitely not trying to make the journal round by polishing, only cleaning it up for the most part if it needs it. I pretty much just want to clean the surface for the new bearing material..not trying to remove metal so maybe polishing is the wrong term. The knock started a week or so ago, very light, been driving it easy and I'm hoping it's very early not to have damaged the journal. I did replace rod bearings in a knocking Iron Duke 2.5L with 190k miles and it kept going afterwards...this 4.2L just started so fingers crossed. Interesting about the direction to polish....makes sense.
 
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1,877
Location
New England, USA
I did this on my MG Midget 1500, a car with a notoriously short lived lower end, that if the warning signs were ignored, would put a rod through the block. This car had an immaculate body but a slightly knocky engine and low oil pressure. Unfortunately, even the nicest one of these late cars wasn't worth putting a new engine in (plus I was younger, dumber and didn't want to spring for a new engine). A friend of mine and I dressed the big end and 2 of the 3 worn main bearings (couldn't get to one...rear maybe?), replaced the oil pump. One of the rod bearings was moving in its seat, good thing we did this when we did. This reduced the knock and brought the oil pressure to the lower end of normal. This was in 1994. A few years later I sold the car to a relative who planned to drive it a bit and overhaul the engine himself when it got bad again. The car is now one of several fun cars in his fleet and still driving on the same engine. Oil pressure is now at the very low end of normal, so it will get O/H'd soon. Not sure what the results will be on a modern engine with current tolerances as opposed to the Triumph 1500 boat anchor. As long as the journals are not scored, nothing you can catch a fingernail on, I'd give it a try. Be conservative with the polishing, don't remove too much material and don't scratch anything. Be crazy obsessive about cleaning the swarf. Good luck. Please report back.
 

i6pwr

Thread starter
Messages
363
Location
Va
Cool story....I will keep this updated. I guess I will just inspect for anything other than a smooth surface, then assess the situation then. I'll get a mic on the journal at a few different spots as well as plasigauge. I've heard that knock a few times before on other engines. I'm always listening to this motor and this started just recently. When hot it's hard to hear it, but it starts after 2-3 min then doesnt go away. It has some slight piston slap but this is a different sound for sure.
 
Messages
903
Location
New Hampshire USA
Not sure it`s a rod, how is the oil pressure, any knock on startup, does the knock appear on slight throttle increase from say 1600 rpm? Have you tried locating the sound with a stethoscope?
 

i6pwr

Thread starter
Messages
363
Location
Va
Oil pressure is good, the pump on these motors are capable of 11 gallons per min @2500, may not see much of a drop with one bad bearing....not sure though. I get a slight, quick piston slap on cold startup, somewhat quiet though. If I run it up near 2000 rpm, I can still hear it, but only when under the with someone else revving it, I don't think it's a wrist pin. I did use a stethoscope and when placed near the #4 coil on the head, I can hear a light knock and goes away with the coil disconnected. This isn't a knock that's been around awhile, tough to say if it's a severely worn piston skirt or not. The rod knocks I've heard in the past in their early stages, tend to get noisy after a min or 2 from startup, then follow the cam RPM...until they get so bad it sounds like the rod is ready to punch through the block. I hope it's not a rod bearing, and if so, I might get lucky with fresh bearings...but I didn't want to drive it any more until I can confirm This is about 5 min after warmup, I was feathering the gas a few times raising the RPM slightly.
The mic on the phone seems to cancel some of the fan noise, the knock sounds slightly sharper in the clip than in person...but very similiar.
 
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Messages
1,062
Location
Minnesota
Ha-My wife has one of those Midgets with only 3 main bearings. There was no audible knock, but at about 25k miles the hot idle oil pressure was down a tad. I noted that a pan drop was easy and soon found that the #3 main shells were blue from overheating and wanting to curl inward. Surprisingly there was no damage at all to the crank journal. New standard shell and thrust bearings were slid in and all has been well for many years and miles.
 
Messages
25,805
Location
MA, Mittelfranken.de
Originally Posted by AITG
I worked in a shop where the Manager tried this years ago. The unfortunate mechanic spend two days with crocus cloth and re-assembled it per instructions. It let go on the road test. If it's bad enough to knock, you have no way to keep it within spec as you remove material.
The question is totally hypothetical, best case. As you say if its knocking the damage may already be done but if caught early it may not have scored it up, if you can hang a fingernail on any scores it then its probably done for and needs to come out and be reground and proper size bearings fitted.
 
Messages
2,363
Location
NY, NY
I tried it once on some four cylinder engine to try and save a friend some money. It was a failed attempt. I ended up taking the crank out and having it repaired and installing new bearings. After that, it ran perfectly... A month later, she wrecked the car.
 
Messages
10,004
Location
Waco, TX
Originally Posted by AITG
If it's bad enough to knock, you have no way to keep it within spec as you remove material.
This is exactly why .001" and .002" undersize bearings are made and for sale
 

Kestas

Staff member
Messages
13,946
Location
The Motor City
In production, grinding on ductile iron crankshafts is done in the opposite direction of crank rotation. This is followed by polishing in the same direction as crank rotation. Done improperly, it can result with the surface that looks like a cheese grater at high magnification.
 
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