Burnt rod bearing

[inlet]

Okay need some second and third guessing on possible causes for this failure.<br />5.7 Mercruisers, ran well, but decided to rebuild because I have the inhouse resources to do the job and the hours were getting up there.<br />Rebuilt port engine developed knock in first hour, limped in, pulled engine and stripped down to find #8 Rod bearing almost completely gone. Can't determine why? Going with a new crank and bearings (every thing else looks good), but really would have liked to have found the root cause. <br />Shop Guru claims everything spec'd out on initial rebuild, he is a good wrench and very attentive to detail. <br />What are the possible causes for that failure, we're guessing debris, but just don't know, which is somewhat troubling.

 

[Laddies]

Re: Burnt rod bearing

Good wrenchs don't leave debris, were the rods rebuilt, was the crank turned if so were the oil passages champfered, were the brgs plastic gauged, did it have a new oil pump, it could be alot of things hard to say without the parts in hand--Bob

 

[inlet]

Re: Burnt rod bearing

Crank was within specs, new connecting rods. Did plastic gauge everything, new oil pump. Believe me nobody wants to think that it was a piece of grit that caused the problem, that's why I'm asking for alternate causes. I'm not a mechanic, but I've employed this man for 5 yrs, and he is both competant and fastidious.

 

[weatherz]

Re: Burnt rod bearing

I've seen very small pieces of shot peen destroy a bearing before. That's what it sounds like. The block must be hot tanked after shot peening and cleaned and blown out very thouroughly.

 

[inlet]

Re: Burnt rod bearing

Thanks, mechanic despite himself is suggesting something like that must be the cause.

 

[cc lancer]

Re: Burnt rod bearing

Insufficient clearance on the rod bearing, after the oil heated up, it closed the bearing to crank clearance.<br />If the oil hole in the crank was stopped up, this would have been evident as soon as the rod cap, and the rod was shoved away from the crank.<br /><br />That is the reason the engine lasted an hour.<br />I don't like plastic guages, my micrometer dosen't lie to me.

 

[inlet]

Re: Burnt rod bearing

Makes sense, The appearance of the crank is that there was no oil film on the mating surfaces, bearing heated up and put a noticible groove in the crank. Would think though, that all the bearings would be equally too tight. Would there not be some evidence on the other rodbearing surfaces?<br />I believe the bearings were too original spec. with the old crank, any wear on the crank would mean extra clearance.

 

[cc lancer]

Re: Burnt rod bearing

Would there not be some evidence on the other rod bearing surfaces?<br /><br />Not necessarily, he could have picked up a used plastic-Gage, while some one was talking to him,<br />distracted him for some reason.<br />We both know the bearing is soft, you can scratch them with your finger nail. The bearing is "floating on a small film of oil",and if the clearance is minimized for a short time, plus the bottom of the rods are generating some friction heat since everything is new and tight. The clearance starts closing up because of metal expansion, and the oil can't get to the bearing.<br /><br />If this is the only mistake this man has made in 5 years, I would cut him some slack, if it happened again soon, I would cut him a final check. There is so many things that must be hung on a marine engine, no one want to do it over.

 

[inlet]

Re: Burnt rod bearing

Thanks for the mechanical imput, human resource advice not required.

 

[cc lancer]

Re: Burnt rod bearing

One other suggestion look at a Starrett Digital micrometer. Check the crank in 4 to 6 places around the crank, put the cap on the rod and use digital caliper to measure the inside. Subtract the difference. This was the way I was taught at one of the race engine shops here in NC.

 

[cc lancer]

Re: Burnt rod bearing

human resource advice not required<br /><br />Nor was it given.<br /><br /> You will notice the first two words of the comment.<br /><br />I would cut him some slack, if it happened again soon, I would cut him a final check.<br /><br />I did not say.... You should.<br /><br />As an employer for 30 years I was stating what I would do.

 

[rodbolt]

Re: Burnt rod bearing

sounds like insuffiecent clearence. usually grit will embed in the bearing but not cause the crank to turn blue. if the crank and rod turned blue it was an oiling failure, either due to a blocked crank passage or insufficient clearence. plastigage is the prefered final assy method of checking but myself the preliminary fit I measure the crank journal in the places for an average and use a dial bore gauge on the assembled cap/rod assy with the nuts torqued and the bearing in place and do the math. the actual engine assy is quick. the prep work for assy takes 20 to 30 hours. no one wants to pay it anymore. if I was putting a stock 350 for grannys grocery getter together a single plastigauge measurement will suffice. when assembling a high performance engine that will be required to run hours at a time at its rated horsepower it wont. worst I ever saw was a friend and his son assembled a 400 sbc and did not clean the crank after having it ground, lasted about 30 min before total loss of oil pressure. took about 6 hours to clean the debris out after it was dissasembled. the only way to clean it is with hot soapy water and a set of brushes. but occasionally it will be debris or casting flash that was not removed prior to assy and no real fault of the assembler.

 

[AaronG]

Re: Burnt rod bearing

Maybe you guys ordered the wrong rod bearings, or the set you bought was mislabeled? I know you said you plastigaged evertthing, but....<br /><br />I'd be checking the other 7. I bet at least one of them shows wear too.

 

[inlet]

Re: Burnt rod bearing

Thanks very much for your expertise guys. Off to work now with a bit better understanding of the problem.

 

[inlet]

Re: Burnt rod bearing

Hi Aaron. <br />Just noticed your comment, my only contribution to the rebuild was to ask a few questions along the way. That said, I have looked at the crank and there is only wear on #8.

 

[Wellcraft 89]

Re: Burnt rod bearing

Ask your wrench if he mic'd the crank for being round.If the rod journal was out of round it would do what it did. Did it actually spin the bearing on the rod ? or just burnish the bearing?<br />When we are rebuilding we always mic'd everything<br />3 times before plastic guage. <br />Rich

 

[inlet]

Re: Burnt rod bearing

Did not use a mic. Mechanic describes it as a spun bearing, bearing is feathered down to nothing on it's edges. Crank has a fingernail thick, full width of the bearing groove worn in to it. There was some serious stress. <br />Discussed with him today my enquiries. <br />His response, and I should say he understands exactly the various scenarios,(much more so than I)<br /> Three possibles:<br />1) Debris.. unlikely<br />2) Bearing clearance .. ????? why only one bearing?<br />3)Whoops, is it possible that the one bearing in the final placement was tightened with the tabs not quite aligned, creating zero clearance. He brought that one up as a suggestion made by fellow Guru.

 

[Dunaruna]

Re: Burnt rod bearing

I have found the wrong bearings packaged in the right box before. <br /><br />My vote is debris.<br /><br />Aldo

 

[LubeDude]

Re: Burnt rod bearing

Being an engine builder of sorts, and mainly small blocks, I have to ask two questions.<br /><br />1. What was used for assembly lube?<br /><br />2. Was the engine oil system primed before you started it?

 

[inlet]

Re: Burnt rod bearing

1 don't know<br />2