Piston thrust face scuffing, Eagle rods?

[jmeager]

I recently rebuilt a Nissan SR20 turbo charged engine. Ran fine for around 2000 miles. Pulled it down due to another failure and found thrust face wear on the forged pistons.
We installed some Arias forged pistons and Eagle H-Beam rods. These rods however do not have a thrust face oil squirter hole, whereas the factory rods do.
Piston number 1 started to scuff lightly, 2 and 3 are perfect, and 4 is quite bad.

I am chasing the cause. Given some cylinders are ok and some aren't has me confused. I assume that as the rods do not have the thrust squirters on them, they rely on the splash from the crank to lubricate the thrust face. Would it therefore be fair to say that two and three's bearing clearances on the conrods might be greater, thus passing more oil to lubricate the bores more?

At no point had oil pressure seemed to be a problem as far as I know.

Any ideas on stopping this happening again?

Run more clearance on the con rod bearings to help pass more oil?
Or drill a tiny hole where the thrust oil squirter should be?

Any advice would be great.

Thanks in advance

Jeff Meager

 

[Metalguy]

Interesting. IIRC, the oil leaving the sides of rod bearings vaies as the square or cube of the clearance, so a diff. of say .0005" could mean a big diff. in oil throw.

Did you set the rod bearing clearance on the tight side? How much side clearance--if too little that will reduce oil throw too.

 

[jmeager]

Not being an expert on such things, and self tought, assumptions were made.

The bearings reinstalled after the rebuild were of the same specification as original. Nissan do a numbering coding system on their stuff and we just replaced same with same. Knowing however that we were using different rods with the same crank, I plastigauged the bearing clearances to be sure they were ok.
Nissan spec the clearance to be 0.0008-0.0018 in.
My measurements came to cyl 1,3,4 - 0.0015, cyl 2 0.001.

That would therefore indicate that bearing clearance to be no issue, as cyl 2 and 3 are not worn, but one of those has tight clearance.....

I did not (my big mistake maybe) measure the side clearance. Not knowing how I could change this anyway, I didn't check. I now will be checking this....

Any other ideas? Could that cylinder be running lean causing more expansion? I would have thought though that the expansion wouldn't matter until such point as siezing occurred between both sides?! And as only the thrust is worn.....

Thanks
Jeff

 

[Metalguy]

Rod side clearance is increased by grinding the side of the rod big-end a little, if required. It can only be decreased by selective fit, but I've never heard of anyone who wanted to decrease it. What oil are you using?

 

[jmeager]

Was using a Valvoline semi syth 10W-40. Since found out it was only a semi too. The engine was putting out 350 hp at the wheels out of 2Litres of capacity, which is basically twice the power output from factory, so it was fairly stressed.

There are many of these outputting much more, so I don't think this is the problem.

The Eagle rods are specifically designed as a factory replacement, so I doubt they would be incorrect, but never assume, I know. I shall check this .....

The oil holes in the pistons are for returning oil from the bore from the oil rings, correct? They are not for PUTTING oil on the bore are they? I'm sure not....

Jeff

 

[SMOKEY44211]

Given the clearances quoted I would say you are a little on the tight side.My experience suggest .002-.0025" to be optimum. This would also promote addition lube to the cylinder walls. The fact that the factory put squirter holes in the oem rods makes the lack of lube suspect. Question for you were the cylinder walls oiled prior to initial fire up? Did oil pressure come up right away? Are the skirts galled up or do they just have some verticle lines? Possible contamination. If you don't want to drill the rods machine a couple of grooves in the thrust face in the same direction the oem hole was oriented. If you are only seeing distress on the thrust face I would say piston to wall clearance is not an issue. Hope this helps.----Phil

 

[Metalguy]

Agree w/Smokey. Yes, the oil holes in the piston behind the oil rings are to remove oil, not supply it.

 

[76GMC1500]

Did the car come originally with forged pistons? I don't think it did. Forged pistons generally require more piston-cylinder wall clearance than cast pistons because they have a higher coeficient of expansion.

 

[patprimmer]

I agree with all the points above and would add that extra boost and variations in mixture or boost for different cylinders could cause scuffing if the clearance is inadequate. The force acting on the side of the piston is always higher on the thrust side, so if the piston expands to a micron or two clearance, it might have enough friction so as to not quite scuff the non thrust side, but still scuff the thrust side.

Also, how clean were the bores on assembly.

How uniform was the block temperature as you bedded in the rings.

Regards
pat

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[Metalguy]

I have knowledge of a 20 cyl, turbo diesel (8,000 +HP) that had several pistons severely scuff/score/ only on the non-thrust side, but it was an unusual case.

The side loads were actually higher on the non-thrust side.

OK, let the "how come" guesses start!

 

[patprimmer]

Excessive pin offset in wrong direction?



Regards
pat

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[willeng]

jmeager:
Although some people like ARIAS pistons we have had several problems associated with these & now use an alternative brand with no problems.
All the suggestions above are good & i'd say your definately tight on your rod clearances for your application.

Where are the scuff marks ie are they in the middle of the skirt or are they offset from the middle or are they both sides of middle etc.

Cooling to No 4 pot could be an issue also, maybe it would pay to test the coolant flow while the engine is torn down.

Do the bearings on No 4 rod journal show any signs of being hot etc indicating low oil pressure or in sufficient oil, also is the radius on the crank rod journal correct.
If you fit a bearing shell to the rod cap & place it on the journal you can check to see if the bearing shell has sufficient clearance to the radius, if not it may interupt the oil flow.

 

[jmeager]

Thanks everyone.
Not factory forged, correct. The block was bored with a torque plate (alloy block with steel liners), and taken out to the necessary clearance as specified with the pistons.
I cleaned and cleaned the bore knowing of the possibility of problems with this and yes, they were as clean as I could get them. I'm sure I rag tested them and they came out clean.
Smokey, we tried to stick within the specs as per the nissan manual for the engine. These engines are known to run very tight clearances throughout the engine. This however may also give the reason for the conrod thrust squirters, as they run tight bearing clearances, more oil is needed.....

Cylinder 4 is very galled up. About the size of your thumb. Cyl 1 has only 1 or two vertical lines.

As for pre-oiling. I would shake and lower my head and say no. We used a tub of redline engine assembly lube and thoroughly coated everything with this stuff. But I must say, the more I think about it, the more it is quite possible that not putting oil/engine lube directly to the piston walls (instead of just the pistons) could have meant there was not enough lube on the walls prior to startup.
The engine wasn't pre-pressurised as I have read about since. We setup the engine, and cranked it to get up oil pressure (disconnecting efi so it wouldn't start). Once oil pressure was up, we went for it.
Is this a bad move, or ok?

Finally in summary. I think even though there was probably a lack of cyl wall lube due to not pre-oiling, the pistons were dripping i assembly lube, and cyl 2 and three are ok so I'm not convinced. (but it's hard to know)

Smokey, can you ellaborate on the machining you are talking about?

Thanks everyone

Jeff

 

[jmeager]

Willeng, thankyou.

I am skeptical. The arias take away a fair bit of the pin offset that is there in the factory units. Ie, it was easy to measure the factory offset, nearly impossible to tell the the arias, but there is a tiny bit.
Scuffing is on the middle of the pistons.

OK, can someone point me to a site with pics of good versus bad bearings please. As far as I know, they are fine. There is no discoloring to any of the bearings. The only signs on the bearings are shiny patches where they are wearing. Mostly they still retain their dull, brand new look.

Finally, as part of the rebuild, I am putting in a higher flowing oil pump. Can I expect this to help the situation in the future? I am not saying that I will use this to "fix" the problem. I am very keen to know more about what could be wrong to fix it the right way, rather than to bandaid it up another way....

Jeff

 

[patprimmer]

The Clevite catalog has some good info and pics

Regards
pat

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[jmeager]

Had a look at the Clevite site and the alli bearing section. They look fine compared. They have spot wear, but as mentioned by clevite, this is fairly normal. There is no scratching, discolouration etc to be seen.
I don't think lack of oil to the bearing is a problem, more likely the lack of oil squirted onto the thrust face by the rod.

How would pin offset affect thrust wear? ie, being the arias have less offset? It still doesn't explain why two cylinders are fine.....

 

[jmeager]

ANOTHER question (what, he wants to know more!)....

ok, as mentioned previously, Nissan bearing clearance specs are 0.0008 to 0.0018in.

I had about .001 to .0015.

If I was to go to .0025in clearance, what negative effects do I have other than loss of oil pressure do to excess oil being lost to spray?
Am I mad to go out of factory specs for this? Although it's not like I'm sticking to factory specs for power etc am I....
Will it cause failure by running larger clearance?

It would be fair to say that the main bearings would have no affect on the splash to the crank as they are not flying up and down?!

thanks

 

[enginesrus]

"We used a tub of redline engine assembly lube and thoroughly coated everything with this stuff. But I must say, the more I think about it, the more it is quite possible that not putting oil/engine lube directly to the piston walls (instead of just the pistons) could have meant there was not enough lube on the walls prior to startup."

Yup, the oil will go where the grease won't.

 

[76GMC1500]

Cranking it until you get oil pressure is good enough. The best way to clean cylinder walls is using soap and water or engine oil. Solvents like laquer thinners, mineral spirits, etc... don't pick up all of the small particles left in the scratches from the honing process. I don't think that is your problem though.

 

[SMOKEY44211]

Let's see if we can't get you back on track with a minimum of expense. Given the information provided so far I think its a pretty safe bet that the problem is related to inadequate splash oiling to the piston skirts. If you can imagine drilling 1/2 of a hole in the big end of the rod where the face meets the cheek of the crank. You can do this on both sides of the rods. I only suggested that as an alternative to drilling a hole ala factory method. Adding another .0005-.001" rod bearing clearance will give you a greater load carrying capability of the hydrodynamic wedge formed by the oil film. Hopefully you are going to introduce greater forces at this location. The additional flow helps carry away some of the heat from this area. I agree that having a lot of oil splashing around robs some of the power and also churns a lot of air into the oil supply. A lot of people think that the oil pressure drops as the egine heats up by making it "thinner". While this is partially true most of the loss in guage reading is due to the airation making the oil somewhat compressable. If the oil pump you are running now is up to the task I would stick with it. Going larger will just pump more oil through the bypass valve. For what its worth I have used Arias and Eagle products in the past with excellant results. So with a new set of pistons, good assembly hygine, drill or groove rod, and a few squirts of oil on critacal sufaces you should be good to go.-----Phil