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  #61  
Old 03-09-2016, 08:25 PM
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Quote:
Originally Posted by BillGrissom View Post
I agree w/ Kestreltom. After reading the link in post 49, the photo in post 53 shows horizontal boring marks which shouldn't be visible after a proper honing.

I have read different things about rings spinning in the piston grooves. One story is that when GM first switched to robot assembly, they installed rings with all the grooves aligned. That caused excessive oil consumption. Might be an apocryphal story since if the rings do spin they would soon randomize the gaps. I can't envision them spinning since seems like the sharp ends would dig into the aluminum to lock them in place, and if it spun seems it would machine away the aluminum grooves like a turning tool would.

The spinning is more of a low speed walking, if the never walked around there would be a unworn area on the piston land where the ring gap resided.

I'd doubt the GM story as why would the robot be set up to make an effort to align the ring gaps?

I remember reading a story how Chrysler measured ring rotation in the 60's using radioactive material embedded in the piston ring. I can't find that article but did find this one.
http://kns.org/jknsfile/v31/A04803285773.pdf

The rotational speeds vary from 0.5 to 3 RPM

An interesting side note is they are using a hydrogen fueled engine that allows real time measurement of oil consumption by monitoring exhaust gas for CO2 and CO.

With this they round that lined up ring gaps increase oil consumption. However I'd say that since rings rotate, increased oil consumption would be limited to the times the rings are lined up.

I'd think that an engine with a 3 piece oil ring would be less susceptible to the top and 2nd ring gaps being lined up since the oil ring does not have a gap.

Also read through this piston ring position - Engine & fuel engineering - Eng-Tips

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  #62  
Old 03-09-2016, 08:53 PM
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Have a read through this about ring gaps.

Institution Of Diagnostic Engineers
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  #63  
Old 03-09-2016, 10:16 PM
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Hope new pistons will not be required. Sad very labor intensive situation. Measuring the land depth on the wrist pin axis compared to the other axis may give some indication of need.

It hopefully should not come to it but that machine shop perhaps has insurance?
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  #64  
Old 03-09-2016, 10:38 PM
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Originally Posted by 97 SL320 View Post
Have a read through this about ring gaps.

Institution Of Diagnostic Engineers
I stopped reading the article when it suggested that all gaps in line on all cylinders could occur in a random nature. Statistically this occurrence is almost equivelant to winning a powerball lotto.

That's if the rings where installed in the normal fashion. Normally if there is rotation it will remain random in nature to some extent. I can think of no exerted forces in operation that would tend to line the gaps up on all pistons.

Even blow by pressure would tend to separate the gaps or be a neutral force. The issue even with that to me is the rings are heavily loaded at that point and cannot rotate anyways. So once again any spinning of the rings is not orchestrated.

Also it would be almost pointless to separate the gaps on installation if any normal tendency to gap group was present. The guy also claimed status as an expert witness.

I think in some cases the litigant probably got thrown under the bus by this guy. Wonder what his fees are as his principal employer would be a defendant. Or perhaps in real life he goes both ways depending on who is paying the freight.

I wondered if I had missed something in my thoughts. In this case I think it unlikely.

Last edited by barry12345; 03-09-2016 at 10:51 PM.
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  #65  
Old 03-09-2016, 10:42 PM
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Interesting article in post 62. It sounds like ring rotation is more of an "inchworm effect", rather than sliding which accounts for not wearing the piston lands. Also interesting that even when tested with a very large gap, blow-by and oil consumption hardly increased. Most likely, the problem in worn engines is from "out of round" rather than increased gap. That can cause blow-by all around the ring circumference. That also means that a hand-hone and new rings won't solve the problem. There are special rings for racers that overlap at the ends (no gap). Sounds like that would give marginal improvement.
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  #66  
Old 03-09-2016, 11:14 PM
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Actually in a worn out engine the rings have lost their ability to follow the sides of the bore due to metal fatigue. Even with the top of the top ring slanted down as it goes towards the center of the piston... which causes the gas pressure from combustion to press outwards on the ring to encourage sealing.... after a certain number of billion compressions and extensions ( in and out with respect to the center of the piston ) it simply loses its springiness ..... so since the bore wears unevenly due to the angle of the crank throw.... it simply spends part of the stroke not in sufficient contact with the side of the bore.
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  #67  
Old 03-10-2016, 01:47 PM
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The machine shop foreman came in today and measured pistons and bore, but did not share his findings. As we listened to his 1st explanation "We idled the engine too long and glazed the cilinder" I stopped the conversation, busted out a phone and said lets start over "I am recording this"

The tones changed, mine went up and theirs went down as the absolute BS was getting DEEP. Again keeping with that we must have idle the engine too long. (The engine was heat cycled a few times and then strapped to the (Loaded) dyno for varied low RPM, high load ring seating.) The idea of litigation was thrown around and then the owner of the car stepped in and struck a deal. While they will not verbally admit fault, they are building us a shortblock with a warranty at their expense Motor is on it's way there now.

Thank you all for your input and insight. At the end I believe the engine was just ever so slightly under-bored, but we will never know for sure.
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  #68  
Old 03-10-2016, 02:03 PM
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You did not ' over idle ' the engine and glaze the cylinder... you are right to go with that deal... you went in thinking this was a ' well respected shop'... I hope you share locally the entire story so others can be properly warned.
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  #69  
Old 03-10-2016, 02:17 PM
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Good guess as to cause. When they drop off a short block you probably will have to use your oil pan. Take a moment and check the crank end play. If excessive I thought you might have heard it but these diesels are noisy.

The BS seems everywhere today. I have a pending litigation issue myself to the tune of 100k+. The more they tried to rectify the original situation they created the worse it got.

They also did it on one previous job but we were able to work around it. These regulatory people actually scare me as to their negligence and incompetence.

They have no actual hands on experience but make decisions that over ride common sense and convention. Give idiots power and you produce powerful idiots and proficient lie fabricators.. An average guy off the street would be more competent.

Last edited by barry12345; 03-10-2016 at 02:29 PM.
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  #70  
Old 03-10-2016, 02:31 PM
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Checking it when it arrives is really too late...
I suggest providing the machine shop with a 617.9????? Which ever engine you have... Factory Shop Manual. That measuring and setting and choosing of thrust bearing size is before the engine is assembled... the closest proper size thrust bearing is chosen based on measuring the crank... THEN the crank is machined to fit that thrust bearing tolerance .......
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  #71  
Old 03-10-2016, 08:02 PM
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Originally Posted by barry12345 View Post
I stopped reading the article when it suggested that all gaps in line on all cylinders could occur in a random nature. Statistically this occurrence is almost equivelant to winning a powerball lotto.

Not really, we are talking about 2 ring gaps matching up, the oil ring is generally 3 piece so where the side rail gaps reside doesn't really matter.

In my years before , during and after running my own shop I've had countless engines apart. I know there is at least one that had the top two rings lined up and likely there were other occurrences.

Given the rings spin at 0.5 to 3 RPM and there are countless minutes an engine will be running, lined up gaps are more common than you'd think. I agree that taking an engine down at the exact time the gaps are lined up would be very hit and miss but that does not mean that lined up gaps don't occur in normal operation.

When assembling an engine I keep the gaps out of the skirt area as the skirt ares is the most vulnerable to wear.

As for the engine being idled too long and walls glazing up. Cylinder glazing comes from carbon filling the honing marks / the peaks being rolled flat. This reduces the sand paper effect to lap in the rings _all around the bore_ not just at the skirts.

With the engine in question , the skirt is what wore the cylinder as you can see the upper and 2 nd ring wear pattern at the top of the bore is above the skirt wear.

You could post the pics on speedtalk.com This is a high end professional engine build site. The only qualifier is that they tend to be race engine oriented though there are some guys with general machine shops.
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  #72  
Old 03-10-2016, 08:10 PM
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Just had another thought. Did the honing RMS match what MB was calling for? What sort of piston to wall clearance was the engine built to?

What if, in the days of less precise machining, MB speced a rough cylinder bore, tight piston to wall clearance and let things wear into fit?

Modern machining practice is to have a smooth bore as low tension rings since rings and bores are rounder and rings can't afford to lose much radial tension.

For this build, tight clearance , smooth shallow honing = honing wore off. Maybe?
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  #73  
Old 03-10-2016, 09:51 PM
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Re: OP's bore diameter information

From posts #1 & 30 this may be derived:
If the piston-to-wall clearance was correct (.0005"-.0015") at a bore of 90.920-90.925mm, then with a actual measured bore of 91.008mm, clearance would be nearly .0048", or more than triple the high side of correct.

The observed wear pattern is consistent with this large clearance; thrust is concentrated in a small area.

Last edited by Frank Reiner; 03-11-2016 at 09:04 AM.
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  #74  
Old 03-10-2016, 10:55 PM
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FYI...IIRC .My diesel 300d 5 cylinder engine bores were honed 2"thou from it's bored size to fit each piston as follows...

#1 piston clearance = 7/10"ths thou.
#2 thro #5 piston clearance = 1"thou.

All piston rings were OE and second rings sent to Total Seal. End gaps as per FSM .

Back in the UK a company called Hepolite ran a piston rotation test. They found the rings would rotate around the ring groove up to 7 rpm depending on engine rpm.

Total Seal second ring modification is well worth doing. I have 1% to 2% leak down on assembly and around 530 psi compression pressures after break in.

I use T/S rings on all engines as of 15 ish years ago in a Ra bore finish of Ra 12 after plateau finishing..Ed at T/S says I should use Ra20 to 25....but hey, what does he know

Honing is critical to oil consumption and ring rotation. Anything more than 45 degree cross hatch increases the ring rotation speed and ring groove wear.

Ra finish = 20 to 25 range, which is about right for most Mo coated rings and OE are down to 0.15 to 0.3 Ra finishes....which is nearly mirror finish....and talking of mirror finish........ I have been re ringing good bores in diesel engines with out first honing or roughing up the bores. Have several VW and Audi motors done this way with zero problems. Have yet to do a Mercedes diesel this way...yet.

Finally.....the bores shown here indicate the piston clearance was too tight or the coolant temp went too high. I would have liked to have seen the piston skirts. I do think the bores also tell a tale of some dirt in the assembly or inadequately cleaned bores after machining...just my 2 cents..

I tend to be super fussy about bore cleaning using MEK on a Kleenex and it must come away clean.



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  #75  
Old 03-10-2016, 11:05 PM
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Honing angle ( cross hatch ) is supposed to be 25 degrees.
" Bore cylinders in two passes. An allowance of 0.03 mm for honing is to be left in the bores. "

Not MB but interesting...
https://www.hastingsmfg.com/Master_Catalog_Pdfs/P7-9.pdf

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