Piston Rings Worn Out, Time for a Rebuild

[74M35A2]

Top ring may have been chrome plated. Second rings are usually just plain iron. Check the new rings you ordered when they arrive, this may be the case there. I think relaxed state does matter partially as it will indicate the ring sealing pressure against the cylinder wall. Again, check the new rings when they arrive. I am really tuning into what looks like carbon around the wrist pin area. That looks like combustion getting past the rings, if the skirts were not Teflon coated like the tops (I'd have to go back and check the pics). Also, what about the white deposit outside of the injection bowl on top of the piston? Is there any polarity to the injectors? I know you have had them out and apart several times, adjusted pop pressure, all that. If the injector hole(s) are somehow now not aimed correctly at the bowl, it could be causing all of this?

 

[Wildchild467]

Top ring may have been chrome plated. Second rings are usually just plain iron. Check the new rings you ordered when they arrive, this may be the case there. I think relaxed state does matter partially as it will indicate the ring sealing pressure against the cylinder wall. Again, check the new rings when they arrive. I am really tuning into what looks like carbon around the wrist pin area. That looks like combustion getting past the rings, if the skirts were not Teflon coated like the tops (I'd have to go back and check the pics). Also, what about the white deposit outside of the injection bowl on top of the piston? Is there any polarity to the injectors? I know you have had them out and apart several times, adjusted pop pressure, all that. If the injector hole(s) are somehow now not aimed correctly at the bowl, it could be causing all of this?

Injectors all have timing marks so they only go together one way. I have also whitnessed them going in the cylinder heads and they align perfectly. Pop pressure was set to the bosch manual for these injectors.

 

[Wildchild467]

I measured the height clearance of the second compression ring and the piston. I measured .010". A quick google search showed me other pistons (not multifuel) that the spec was between .0015" and .003". I have to search the TM's to see if I can find the specs on the pistons. I have never seen them before, but I am sure they are out there. I think this huge clearance issue is the reason for my huge loss of compression on this cylinder. .010" seems huge. I need to get the specs so I can then see what is out of spec (if that is out of spec), the ring or the ring groove.

 

[gringeltaube]

You can't measure these with a feeler gauge like that, they are tapered, both sides (= trapezoidal section).
See TM 9-2815-204-35; page 212, for the LDS427-2 which has a very similar top compression ring.

......
Picture 4: This is what I am most curious about. The top ring has (2) dots, while the second ring only has (1) dot. Are these backwards of what they should be? I need to research this to be sure but it seems like the (1) dot should be on the top and the (2) dots should be on the second groove. I need to pull the #1 piston out which shows only 15% leak down and see what the piston ring dots are on that one....

I am thinking the rings are not in the correct order from the factory and that messed everything up. Other pistons will tell if they are in the same order.

I'm afraid you will find them all in exactly that same order because I just pulled two of mine out of the box and they were assembled like that: N.1-top ring = 2 dots; N.2 compression ring = 1 dot.
I don't think there is a TM that would show this particular arrangement so I'm just assuming they all came that way, and that that is the way it's supposed to work.

They were not clocked correctly, that was for sure... but I fixed that before I put them in. I clocked the compression rings 180 degrees apart. When I pulled them out, I discovered the compression rings were at about 10:00 and 1:00 maybe.

That is very strange. What can make a piston ring rotate, once it's been installed?

I took the top ring off and put it inside the cylinder to measure the ring gap. It measured about .010"

Top rings are supposed to have more gap than the second ring because they get more heat and will expand more...

Agreed but I have measured the ring gap on both rings in question: 0.025" (+/- 0.0005”) for both. And just for comparison, also measured - put in that same spot, same liner- the top ring out of an LDT with very low miles: 0.029".
0.010" seems a bit on the low side to me.


G.

 

[Wildchild467]

Are there any TMs showing these style pistons or any dimensions/wear limits for them? I never saw any, only for the wrist pin hole wear limits.

I wonder what the rings are made out of... is the top one chrome? Are new piston rings available for these new style pistons?

Any ideas what my issue could be so I do not run into the same problem?

 

[JasonS]

Yes, piston rings rotate. I'd guess that the cross hatch is the cause. What is also interesting is that the rings will rotate at different rates. Accordingly, no matter that you carefully you space out the gaps, they will eventually line up.... and then move out of alignment....

 

[74M35A2]

Maybe it didn't like breaking in on WMO? I don't get it that #2 had 95% leak down rate, and the rings are not shattered. I just can't wrap my head around it. Your other cylinders were all over the map, from 15% leak to 95%. Maybe test your injectors once more to make sure none are leaking? Pintles can be lapped in with compound if they are, that is what Superior did with my Bosch injectors on my Kubota engined light tower, runs great now. I'm at a loss, and don't know what to add except to say break it in on diesel and hope for a better result. Maybe it can handle WMO once the rings are seated.

 

[rustystud]

Maybe it didn't like breaking in on WMO? I don't get it that #2 had 95% leak down rate, and the rings are not shattered. I just can't wrap my head around it. Your other cylinders were all over the map, from 15% leak to 95%. Maybe test your injectors once more to make sure none are leaking? Pintles can be lapped in with compound if they are, that is what Superior did with my Bosch injectors on my Kubota engined light tower, runs great now. I'm at a loss, and don't know what to add except to say break it in on diesel and hope for a better result. Maybe it can handle WMO once the rings are seated.

I'm starting to think along these lines also. Who knows what is in that WMO your using. You could be injecting "cleaning solution" down those cylinders for all we know. Like I said earlier. There is to much carbon on that piston (top and sides) !
Did you find out which direction the rings taper was pointing ? The tapered side should be facing the combustion chamber.

 

[gringeltaube]

The tapered side should be facing the combustion chamber.

Dot markings always go facing up, or? Plus the second ring (bottom in picture) even says TOP, on the face which is stepped.

And BTW, what if both sides are tapered, same angle (top in picture)?

 

[rustystud]

Dot markings always go facing up, or? Plus the second ring (bottom in picture) even says TOP, on the face which is stepped.

And BTW, what if both sides are tapered, same angle (top in picture)?

If the manufacturer puts "TOP" on there ring then I would go with that. Looking at that ring they must have some way of pressurizing the ring to have it "push" against the cylinder. I cannot tell from the picture which side is level and which is tapered. On all "unmarked" rings though you go with the taper facing the combustion chamber.

 

[welldigger]

What engine oil did you use?

Don't know if what brand or type matters a whole lot. Just curious.

 

[welldigger]

I re-read the parts of this thread where you fired it off for the first time after the rebuild.

Honestly, I'm starting to think you were too gentle with it. Diesels need a good bit of load on them to seat the rings correctly with as little idling as possible.

 

[74M35A2]

He pulled the remaining 5 pistons out last night, no broken rings, and all rings were consistent with dots facing up. 2 dots on top ring, and 1 dot on bottom ring. I was wrong on my broken ring guess. I have never seen 95% leak down rate with new rings intact. Starting to look like they just did not seat correctly for whatever reason. His plan is to ball hone the new cylinders, clean up his previous pistons which have one additional ring on them, re-ring and install those pistons, and then attempt a break in per procedure, though that will be difficult because procedure uses dyno loading values of hp and RPM (15hp @ 1000rpm for the first hour or whatever). All parts are ordered or have been received already. He will break it in on 100% diesel fuel, and if leak values stay proper, then later decide whether to try WMO oil again, well after the rings have broken in. Or, maybe just run WMO from known sources and not the local city recycle center.

Here is an interesting read on various ring type from an OE ring manufacturer: http://korihandbook.federalmogul.com/en/section_18.htm

 

[JasonS]

I forgot to add, rings will not rotate once they are broken in. They will barely rotate even before then.

LOL. I know you love to argue EVERYTHING. You are correct that they will stop rotating if the cylinder becomes out of round. Otherwise, they will normally rotate. This has been studied quite a bit by engineers and published; read up. Let me know if you need some hand-holding and I'll even do the search for you.

 

[daytonatrbo]

If the manufacturer puts "TOP" on there ring then I would go with that. Looking at that ring they must have some way of pressurizing the ring to have it "push" against the cylinder. I cannot tell from the picture which side is level and which is tapered. On all "unmarked" rings though you go with the taper facing the combustion chamber.

(Rusty, I know you know this, but just to clarify) The taper that matters on the 2nd ring is the angle of the face that meets the cylinder wall. It has to go /| |\ so that it drags the oil away from the combustion chamber.

Judging by the wear marks on Wild's 2nd ring, they were installed that way.

 

[daytonatrbo]

LOL. I know you love to argue EVERYTHING. You are correct that they will stop rotating if the cylinder becomes out of round. Otherwise, they will normally rotate. This has been studied quite a bit by engineers and published; read up. Let me know if you need some hand-holding and I'll even do the search for you.

https://www.highpowermedia.com/blog/3954/rotation-of-piston-rings

Interesting.

I guess that's why in 2 stroke power sports motors, there is a dowel pin to keep the rings properly clocked.

 

[Wildchild467]

What engine oil did you use?

Don't know if what brand or type matters a whole lot. Just curious.

Rotella 15W-40, white jug, regular stuff.

 

[Wildchild467]

I looked a the cylinders and they do look awful shiny. I am wondering if the cylinders are glazed? I can still see cross hatch though and I thought glazed cylinders are when you cant see any cross hatch.

 

[JasonS]

Rusty, please see here, right from the ring manufacturer: http://www.ms-motorservice.com.tr/fileadmin/media/MAM/PDF_Assets/ks_50003958-02_web_53094.pdf

1.6.11Piston ring movementsRing rotationIn order for piston rings to run-in andseal properly, they must be able to rotatefreely in their grooves. The ring rotationis the result of the honing pattern (crossgrinding) on the one hand, and the rockingmovement of the piston at the top andbottom dead centre on the other hand.Flatter honing angles cause fewer ringrotations whereas steeper angles resultin higher ring revolution rates. The ringrotation also depends on the enginespeed. 5 to 15 revolutions per minuteare realistic figures to get an idea of thedimension of the ring rotation. On twostrokeengines, the rings are securedagainst rotation. As a result, both ringrotation and rebounding of the joint endsinto the gas channels are prevented.Two-stroke engines are mainly used inmotorcycles, gardening machines andthe like. The irregular wear of the rings,a possible coking in the ring grooves anda restricted service life due to inhibitedring rotation is tolerated in this regard.In any case, this type of application isdimensioned for a shorter service lifeof the engine from the outset. Therequirements made on a normal fourstrokevehicle engine that is in road use,are higher by far where the mileage isconcerned.The twisting of the ring joints by 120 ° toeach other is only intended for facilitatingthe start of the new engine. After thisperiod, any conceivable position of thepiston ring inside the ring groove ispossible, provided the rotation is notinhibited by design (two-stroke engines).


If you are interested in reading up in more detail, do a google scholar search.

 

[gringeltaube]

I cannot tell from the picture which side is level and which is tapered...

I said it already: both sides are tapered!


(drawing based on TM 9-2815-210-35, fig. 286 (Sep1964))