Cummins 5.9 UNEVEN ring end gaps

[fitterski]

i noticed that the nanual gives end gaps as folows

page V-30

oil control: .010-.022

page V-31

top ring: .012-.018
intermediate ring: .032-.047
bottom (oil) ring: .010-.023

The oil ring spec max is incongruent .022 vs .023, this would make me lose confidence in the Cummins manual if I still had any left. In addition to that there are quite different values found on the net. I'm concerned mostly with the intermediate ring for which some widely divergent values can be found, down to as low as 1/2 the spec. The as-is gaps measured all fall within tolerance EXCEPT the intermediates ALL of which measure at around 1/2 the spec. On reaction this would prompt me to file down all the intermediates ...but wait a minute, something seems to be sticking out like a sore thumb here.

I've learned a long time ago to swing into paranoid verification mode on any doubt and here I already have 3 grounds for doubt. To begin with why would the middle ring need a much wider gap? And then why would the replacement rings all fall within spec as-is except the middle one but that in every single set?

 

[CincyFlyer]

0.001" difference for a ring end gap can safely be ignored, the only real danger is not enough ring end gap; if the ends touch, they will shortly destroy the piston, and perhaps many more parts.
Are the intermediate rings much more massive than the top ring?

 

[KWentling]

I would call Cummins with your CPL number (on the timing gear cover by the injection pump) and see what the spec is for that particular engine configuration.
The only book I have left here is for a 2004 Dodge and it calls for the intermediate ring as .014-.0249" with a maximum of .029. I'm not surprised you find many differing specs because this engine was used in many applications at differing outputs. Some turbo, some NA, etc. Lots of CPL numbers.

 

[fitterski]

0.001" difference for a ring end gap can safely be ignored, the only real danger is not enough ring end gap; if the ends touch, they will shortly destroy the piston, and perhaps many more parts.
Are the intermediate rings much more massive than the top ring?

No problem ignoring the single thou, but it IS a DISCREPANCY which in my book (not this manual) is inexcusable when all you make is engines and you charge $250cdn for your own publication. Otherwise no, the ring section is the same ballpark as the top one. I always presumed (never presume) that the sole purpose of a gap was to assure that the ends never touch in service. Maybe there IS a requirement here for an actual relief gap for some obscure reason?

 

[fitterski]

I would call Cummins with your CPL number (on the timing gear cover by the injection pump) and see what the spec is for that particular engine configuration.
The only book I have left here is for a 2004 Dodge and it calls for the intermediate ring as .014-.0249" with a maximum of .029. I'm not surprised you find many differing specs because this engine was used in many applications at differing outputs. Some turbo, some NA, etc. Lots of CPL numbers.

What blows my fuse is that they took $250 cdn from me for manual that is physically falling apart and is poor in every other respect and for which I had given them the ser# of both my "12-valves" to make sure I get the right publication. Here I am, was all set up to do the crank and pistons on the weekend but can now sit on my hands till Monday instead. That 2004's gotta be a 24-valve but that's the range of numbers I've seen spread around for 12-valves also. The last thing I wanna do is file down if I don't have to. The rings came in PAI piston kits (111336, oem cum3802494), they're a major supplier which is why I stopped upon seeing the sore-thumb difference. BTW i see no other gap spec in the manual except 032-047.

 

[fitterski]

What blows my fuse is that they took $250 cdn from me for manual that is physically falling apart and is poor in every other respect and for which I had given them the ser# of both my "12-valves" to make sure I get the right publication.

Sorry, I don't wanna be ambiguous, it'e not the truck manual but the Cummins engine manual (#3666087)

 

[PJ The Kid]

The ends will only touch under high operating temps. As mentioned before, the CPL# makes a huge difference. and engine running at power band RPM for days at a time in a power unit or generator will have different needs/specs from one being used for automotive purposes. A naturally aspirated engine can have tighter ring gap tolerances than that of a turbo'd engine due to different combustion chamber temps. Also, don't act like you have never made a typo, the desk jockeys typing the service manuals are only human as well, a .001 difference is so negligible it's almost laughable.

 

[KWentling]

Cummins Rocky Mountain in Gillette, Wy is open 24 hrs. 307-682-9611. The first question they ask will be the CPL number. Get your CPL number and call them. They have been very helpful to me. 24 hr parts is great too.

 

[DaveOmak]

I've never re rung a diesel... Some piston groves have a "proud" ridge for the ring notch to fit in... Do the rings have a notch for that ridge ?? Do the ring grooves have a "proud" ridge ?? If so, measuring the ring gap, the ring notch must be where the groove ridge is located...
If my head is up my b#tt on this, sorry...

OK.... pulled my head out and searched...
When measuring piston ring end gap, check the gap with the rings at the top and the bottom of the bore. If the bore has taper wear (bores typically wear most at the top), the end gap will be larger at the top and smaller at the bottom of the bore. Use the bottom position to set the end gap.

 

[fitterski]

I've never re rung a diesel... Some piston groves have a "proud" ridge for the ring notch to fit in... Do the rings have a notch for that ridge ?? Do the ring grooves have a "proud" ridge ?? If so, measuring the ring gap, the ring notch must be where the groove ridge is located...
If my head is up my b#tt on this, sorry...

OK.... pulled my head out and searched...
When measuring piston ring end gap, check the gap with the rings at the top and the bottom of the bore. If the bore has taper wear (bores typically wear most at the top), the end gap will be larger at the top and smaller at the bottom of the bore. Use the bottom position to set the end gap.

I've built maybe half a dozen gas engines but only one other diesel (a Deutz 4 holer) and I never heard about that proud ridge. The bores are 20 over and straight but I measure 3-1/2 inches down in the hole anyway for no other reason in this case than that is the Cummins procedure. So reading your comments I immediately went and looked but didn't see anything of note except that the top ring groves seem to be treated, they are black in the grove as well as around it on the piston side.

We NEVER stop learning. I just came across another little tidbit last night, that the ring manufacturers use robotic machines and control the gap size down to 0.0001" which I thought was really nice of them. But I wondered why two of my rings would then only allow a 0.009 feeler gauge to slide in while it was obvious that the gap itself is easily 0.015 or more just eyeballing it. So I took out the magnifying glass on a hunch and there they were: little specs inside the gap on one ring end. So much for automated machines doing 0.0001

The weekend is over, I'm gonna sort this gap business out tomorrow. My curiosity insists on finding out why the middle ring would need double the gap which only some plastic material would justify on the basis of thermal expansion IF that figure is right in the first place.