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Northstar engine piston ring discussion.

9K views 0 replies 1 participant last post by  JimD 
#1 · (Edited by Moderator)
Re: Engine removal out the top. 2001 Deville

Moderator edit: This post copied from an engine overhaul discussion. More information in this vein can be had by studying the Cadillac Technical Archive way up ^^^ in the top black bar. All written by a Northstar Powertrain Engineer and by the engineer who researched and developed the OLM algorithm. Thanks to CF member JimD for archiving these following notes.



...One thing I notice when the pistons came out was I paid attention to the radial location of all the compression and oil ring end gaps. On several of them they were very close to one another and on a couple of them they were almost inline with each other, including the oil control rings. The FSM is very clear on the location of these ring gaps in relation to one another and they are not supposed to be close to one another at all. So either the rings rotate some over time or whoever put these on at the factory was not paying attention that day. I wonder if this has something to do with the excessive oil consumption I was seeing on this engine. Some of the oil control rings were all but gummed up with carbon so I'm not sure how effective they were also.
Intro posted by JimD: The text below was written and posted here by a Cadillac Powertrain Engineer back in 2005. It's a long read but will put your mind at ease about positioning the rings on the pistons as you assemble your engine. There is also anecdotal evidence regarding the effectiveness (and necessity) of WOT therapy.

Sorry it took me so long to locate the text file of his post. I saved many of his posts and the files are not very well organized. I'm still looking for his comments on oil pump assembly / installation.

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Cadillac Owners - Oversize pistons? Undersize bearings?

03-25-05, 01:30 PM #27


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Ownership: 2005 Bonneville GXP
Re: Oversize pistons? Undersize bearings?



There is no question that rings turn on the piston. At low speeds
and low loads the rings stay pretty stable and do not move....that
is not a good thing and is one reason the Northstar engine likes an
occasional WOT blast. It keeps the rings moving and free and
"exercised".

I have seen studies at GM where the rings were tagged with a
radioactive tracer in one spot. In a running engine the rotation of
the ring could be measured/tracked/quantified by using a counter
outside the engine that would sense the proximity of the radioactive
tracer on the ring. At 6000 RPM and full load the rings walk around
the piston quite freely. Under no load, at high RPM (an overrun or
engine braking situation) the rings will move even more and you can
actually assign an "RPM" to the ring movement around the piston.
This movement is a good thing as it keeps the rings free and moving
so that they can stay in contact with the cylinder walls.

Cylinder walls are never "round" in practice and the rings are
designed to move in the ring lands and conform to the cylinder
walls. If they are not free to move they cannot conform and the
engine will not seal up.

This is very evident if you see the results of testing with
cylinders that are deliberately honed with taper in them. The ring
is constantly moving in and out of the ring groove to conform to the
slightly varying iffective bore diameter as the piston traverses the
tapered range of the bore. It will seal fine..... The piston ring
manufacturers have extensive data bases on how rings respond to
certain bore irregularites and distortion and amounts of taper.
Moving in and out due to taper constantly exposes the ring face to
different parts of the cyilnder wall and it seals fine anyway so the
ring face doesn't mate to that particular part of the cylinder wall
at all.

I have seen evidence of this in teardown engines where an
irregularity in a cylinder wall caused by a scuff or debris caused a
vertical ridge in the cylinder wall that was sufficient to catch the
open end of the ring as it turned and index it....all the rings on
that piston were aligned so that the gap was directly in line with
the scratch!!! If they hadn't been moving around they could never
have found the scratch.

The main thing that happens at break in is that the rings wipe any
"high spots" or asperities off the cylinder wall surface making the
cylinder wall surface smooth and "flat". They also seat to the sides
of the ring grooves to make the seal perfect between the sides of
the ring and both sides of the groove because that is where the ring
seals to the piston and it changes sides depending on the piston
direction travel. Also, any high spots or surface asperities on the
rings themselves will be polished or burnished down to the plane of
the cylinder wall surface but this is pretty minor considering that
the ring faces are sized, honed and lapped at the ring manufacturer
so the surface of the ring face is pretty precise.

The thing that gives the ring seal is the oil surface tension
between the ring face and the cylinder wall. There is a hydrodynamic
bearing film established between the moving ring face and the
cylinder wall so the ring never really "touches" the cylinder wall.
The more oil retention the better the seal and the longer the ring
life as the ring is suspended on the hydrodynamic bearing film away
from the cylinder wall. That is basically the reason that people
take the Northstar engine apart at 150K and see the rings are barely
worn, the cylinder wall looks perfect with the hone pattern still
fresh and intact and the piston skirts looking like new. The down
side...???...more oil consumption due to the oil retained on the
cylinder walls by the aggressive plateau honing. The thought was
that it is a good tradeoff for engine longevity but I guess we
underestimated how much a little oil consumption would upset
people....LOL LOL.

Most pistons in modern engines have hard anodized top ring lands so
as to provide a harder wear surface to allow the rings to be moved
higher on the piston to reduce crevice volume. One of the downsides
of hard anodizing is that the anodized surface looks "pebbley" under
a high powered microscope so the ring must polish or burnish the
high spots of the "pebbles" down to effect a side seal. This takes
some load and ring movement....which is why the engine needs some
gas pressure load on the piston/rings to fully seat and seal during
breakin. That is the basis of the recommendation to take a Northstar
on the expressway at 55 in manual 2nd gear and do repeated full
throttle runups to 70/75 MPH and then engine braking decels back
down to 55. Do this 10 or 12 times. Drive normally with the trans in
D for several miles and repeat. This load and unload cycle will load
the rings against the sides of the ring lands and seat them and also
cause the rings to walk aound on the piston and free up and burnish
the ring lands as is required for breakin. I have seen 30K customer
Northstar engines that had oil consumption complaints that the side
of the ring lands were not even broken in due to lack of
load...i.e...gentlemanly driving to the lodge meeting and back just
don't hack it....LOL. Pound on it
occasionally...er...ah...frequently.

Funny how none of our fleet or company cars never, ever show oil
consumption complaints or cold carbon rap..... They always looked
perfect upon dissassembly. Even a cold carbon rap engine was usually
"cured" by the time it was bought back and shipped to Detroit as the
truck driver would rod it onto the haulaway and "fix" it....or our
tech would rod it driving it back to Detroit for inspection and
"fix" it....


It's good practice I suppose to "misalign" the gaps on the rings at
assembly so that at least the engine starts out with the ring gaps
staggered but it is really not something that is viewed as terribly
important by the engineers as they know that the rings will move
anyway.


One thing that you can look at when the rings are out of the engine
is to look carefully at the face of the top and second ring and see
how much of the ring face is polished or worn or burnished. With new
rings that just run for breakin there is just a fine line of contact
or polish on the face of the ring due to the ring face contour
and/or taper. As the ring wears and ages the contact face gets wider
and wider. On the second ring in many of the early Northstar engines
and latest ones you will notice that the ring is a stepped ring or
scraper ring design so look at the ring surface above the scraper
land and there will always be a sharp line of contact just at the
scraper land. This will slowly widen with miles and eventually cover
the entire face of the ring surface that contacts the cylinder wall.
Even though the ring will still seal quite well the general rating
of the ring would be that the ring is nearing the end of it's
service life if it has worn "full face" and should be replaced. If
the ring is not full face then it is still fine and would simply be
reinstalled as it would work like a new ring. I see a lot of
Northstar engines with over 100K on them that the rings are FAR from
"full face" contact indicating the estimated ring service life would
be 250K-300K or more. Not to say the ring would just give up then
but that it would take that long to wear to full face contact. No
way would the ring last that long if it was actually contacting the
cylinder wall and wearing directly against the cylinder wall.



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EDIT:
Also found these words by the same engineer regarding 2000+ engines specifically. This is not the complete post.

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"At high RPM and WOT the rings move around on the piston...they actually
rotate on the piston and will polish away any carbon and seat themselves to the
sides of the ring grooves. This is especially important on the 2000 and later
Northstars which had hard anodized top ring lands on the pistons. Very hard and
wear resistant...also harder to breakin and seat the rings to the sides of the
ring lands to promote the best possible seal. Many oil consumption complaints on
the 2000 and later engines are related to some extent with the rings never
seating to the sides of the ring grooves due to lack of load as the engine was
babied around forever. Even engines with rings stuck in the ring grooves due to
carbon build up can eventually be freed up with enough high RPM operation."

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