Hi,
A little history, I bought this 02 zo6 with a 440ci ls2 in it.
When ever I down shift, say coming off the freeway I get blue smoke and bad burning oil smell. It uses about a qt every two tank fulls. Anyway it only has about 20k miles on the engine. Recently it developed a lifter noise so I decided to pull the engine and go through and fine the oil burning problem. I found the piston ring gaps are-
Top ring .036”
2nd ring .030
oil ring .034 (when I put the oil ring in the cylinder it almost falls through with no tension)
Would these gaps cause my oil burning issue?
It’s a 4.155 bore.
This was the first and only set of rings I have check so far.
I thought the ring gap should be around .020-024 or so?
Also do you see how the oil ring cuts through the wrist pin area? Can that
cause the oil issue? Someone had mentioned that but I can’t believe they would
make a piston that have such a oil problem.
Last question, Do you know what brand piston this is?

Thank in advance for your help!

 

I would say you need to also take a look at the valve guides and seals. Usually blue smoke under deceleration is due to bad valve guide seals and worn guides......

Mark

 

Thanks for the reply Mark!
I installed new valve seals when I got the car thinking that might be it. The ls7 heads have been reworked with bronze guides and everything looked good. I installed new seals anyway.

 

Sac City,

I haven't built an engine in a few years, and never an aluminum block engine, but it used to be that you allowed .004 gap for every 1.00" of piston diameter. That would mean that a 4.155 bore engine should have top and second end gaps of around .016. Oil rings, if I remember were not as critical.

 

Red06 your pretty close. According Totalseal the gap should be around .019.
Hope the big ring gaps i have are the oil consumption issue.

 

I would also do as 08VR suggests and check for valve guide wear and seal integrity. On decal most engines pull a lot of vacuum and suck oil past valve seals and stems if they are worn.

 

Piston is probably made by Diamond Racing Pistons DRP-171 is probably the old part number. Picture shows oil control rings over the end of the wrist pin like your photo does. http://www.diamondracing.net/pdf/Chevy-Section.pdf
http://www.diamondracing.net/pdf/Catalog-TOC-Intro.pdf

I would be curious as to what the current piston to cylinder wall clearance is and wonder if the engine has been hot enough to take the temper out of the oil control rings.

 

Hi 1fastbob,
I think your right about them being Diamond Pistons.
I don't know for sure if it ever got to hot but I don' think i did. It always runs cool with the Ron Davis radiator. I Checked the piston clearance with a long feeler gauge and its .006-.007". Its pretty cool in the shop, about 50 degrees if that matters.

Thanks!

 

If the guides and seals are good maybe because of the excessive ring end gap your getting more crankcase pressure and the pvc system with the higher vacuum on deceleration is pulling into the intake and into the combustion chambers that way......

Let us know what you find......

Mark

 

It's hard to check piston to cyl with a feeler gauge because the curve of the cylinder. Usually a bore gauge(that most people don't have) is the way then measure the piston and do the math.

 

I'm going to send the block and pistons to the machine shop and get it checked.

 

I have had good results getting a ballpark figure with just the piston less rings and a feeler gauge, but stanger383 is correct a bore gauge is best. If you actually have .006 to .007 piston to bore clearance that sound way too loose to me. I am guessing.... but thinking .004 would be about the max. Maybe someone that has built an LS motor will chime in. I've built a few small block Fords and .004 is too much.

 

Quoting myself here. Did some research and maybe your piston to bore clearance is not that far out of whack. See link for info.

http://www.kb-silvolite.com/article....n=read&A_id=32

 

Good info 1fastbob, I'll see what the machine shop says.
Thanks!

 

I was talking to Diamond tech support about rings and he is recommending a Gapless second ring. But I'm not reading a lot of good things about running them. Seems like a lot of engines burn oil using Gapless. I'd hate to chance that.

 

The oil rings with little or almost no tension as you've described are low
tension for reduced friction. Typically racers that use them also swear
by using a VAC. pump to help control the blow bye. That particular type
of short skirt, long stroke/rod piston will inherently get oil past the lower
rail and into the expander (between oil rails) because of where the wrist
pin is located. The particular alloy used will dictate the proper piston to
wall clearance. Most forged pistons call for .004"-.0055". Yours being at
6 or 7 means it has wear but isn't junk. When a machine shop re-hones
the block they will remove another .001-.0015 in order to get a good
crosshatch for the new rings to seat. Then they can remove the pistons
from the rods, tape off the ring lands, and bead (not sand) blast the
skirt surface to kinda knurl it to keep the PtW clearance in check. This
will also hold oil better to reduce piston rattle noise. Lastly your ring
gaps are a dead give away that the engine was built to get sprayed.
Perhaps the previos owner removed the kit or just never ran one but
the builder had this detail in mind during assembly.
FWIW I'd run a gapless primary ring/napier second type combo with
standard tension oil rings and consider running a vac pump as it not
only prevents oil leaks but makes power by virtue of reducing ring flutter
which causes lost compression AND oil blow by.

 

Thanks A.R. Shale Targa,that really good info! I spoke to Diamond s tech guy and he recommended a gapless 2nd ring and napier top? He also mentioed using a vacuum pump. Doing a lot of Google searching I see some guys have good luck with gapless and others have oil issues. I think it depends on cylinder prep. The main reason in tore it down was to stop the oil burning problem, i would hate to have the same problem with gapless rings. Diamond also recommended the Diamond Pro Select with a Napier 2nd ring upgrade which I think I will go with.

From there add.
Diamond Pro Select Piston Rings
Diamond plasma moly rings, designed for street and race track performance, offers features that maximize horsepower, including: Plasma-moly facing on the top ring for instant seating and long life Cast iron second rings, with tapering face profile and open-gap design for lower inter-ring pressure buildup and excellent oil control at even the highest RPMs.

 

Those Diamond pro select rings sound like they would work real well !!!
The reason I suggest a topless primary is because of the reverse gap
theory (for NA apps., tighter primary gap than secondary). Try to trap
as much compression as possible with the primary ring because any that
gets past that and elevates inter ring pressure flutters (and unseats)
the rings. The 4 page article I read on this measured the number of
degrees of crankshaft rotation past TDC (power stroke) before the
primary ring unseats itself....conclusion....that number is increased by
running a reverse gap....MO power

 

A.R. that sounds good, but the Diamond tech said the gapless raise manifold vacuum a lot can pull oil if everything isnt perfect. Thats why I'm going with the Pro Select. Trying to eliminate oil burning and don't want to chance creating another oil issue since this tear down was to fix the oil issue it already had.