Sorry if it's in bad taste to put this up in addition to my build thread, but I figure I'd toss this one out there for the non 3.4 TDC engine swap crowd as well. The background story is that I've been building a 3.4 TDC, and am at the part where I slap the rings on my pistons and drop them in. Only problem - there's no mention in any of the ring or piston literature about ring filing/fitting and whatnot, although they don't line up with GM factory specs. GM factory specs also seem to fly in the face of all common ring end gap practices as well, so what's the deal? I'm asking for any input from anybody who's done rings before on any motor, but the 3.4 TDC would be the best. Thanks guys.

[Cross post]
A little more on piston rings and the big, headache inducing, head banging on hardwood desk causing lack of information regarding ring gap on these motors. I took them back off of my pistons (I do everything backwards...) and test fit them in the bore and measured their clearance.
1.) Using a flat top piston as a guide, press the ring (with the top side of the ring up, obviously) into the bore about where it would be at TDC. Make sure it's dead square.
http://img.photobucket.com/albums/v249/pyromaniacal/TDC%20Swap%20Project/Engine%20Rebuild/Assembly/Piston.jpg
http://img.photobucket.com/albums/v249/pyromaniacal/TDC%20Swap%20Project/Engine%20Rebuild/Assembly/Ring_In_Place.jpg

2.) Using a feeler gauge, measure how much end gap is present - consensus seems to be that the best reading is where the gauge slips in just snug enough to barely hold the gauge in place, without excess force required:
http://img.photobucket.com/albums/v249/pyromaniacal/TDC%20Swap%20Project/Engine%20Rebuild/Assembly/Measuring_Gap.jpg

My end gaps with rings straight from the box as measured by a brand new feeler gauge are as follows:

#1 Top: .015" Second: .013"
#2 Top: .014" Second: .013"
#3 Top: .015" Second: .013"
#4 Top: .015" Second: .014"
#5 Top: .015" Second: .013"
#6 Top: .014" Second: .013"

... Ok. GM (96+) spec says .008" - .018" for the top ring, and .022" - .032" (!!!) gap for the second ring. That's a big ass gap, from what I know. My machinist also says that even .024" gap on certain KB hypereutectic pistons is extreme, and .032" is nuts. I called the Federal Mogul tech line and after telling him it was a basically stock rebuild, he said something along the lines of "well um... just put them at GM spec, or something, yeah." Great. The most interesting thing was when I looked up rules of thumb for high performance motor building (SBC's mostly). They say .004" per inch of bore for the top ring, and 80% of the top ring gap for the second ring gap. Guess what that turns out to be? .0146" --> .015" for the top gap! Second gap computes at .01168 --> .012" which is really close to the .013 I'm getting! Are these things already ready to drop in, with performance use gaps already set up? There's no mention of ring gapping on the box, or anywhere. In fact there's no indicator that I should file at all, except for the glaring mismatch between what GM wants and what the second ring is set up at. I'd personally rather not file if I don't have to, but I gladly will if that's what I gotta do.

I'm so incredibly confused that I don't know what to do. I'm not going to take a chance with what has to be one of the most critical components in an internal combustion engine. Can anybody give me a couple pointers here, and perhaps shine some light on the big ring gap mystery? I'm pretty much out of places to ask, my machine shop says stay where they're at, and perhaps file a little on the second ring. Another machine shop (that didn't rebuild the motor) says file them to GM monster gap specs. The rings themselves are perfectly gapped for performance specs that may or may not apply to my engine. I'm totally out of places to ask and have no clue who to believe. Anybody have any ideas? This is the only thing holding me back at the moment.
[/Cross post]

Check with the manufacturer of the rings for advice, to my knowledge all you need be concerned about is making sure the ring ends don't butt after expansion during the extremes the engine may be exposed to, they should be able to tell you the minimum gap required to keep that from happening. The specs may vary depending on the source, but the thermodynamics will be consistent no matter what the recommendation. I have filed rings before and if you don't have equipment designed to do the job it can be a lot of work, especially if you have to file fit every ring and oil rail like I did.

I've built lots of engines... "grocery getters", performance street, and race engines.... cast pistons, hypereutectic, and forged pistons. (Mostly I build normaly asperated 2.0L to 3.0L race engines that rev to 7500 or more... but never a 3.4L TDC). The difference between cast. hypereutectic, and forged pistons has always been in the piston to cylinder wall clearance... not the end gap. I have never seen big gaps like 0.022 -0.032" for the second ring except as a service limit or for forced induction / nitrus. The 0.004" per inch of bore for the top ring and 80% for the second ring is the standard SAE recommendation.

I use Total Seal gapless rings in all of my engine builds (grocery getters to full tilt race engines), so here is my recommendation. Buy a set of total seal gapless 2nd rings for your pistions. (Yea, I know... more money spent, but the gapless rings are well worth it.) Although Total Seal only lists full sets of rings, they will sell you just the second rings if you call them and ask for them... PM me if you have problems. When you order the rings you will need to know the ring groove thickness and the ring groove depth. You can measure the rings for thickness, but measure the actual depth of the groove in the piston.

Total Seal also has a good support tech line... just don't use their 800 number for tech questions. Then go with the Total Seal recommendations for their gapless second ring. (Their rings come with a good data sheet on gaps.)

The Total Seal web site is http://www.totalseal.com/

Install them as they are and you will be fine.
Many engines under my belt for street and higher compression for fun.

(*)
Make sure you get the ring spacing correct.

[This message has been edited by Indiana_resto_guy (edited 06-23-2005).]

I just got done ringing two differant engines and my ring literature said to have >.004" per inch of cylinder bore. If you didn't have your block bored to fit bigger pistons then you're probably gonna have a slight ridge at the top of the bore. Just under the ridge the bore will open up, and as you go deeper down the bore it will slightly decrease back to the original bore specs (+ a couple thousandths for wear). You want to measure down in the bore a bit more than TDC. If you are at bare minimum at TDC then your end gap will decrease as the piston travels down the bore . You might end up closing your ring end gap totaly and put alot of strain on the rings which will cause them to wear faster than normal.

GM recommends those gaps for a reason.

The top ring is loaded by and seals because of pressure difference across it. It will obviously have blow-by through the ring gap. If the 2nd ring gap is smaller than the top ring gap, this blow by won't blow past the 2nd ring as fast as it blows by the top ring. The blow-by gasses can build up between the top and 2nd rings, decrease the pressure difference across the top ring and cause it to unload under certain conditions. Early LS6 Corvettes had a problem like this... at high RPM and very light load the rings would unload and flutter and the engine would consume large amounts of oil.

For the same reason I recommend against gapless 2nd rings.

Gapless top rings are great, IMO.

Ring side clearance is critical. MEASURE IT. Mic the rings for thickness and get a machinist to measure the width of the ring grooves to the 0.0001". I recently had a problem with rings being made too thin. The resulting ring side clearance was 0.0034 top and 0.0038 2nd. These clearances should be 0.0015 top and 0.001 or so 2nd.

Thank you for the input, gentlemen.

For those of you not following my thread, I had the block CNC bored (they even realign the bores and correct the errors in their alignment!) and then honed on your basic Sunnen machine. It's .030" over, with corresponding oversize Sealed Power H684CP pistons and E928K rings.

The rings themselves make no mention of sizing at all, not even a warning to check that they are within reason. My shop tells me to basically leave them where they're at, and maaaybe file a couple thousandths off the second ring. The first time I called the Sealed Power tech hotline, the guy really seemed uninterested and told me to put them at factory spec. I doubt he even checked to see what parts I was talking about. The second time I called, another guy answered - he actually asked me my application and told me that they're not a file to fit set - those end in like, 25 or some other number, he says. He went on to say that basically, if my bore is truly .030" over and the rings are too, then they should be good to go. I also mentioned that they seem to match up to the old hotrodders rule(s) of thumb, too. If I were doing rings by commitee, "Drop em in as is" would be the most popular vote at the moment...

So why not just drop them in? Will talked about exactly what I've been wondering about - ring flutter. So the question is, is it more likely in a high RPM capable motor (LS6, LQ1, Northstar?)? Somebody will slap me for mentoning the unholy C word, but my Chilton manual shows similar monster gap specs on the second rings on a couple other engines in the W carline. I suspect two reasons GM specificed a monster gap in the 3.4 - ring flutter, and extra room for abuse or any concievable operating condition. So what's the deal? Is GM retarded, or am I gonna get ring flutter?

Gapless top ring... a little off subject, but I too suspect that this would be a good performance move. Most of my experience is with the gapless second rings on street cars, so I am a little reluctant to recommend that yet. I've been using gapless in all out race engines for a couple of years, but those get changed every 6-7 hours of running. However, I just rebuilt my Iron Duke "grocery getter" with gapless top rings (Total Seal). I have about 5,500 miles on it and everything has gone very well.

I'd say that as long as you're not going to use N2O or forced induction (or Nitro or Methanol or anything else weird), leave the top ring gaps as-is and file the 2nd ring gaps out to the 0.020 range.

[This message has been edited by Will (edited 06-25-2005).]