Stock Piston Ring Gap

[idkface]

Hey guys I have a dilemma.

This is for a stock bottom end rebuild. I'm using Grant rings.

VAG ring gap specs: 0.0079" to 0.0157"
Ring gap specs for stock application according to everyone on the internet: 0.016 (top ring), 0.018 (second ring). +0.002" on both rings for big turbo application.

Can someone please shed some light as to why people are recommending a gap outside of VAG specs and why the top rings is .002" smaller than the second ring?

Also, oil control ring: 0.0098" to 0.0197"
Mine are waaay passed being out of spec if these are the numbers I need to follow.

Thanks!

Edit: I think this belongs in engine tech, but I can't figure out how to delete this.

[Seerlah]

Haynes manual has ring specs. You don't run the rings out of spec. You have different spec ranges. You run a wider gap for higher boost applications. More boost, the more ring expansion.

[Kevin C]

More boost = more heat in the rings, so you need a bit more gap to prevent the ends from butting. You want to make sure that the top ring always seals. By having a tighter gap than the second ring it (the top ring) always has a good pressure differential. If you don't have pressure behind the ring to push it outwards, it won't seal. When you have a tight second ring gap, the leakage past the top rings gap can be high enough that it does not build sufficient back pressure to properly seal against the cylinder wall and the second ring ends up being the seal.

The top ring is where you want the best compression seal.

Most heat is transferred from the crown through the top ring and into the cylinder wall (it has the best thermal path from the crown). With a poorly seating top ring the piston and the ring run hotter. That can (will) lead to coking up of the top ring groove, piston damage as well as ring damage. The second ring often ( almost always) has a reverse taper and helps the oil ring to scrap the cylinder wall on the down stroke. Its part compression ring and part oil scraper as well as a better seal on the intake stroke (reverse taper).

Lots of dyno data supports that a wider second gap makes more power (Ford , GM etc).

The second ring as a scraper was invented in the 1930-40's. It made a large, immediate reduction in how much oil engines use.

[idkface]

More boost = more heat in the rings, so you need a bit more gap to prevent the ends from butting. You want to make sure that the top ring always seals. By having a tighter gap than the second ring it (the top ring) always has a good pressure differential. If you don't have pressure behind the ring to push it outwards, it won't seal. When you have a tight second ring gap, the leakage past the top rings gap can be high enough that it does not build sufficient back pressure to properly seal against the cylinder wall and the second ring ends up being the seal.

The top ring is where you want the best compression seal.

Most heat is transferred from the crown through the top ring and into the cylinder wall (it has the best thermal path from the crown). With a poorly seating top ring the piston and the ring run hotter. That can (will) lead to coking up of the top ring groove, piston damage as well as ring damage. The second ring often ( almost always) has a reverse taper and helps the oil ring to scrap the cylinder wall on the down stroke. Its part compression ring and part oil scraper as well as a better seal on the intake stroke (reverse taper).

Lots of dyno data supports that a wider second gap makes more power (Ford , GM etc).

The second ring as a scraper was invented in the 1930-40's. It made a large, immediate reduction in how much oil engines use.

Great information. It all makes sense to me now.

Why though, are some people running ring gaps .002" over spec for new rings on stock applications (stock HP)? I mean according to VAG, the wear limit is 0.0314" so I guess it's technically still okay? The reason I ask is because there seems to be a bit more wear in cylinders 3 and 4. Enough wear that my top compression rings are right at 0.0157" without ever being filed. However cylinders 1 and 2 top compression rings are were too tight and had to be gapped. I brought them to 0.0118". This means that cylinders 1 and 2 second compression rings have room to be gapped wider than the top compression rings, but not cylinders 3 and 4.

[Seerlah]

Copy and paste. I used these for my awm, which is essentially the same short block as the amb.

[idkface]

Did you do anything like gap the second compression ring bigger than the top? I'm right at the limit with the top compression rings on cylinders 3 and 4. I can't go any bigger with the second compression rings without being out of spec. Here's where I'm at right now:

Cylinder 1: Top: .3mm; Second: .3mm
Cylinder 2: Top: .3mm; Second: .3mm
Cylinder 3: Top: .4mm; Second: .3mm
Cylinder 4: Top: .4mm; Second: .3mm

Oil rings are way out of spec. I didn't measure but it looks like something close to .9mm or more on all cylinders.

[idkface]

Never mind what I said about people saying they're running .016" to .018" on stock turbo. I misread. It's actually .014" for top compression and .016" for second compression. Both of which are in spec. I understand the reason behind gapping the second compression ring a bit bigger than the top compression ring thanks to Kevin C. Since my top compression rings for cylinders 3 and 4 are already at the end of their spec (.016"), what should I do with the second compression rings? Do I file them to .016" as well? do I go .002" out of spec but still within the wear limit of .8mm?

[diagnosticator]

The specified ring end gaps are: 0.20mm > 0.40mm 1st and second rings,

0.25mm > 0.50mm Oil control ring.


This specification is not debatable. I don't understand why you are having trouble understanding this spec.

[Seerlah]

How far into the bore are you pushing the rings down before guaging them? And how are you making sure they are placed into the cylinder properly for correct gap testing? Don't measure near the top of the cylinder. Go down enough to where the rings would be. Sorry if this post comes off condescending.

[idkface]

Not condescending at all. Even the FSM says to measure ~15mm from the bottom of the bore. I'd be lying if I said I was doing it that way though. I'm using an old ring in the bottom ring land to square it up. The ring sits down passed where the old ridge would've been. I actually decided to measure my old rings. The ones I saved were all well within spec. Even the oil rings. I've come to the conclusion that my brand new Grant rings are just shit. I haven't measured the bores but I don't suspect so much wear that brand new rings would be so far out of spec. I can fit a .75mm feeler gauge in the gap of all the brand new untouched oil rings.

Gonna call Urotuning tomorrow and see if they can help me out.

[Seerlah]

Wanted to mention that earlier. Grant rings suck. Get Geotze rings. Cost more, but you get what you pay for.

[idkface]

You live you learn. Don't do Grant. Not even once.

They came in a UroTuning bottom end rebuild kit. The bearings they shipped me are Glyco. Am I going to have a problem with those too?