Why do worn rings lower compression?

[Vetal]

Do gases escape through the ring gaps, or between rings and cylinder walls, or between rings and piston?Please enlighten me on this a little bit

[Ubernoober]

A large amount of the sealing characteristics of rings comes from the pressure of compression pushing the rings against the cylinder wall. As the rings and the wall wear, the gap behind the rings changes, reducing the ability of the gasses to properly push the rings into place.In addition, the ring gap widens, and the face of the rings becomes less able to seal against the cylinder walls. As you can see, its not just one thing. Don't forget that the oil control rings wear as well, leving a thicker film on the walls, which the compression rings now have to scrape off. This along with their worn faces lowers their capacity to seal. So you burn unscraped oil, have inconsistent or dropping copression and increased gas bypass.

[240sx2nr]

pretty much what ubernoober said...between the rings and the walls... rings press up against the cylinder walls creating a seal... therefore holding compression... if u have low compression... hella blowby occurs cause of the worn rings not sealing it in the chamber...

[Vetal]

In that case I don't quite understand why rings' wornout is determined by measuring end gaps end clearance between ring and piston lands...

[Red coupe]

measuing ring land to ring clearance isnt to check for worn out rings, its to check for worn out ringlands, they tend to get beat by the ring as it twist, and if they get to big the ring will flutter at higher rpmsIm dont remember why end gap measurement is used though it might be because as the ring wears thinner the gap would get smaller(although that would assume material wears off the inside not just the outside, so im not realy sure)

[Vetal]

Im dont remember why end gap measurement is used though it might be because as the ring wears thinner the gap would get smaller

I guess gap is measured because as rings wear the gap increases and more and more gases will escape from combustion chamber into crankcase

[sanioll]

I guess gap is measured because as rings wear the gap increases and more and more gases will escape from combustion chamber into crankcase

obviously i have a zero knowledge in engine internals, but what happens when pressure builds up in the crankcase?? will the seals go bad? does the engine have special "hole" to relieve the pressure?

[slide240slide]

Thats where a PCV (Positive Crankcase Ventilation) system comes into play if your motor has no external vent, It's oil leak time the pressure will find a way out by means of a gasket being pushed out to relieve the pressure. and every motor ive rebuilt i have never ran into any problems with ring gap because if you don't have some weird "Custom" Pistons and file fit rings it's really not something to worrry about. Stock vehicles run a PCV valve on top of the valve cover then the line from the valve to the TB or Carb (on older vehicles) to Burn/ Recycle the gasses instead of venting them into the air. Most Race only vehicles have a cetch can with a small filter with the line ran from the can to the valve cover, that way it's not contaminating the Clean A/F mix with already burned gasses. just make sure it's vented.

[The Mic]

I would recommend performing a leak down test , this will help to lacolize the problem. If you hear air leaking out the intake, its your intake valves, if its coming out your exhaust, its your exhaust valves, dip stick tube then its your rings, etc.

Forgot to mention you should also try a wet compression test, add a tablespoon of oil through the spark plug hole into each cylinder prior to compression testing it, if your number go up a lot, its your piston rings...