ring to groove clearance, minimum?

[Warp Speed]

Small ring:1.1mm ring isn't much smaller than common OEM these days.
And on ring land clearance "tight" is a relative term also.
If you have nothing better to do, it may be an interesting test to some..........?
Many already know the answer, as "small" rings are getting more common place today. The higher rpm the tighter you need to keep it, thus gas ports.......

[Warp Speed]

On another note, leak down tests can tell you if something is broken, or give you a clue as to the leak path, but won't necessarily be a good indicator of how well the engine will seal while running, especially when using small rings, gas ports, crankcase depression ect.
The zero gap can make things seem better than they are also!

[MadBill]

On another note, leak down tests can tell you if something is broken, or give you a clue as to the leak path, but won't necessarily be a good indicator of how well the engine will seal while running, especially when using small rings, gas ports, crankcase depression ect.
The zero gap can make things seem better than they are also!

True, although they help when categorizing valve leakage.
After the initial build, we pay more attention to the crankcase vacuum gauge readings on track and how long vac. is maintained on shutdown.

Re ring widths, I was looking at the cast iron piston out of a early REO a couple of days ago. It had three 3/8" wide compression rings! (with elaborate Childs & Albert/BWE-style ZGS end gaps) The evolution continues...

[engineguyBill]

This is getting away from the original subject matter of this thread and I apologize for that, but:
The "Z-gap" ring that was marketed by Childs & Albert as a gapless second ring is actually a copy of air compressor rings that were developed in the 1930's for use in the railroad industry for their air brake systems . . . . . . .

[Kevin Johnson]

This is getting away from the original subject matter of this thread and I apologize for that, but:
The "Z-gap" ring that was marketed by Childs & Albert as a gapless second ring is actually a copy of air compressor rings that were developed in the 1930's for use in the railroad industry for their air brake systems . . . . . . .

Are you referring to http://www.google.com/patents/US2092413 ?

[engineguyBill]

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Very possibly so. WABCO is one of several manufacturers who have used this design in the air compressor industry.

[piston guy]

Perhaps a little math is in order.
Using a ring that is 4.0" x 3/16" x 1/16, and a TCR of 11.1 (approx 14-1 CR) we will find a compression pressure of 362.6 psi at TDC.

Given the ring dimensions above, this produces a ring face force against the cylinder wall of 257 lbs, and a ring force against the lower ring groove of 442 lbs. These ring forces are increasing as the piston moves upward prior to combustion. With combustion gas pressure, the ring forces are increased substantially.

The questions then become:
how much ring force is needed to seal the ring against the piston and cylinder wall?

how much force is excessive and begins a destructive process?

Does in increase in piston ring force due to combustion produce any better ring sealing than already achieved by combustion pressure?

David ,
I am not being a smart a$$ but the combination of "natural" ring tension and back pressure must be higher than just simple cylinder pressure ( at any given point in motion) to stay "sealed" under compression and combustion cycles. Otherwise higher cylinder pressure "could" blow the ring off of the cylinder wall. Gas porting improves ring seal (IMPE) because of faster pressurization of the back of the ring and improved seal against the bottom of the rings groove. Too many gas ports ( to me) is not possible from a "sealing" point of view because the "available" pressure isn't changing, but "reality" shows specific amounts vary with ring style and design. The draw back from too many would be weakening the top ring land and overheating the top ring plus potential premature ring wear. Dyke rings ( head land) or "L" shaped rings get combustion pressure for 360*s by there design and are favored it extreme environments like Blown Nitro fueled engines.

[MadBill]

... The draw back from too many would be... plus potential premature ring wear...

Exactly, and that premature wear could only mean excessive friction and thus power loss.
Also, one benefit of gas porting is that the more sealing done by the gas pressure, the less the necessary ring tension and thus reduced friction on the other three strokes and more power.

[Kevin Johnson]

Vertical or radial/lateral gas porting?

viewtopic.php?f=1&t=24762

[frdboy]

Great info in here guys

Questions....

If one had to run the rings across some fine sandpaper to gain a little would it be best to sand the top side of the ring?

What about the second ring? We don't have to worry about gas pressure on it so would .001 be fine?

[engineguyBill]

Touch both sides of the ring lateral surfaces when lapping the rings.
The second ring will survive fine with .0010" side clearance. The second ring does not encounter nearly as much heat and pressure that the top ring does. A second ring function is 80% oil control and 20% combustion pressure control . . . . . . .

[Kevin Johnson]

Kolbenschmidt has a nice graphic of the top ring heat load on page three.

[piston guy]

Great info in here guys

Questions....

If one had to run the rings across some fine sandpaper to gain a little would it be best to sand the top side of the ring?

What about the second ring? We don't have to worry about gas pressure on it so would .001 be fine?

Just use some caution and common sense when lapping rings. The phosphate coating is there for break in purposes. Revoving it completely has led to micro welding as well. A super flat ring and groove "can" create a "vacuum seal" similar to two pieces of glass together. The phosphate wears quickly so the clearance increases at the same rate. Remember, rings are rotating so they are "lapping" in all the time. Second rings deal with oil so they are "lubed" more than top rings and run cooler so there is less chance of them sticking in the piston. With rings on the piston and the piston horizontal, the rings should always be free to "fall down" to the limit of the groove.

[Barry_R]

We could always muddy the waters further by noting that the top ring also needs to seal during the intake stroke where there is no "combustion pressure" helping things out. Might be some risks/rewards/opportunities there for various gas port designs...along with some minimum cross section and tension considerations.

[Kevin Johnson]

Might be some risks/rewards/opportunities there for various gas port designs...along with some minimum cross section and tension considerations.