Ring Seating- What Actually Happens?

[padam]

This may be a dumb question, but I’m asking anyway.
What is the physical process of piston rings seating in?
There really isn’t anywhere for them to go.
So what is the difference between rings that are seated and those that aren’t?

 

[t.flatt]

I'm not an expert or an experienced engineer builder but I think I can answer. Over time, the rings hone themselves to the exact profile of the cylinder and will stay in that position. They can rotate around the piston and once they are seated all the ring gaps will stay in the same spot and be able to compress the air-fuel mixture much better than new rings. Someone please correct me if I'm wrong but I think that is what is going on.

 

[Cranky]

In a nut shell, the face of the ring takes a set to the cylinder wall. Cast rings take longer than say a Moly filled ring and each one takes a different grit finish hone on the cylinders.

 

[snakeoil24]

In essence they polish into each other

 

[69Bee]

Ring seating today is not the same as "yesteryear". Today's rings are usually "Pre-Lapped" to aid in seating. If a Torque plate is used during honing the cylinder, it is honed in the head installed/stressed condition, and the cylinder will be rounder than a cylinder honed without a torque plate. I have actually measured a cylinder, installed a torque plate, and re-measured and have seen as much as 0.002"-0.003" larger at the top due to stresses created by the head bolts.

Now, that being said, when the rings travel in the cylinder, they wear and eventually match the cylinder providing the best seal. This wear-in process is due to the surface finish of the cylinder (final hone), and if too fine, rings can't wear to break-in. If an engine is over fueling, the oil can be washed off, and the cylinder will "Glaze", and the rings will NEVER break-in. If a cylinder finish is too rough for the ring type, it can destroy the rings and burn oil. Cam break-in happens in the first 20-30 minutes, while cylinder/ring break-in happens up to about 500 miles. The best way to seat rings is to get the engine up to speed, then take your foot off of the gas and allow the engine to slow itself down (engine braking).

Essentially, ring type and stones used for proper cylinder finish is:
Cast rings: 180/220/280 then use Plateau brush ~10 strokes
Moly rings: 220/280/320/400 then Plateau brush ~10 strokes
Chrome rings: 180/220 then Plateau brush ~10 strokes

Then, if you use a CNC Diamond Hone like I do:
Cast & Moly rings Diamond 14J 275/325 grit stones then CBN 35D 400 grit Plateau ~6-8 strokes
Chrome (Diesel) Diamond 14K 170/200 grit stones then CBN 35D 400 grit Plateau ~6-8 strokes

Diamond stones do not correlate to Vitreous stones, as it is all about surface finish.

 

[Wietse]

Now comes the next thing, depending on the hone grit and material in use, you only have a limited time to seat the rings.
The "rough" cylinder wall will provide the abrasive surface for the rings to wear in to (seat), but this rough edge wears off.
If you take too long the rings will not have seated properly and have "more" blow-by than rings that are properly seated.
You can actually even measure your rings are seating or not, by going by cylinder pressure figures you can see an increase as the rings seat.
There will be a point where the cylinder pressure no longer rises, which means the rings are finished seating and the cylinder pressure is set.
Taking it very easy the first 500 miles is going to give low cylinder pressures so only a slight force to push the rings against the cylinder wall, the seating time will be long and you take the risk of running out of time as they cylinder wall grit has reduced so far where the rings no longer wear.
Ensuring a good/high engine load increases cylinder pressure and therefore applies more force behind the rings to force them against the cylinder wall and make them wear a little faster, now you allowing the rings to use the abrasive walls to get them to seat fast and properly.
Coming off higher rpm, having the throttle closed will ensure the vacuum in the combustion chamber will pull up a lot of oil to cool and flush the cylinder and rings, this is another important factor of seating piston rings.

So, you gonna drive like a grandma the first 500 miles or you gonna put it to the pace?

 

[snakeoil24]

Ring seating today is not the same as "yesteryear". Today's rings are usually "Pre-Lapped" to aid in seating. If a Torque plate is used during honing the cylinder, it is honed in the head installed/stressed condition, and the cylinder will be rounder than a cylinder honed without a torque plate. I have actually measured a cylinder, installed a torque plate, and re-measured and have seen as much as 0.002"-0.003" larger at the top due to stresses created to the head bolts.

Now, that being said, when the rings travel in the cylinder, they wear and eventually match the cylinder providing the best seal. This wear-in process is due to the surface finish of the cylinder (final hone), and if too fine, rings can't wear to break-in. If an engine is over fueling, the oil can be washed off, and the cylinder will "Glaze", and the rings will NEVER break-in. If a cylinder finish is too rough for the ring type, it can destroy the rings and burn oil. Cam break-in happens in the first 20-30 minutes, while cylinder/ring break-in happens up to about 500 miles. The best way to seat rings is to get the engine up to speed, then take your foot off of the gas and allow the engine to slow itself down (engine braking).

Essentially, ring type and stones used for proper cylinder finish is:
Cast rings: 180/220/280 then use Plateau brush ~10 strokes
Moly rings: 220/280/320/400 then Plateau brush ~10 strokes
Chrome rings: 180/220 then Plateau brush ~10 strokes

Then, if you use a CNC Diamond Hone like I do:
Cast & Moly rings Diamond 14J 275/325 grit stones then CBN 35D 400 grit Plateau ~6-8 strokes
Chrome (Diesel) Diamond 14K 170/200 grit stones then CBN 35D 400 grit Plateau ~6-8 strokes

Diamond stones do not correlate to Vitreous stones, as it is all about surface finish.

I have to say bee this is very interesting !

 

[RemCharger]

A good example of ring seating;
Race engine. Build it. Break cam in. Go straight to track and beat on it.
That's the best case scenario for modern rings.
And rings do rotate. Some high end guys actually have an rpm number for that.

 

[snakeoil24]

I thought rings rotated

 

[markrr]

I have to say bee this is very interesting !

I agree.

 

[t.flatt]

If the rings rotate, why do we spend all that time getting the ring gaps positioned correctly? To help with the very first fire-up?

 

[RemCharger]

If the rings rotate, why do we spend all that time getting the ring gaps positioned correctly? To help with the very first fire-up?

Yes. I've done a lot of researching on gap positions, without finding rhyme or reason to different manufacturers specs .
But most say to put oil rings an inch apart, near the wrist pin ( I put them at rear of cyl) and I put the top ring by the intake valve, and 2nd ring 180 from that.
You don't want the top ring closest to the exh valve.
Just what I've found, always willing read more about it though.

 

[steve340]

Rings do actually rotate around the piston ring land for sure.
Where they end up does not seem to matter much if they are good parts, no overheating, bedded in correctly etc.

 

[69Bee]

I agree that the ring gap position is subjective, and as long as you don't just line up all of the gaps, it should be fine. I stagger all of the gaps to prevent gas leakage.
This is how I do it:
1) I put the expander gap to the front so I can put my thumb on it (this will be 6 O'clock)
2) I install the lower oil rail at approximately 10 O'clock
3) I then install the upper oil rail at approximately 2 O'clock
4) I then install the 2nd compression ring at 9 O'clock
5) I then install the top compression ring 180 degrees opposite at 3 O'clock

 

[mopar 3 B]

I just follow the Mopar engine rebuild Bible. It's called the FSM for short.

 

[Wietse]

A good example of ring seating;
Race engine. Build it. Break cam in. Go straight to track and beat on it.
That's the best case scenario for modern rings.

Or engine builders, 20 minute run to break in the cam on the dyno and after that a good amount of dyno pulls for testing and adjusting.

 

[Ranger16]

If the rings rotate, why do we spend all that time getting the ring gaps positioned correctly? To help with the very first fire-up?

A question I have often pondered.

 

[Wietse]

With the ring seated in position there is only vertical forces applied on it from the piston going up and down.
Nothing applies a force that makes them rotate so they should remain where they are, though I am quite sure they will move a little.
But even after some movement, the clocking of the ring gaps will still have a positive effect obviously.

 

[t.flatt]

I just follow the Mopar engine rebuild Bible. It's called the FSM for short.

I'm about to get the pistons in the block so I've been contemplating ring gap position. I've got two engine building books, the instructions that came with the rings and the factory service manual. They all say something different! The FSM is rather vague, stating, "Be sure that the compression rings are staggered so that neither are in line with oil ring rail gaps. The oil ring expander ends should be positioned towards the outside V of the engine. The oil ring gaps should be positioned opposite each other and above the piston pin holes." I guess that's pretty exact for the oil rings. Use your imagination for the compression!!

 

[RemCharger]

With the ring seated in position there is only vertical forces applied on it from the piston going up and down.
Nothing applies a force that makes them rotate so they should remain where they are, though I am quite sure they will move a little.
But even after some movement, the clocking of the ring gaps will still have a positive effect obviously.

What makes them rotate is the cross hatch. Kind of like gun rifling.