Purpose/Importance of 4t Piston’s oil ring gap orientation

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1/25/2019 6:10 PM

As the title suggests, how important and what is the purpose of positioning the oil ring gap as shown in this screenshot from a KXF manual?
I’ve always done it this way, but it’s a pain to get the end gaps aligned in these positions. Let alone trying to get the gaps to stay in place when compressing the rings & slide the cylinder on. When I broke down a brand new KXF, I made it a point to see how the expansion ring and the two oil rings were oriented. They were NOT positioned as shown in the manual. I’m not talking about the top ring, that one is easy to get right.
Photo
I’m just curious if I really need to stress this for future builds. Thanks in advance.

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1/25/2019 7:21 PM

unless there is a pin to locate the rings, they will constantly rotate on the piston.

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1/25/2019 9:29 PM

The thought process is by staggering them it will reduce oil consumption and blow by. Most engines builders will tell you it doesn't make allot of sense as the rings will rotate and only remain aligned as assembled for a short period, the three piece oil rings are supposed to rotate as an assembly (which would mean they maintain the stagger).

The reason for rings facing forward and rear is because they also tell you not to align them with the wrist pin bores.

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50

1/26/2019 5:48 AM

The top ring should have its gap on the rear side of piston as when the engine turns forward, the rear side of piston gets pressure against the cylinderwall, thereby closing the gap further. Every other ring and spring expander ring should have the gap 180 degrees different. Thus you get the longest distance from one gap to the next, This gives the best compression and lowest blow-by/ oil consumption. Never align ring gaps to the wrist pins as the piston is smaller in this area and the gap more exposed to blow-by and oil.

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1/26/2019 7:22 AM
Edited Date/Time: 1/26/2019 8:34 AM

WEAL wrote:

The top ring should have its gap on the rear side of piston as when the engine turns forward, the rear side of piston gets pressure against the cylinderwall, thereby closing the gap further. Every other ring and spring expander ring should have the gap 180 degrees different. Thus you get the longest distance from one gap to the next, This gives the best compression and lowest blow-by/ oil consumption. Never align ring gaps to the wrist pins as the piston is smaller in this area and the gap more exposed to blow-by and oil.

"The top ring should have its gap on the rear side of piston as when the engine turns forward, the rear side of piston gets pressure against the cylinderwall, thereby closing the gap further."

That is incorrect, the ring gap size does not change with upstroke or down stroke. It only changes with heat.


Every other ring and spring expander ring should have the gap 180 degrees different. Thus you get the longest distance from one gap to the next,

This is mainly relevant on the oil ring set.


Never align ring gaps to the wrist pins as the piston is smaller in this area and the gap more exposed to blow-by and oil.

"unless there is a pin to locate the rings, they will constantly rotate on the piston."

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1/26/2019 8:17 AM

I have noticed my ring stagger (ring clocking) stay mostly intact while running. At service intervals, I would always check how the piston ring stagger was when it was time for a new top end and I was surprised the rings were still in the same basic orientation (give or take 10 degrees or so). Maybe I have magical bikes. Lol

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1/26/2019 3:01 PM

Barnett,sorry for my bad explanation - will clarify herewith: think of the ring gap exposed by a piston off the cylinder wall or covered by a piston close to the cylinder wall. I think you will agree that a ring gap that is mostly covered by the piston will allow less blow by.
Anyway, having built engines for all sorts of racing (even supercharged land speed stuff) and testing engines on a very accurate Schenck dyno, measuring power output at the crank, I do find that it does matter where the ring gaps are.

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1/26/2019 3:20 PM

Good to know, thanks for the comments. I will continue to do my best to get them lined up correctly, but it seems once they fall into the groove it’s hard to move one ring without moving one or both of the other rings. I use a pick to move them around. Any tips from the experts or is it just a necessary tedious task?
Yesterday I bent the edge of one of the oil rings closest to the gap when trying to slide the cylinder on. I’m glad I could tell something was wrong, it required more force than usual for the piston to slide into the bore. If I didn’t notice I’m sure it would have trashed the cylinder.

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1/26/2019 5:41 PM

Try this method. I used it every time and it works extremely well. Had your problem, and came up with this. Much easier. With the piston and cylinder on the bench, put 1 split ring in the piston. I always did the left one, shifter side of the bike. Now put the rings on the piston, orientated correctly. The bottom of the cylinder is cut at an angle, to be able to slide it over the piston with the rings in it. So fit the piston into the cylinder on the bench. The rings will seat, and there is enough space to leave the wrist pin hole exposed. Then slide the cylinder with the piston in it over the rod and put the wrist pin in and the circlip, and slide it down. Be sure to put the gasket in beforehand . Move it up and down, and you are done.

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1/26/2019 5:58 PM
Edited Date/Time: 1/26/2019 6:06 PM

WEAL wrote:

Barnett,sorry for my bad explanation - will clarify herewith: think of the ring gap exposed by a piston off the cylinder wall or covered by a piston close to the cylinder wall. I think you will agree that a ring gap that is mostly covered by the piston will allow less blow by.
Anyway, having built engines for all sorts of racing (even supercharged land speed stuff) and testing engines on a very accurate Schenck dyno, measuring power output at the crank, I do find that it does matter where the ring gaps are.

"I do find that it does matter where the ring gaps are."

Not when they rotate on the piston. The only way to guarantee a ring will never rotate on a piston is to put a pin in the ring land.

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1/26/2019 6:08 PM

pCp 252 wrote:

Good to know, thanks for the comments. I will continue to do my best to get them lined up correctly, but it seems once they fall into the groove it’s hard to move one ring without moving one or both of the other rings. I use a pick to move them around. Any tips from the experts or is it just a necessary tedious task?
Yesterday I bent the edge of one of the oil rings closest to the gap when trying to slide the cylinder on. I’m glad I could tell something was wrong, it required more force than usual for the piston to slide into the bore. If I didn’t notice I’m sure it would have trashed the cylinder.

"Yesterday I bent the edge of one of the oil rings closest to the gap when trying to slide the cylinder on."

I would not reuse it.

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1/27/2019 11:22 AM

WEAL wrote:

Barnett,sorry for my bad explanation - will clarify herewith: think of the ring gap exposed by a piston off the cylinder wall or covered by a piston close to the cylinder wall. I think you will agree that a ring gap that is mostly covered by the piston will allow less blow by.
Anyway, having built engines for all sorts of racing (even supercharged land speed stuff) and testing engines on a very accurate Schenck dyno, measuring power output at the crank, I do find that it does matter where the ring gaps are.

barnett468 wrote:

"I do find that it does matter where the ring gaps are."

Not when they rotate on the piston. The only way to guarantee a ring will never rotate on a piston is to put a pin in the ring land.

I am interested what makes them rotate?

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1/27/2019 1:05 PM

WEAL wrote:

Barnett,sorry for my bad explanation - will clarify herewith: think of the ring gap exposed by a piston off the cylinder wall or covered by a piston close to the cylinder wall. I think you will agree that a ring gap that is mostly covered by the piston will allow less blow by.
Anyway, having built engines for all sorts of racing (even supercharged land speed stuff) and testing engines on a very accurate Schenck dyno, measuring power output at the crank, I do find that it does matter where the ring gaps are.

barnett468 wrote:

"I do find that it does matter where the ring gaps are."

Not when they rotate on the piston. The only way to guarantee a ring will never rotate on a piston is to put a pin in the ring land.

WEAL wrote:

I am interested what makes them rotate?

my bet would be on vibration... there are several forces acting upon a piston ring so the odds of all the forces being in perfect balance is very slim... take anything set it on a vibrating surface you will notice movement/ or rotation... this is just a guess though... I’m no physicist...

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1/27/2019 5:43 PM
Edited Date/Time: 1/27/2019 5:45 PM

WEAL wrote:

I am interested what makes them rotate?

As far as I know, nobody has done a scientific test to try and determine that, but it seems logical to speculate that vibration does. The top ring is exposed to much higher pressure from the combustion process so it seems reasonable to think it might rotate either more or less then the rings below it, but here is a somewhat scientific test on the rotation of piston rings and the results. It is also theorized by some that rings that apply more pressure against the cylinder walls will rotate less than "low tension" rings.

https://www.highpowermedia.com/blog/3954/rotation-of-piston-rings

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1/27/2019 11:35 PM
Edited Date/Time: 1/28/2019 2:24 AM

Thanks for the link - now I wonder if the results are the same at our revs and combustion pressures - I´m not sure as my engines have the gaps more or less where I placed them when tearing into them again.

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