Piston ring orientation –use again?

[Laverda Dave]

Hi all, I’m looking for a bit of advice regarding the piston ring orientation on the Kawasaki Z400j I am currently restoring for the BIL.

When I removed the barrels during the strip-down I made sure I didn’t disturb the rings or pistons. Once the barrel was off I took notes of where each piston ring gap was in relation to the piston. I was a bit surprised to see nearly all the ring gaps were very close to each other and in one case, the gap for the top and second ring were virtually on top of each other!

The engine has 380 genuine miles from new and has never been apart, I can only assume this is how the piston rings were installed at the factory unless the rings move in service although I’ve never seen this happen before.

So, the question is and bearing in mind there are still feint honing marks on the barrels, can I reuse the original rings and before replacing the barrels re-orientate the ring gaps to what they should be at 120deg intervals (but not directly above the gudgeon pins) and with the gaps of the three piece oil rings spaced at 20mm apart? I really don’t want to fit new rings as that will mean a hone and these rings are impossible to get!

Any advice or thoughts welcome 

[McCabe-Thiele (Ted)]

On cars I've always fitted the compression ring gaps as far apart as possible so 180 deg with the oil control ring opposite the lower compression ring. Most of the pistons I've fitted have had just the three rings with the exception of one piston set that had four rings.

With that low mileage I would use the same rings I can't see them loosing much of the spring factor with so few heat cycles or have really bedded in.

Never heard that the gudgeon pin alignment is not good but avoiding the thrust side seems it might have benefits - my late father was a RR / Bently trained mechanic I used his advice back in the day.

[K2-K6]

I could see there's not an "absolute" answer to it.  Strange the distribution as it appears in your findings.

I've always kept the top two away from front side of bore to avoid the ring ends being on principle power thrust face.  A bit like a two stroke where they are ordinarily pegged at twenty past eight.

The hone marks should survive even after more elevated mileage as they are grooves in the cylinder wall that help oil retention.  The swarfy? edges of the hone marks are what usually cut the rings until they get worn off.
With the bores dry,  can you feel a fine sandpaper feel to them if you run your fingernails up and down them? That would give an indication of if they'd work to normalize the rings during running.

If that seems to be in place,  then you could orientate the rings as you want to place them. I doubt it would give any problem at that mileage.

[Bryanj]

Yes rings do rotate arround a piston,Doxford found on aest engine in the late 50's that only ran at 110 rpm rhat theringson the marinediesel rotated between2 and 4 times per hour dependingon load and rpm.
I would reuse and orientate normaly

[robvangulik]

Bryan is right, rings rotate and can at quite some rate in these high revving engines, you'll never find them back in the same position you put them in.

[K2-K6]

I feel enquiringly sceptical about this, by reading things like this

"Rotation of piston rings
Piston rings lead a much more active life than might be thought. While we usually imagine that they only reciprocate in a motion defined by the piston, they also rotate during service. Such rotation has been the subject of many experiments and studies over the years, but this is not simply a matter of academic interest; there are implications for engine performance and efficiency.

With few exceptions, race engines use piston rings of the conventional gapped type – that is, there is a carefully controlled gap between the ends of the piston rings. A report by Jung and Jin (1) shows that, at one engine speed, the speed and direction of piston ring rotation depends on load, and it is not necessarily the case that both rings rotate.

Their study, although weighted toward the technique used in the measurements, monitored oil consumption and was able to correlate this with the relative positions of the piston ring gaps. It is perhaps unsurprising to find that oil consumption rose to a maximum when the two ring gaps were aligned, and fell to a minimum when the ring gaps were spaced 180° apart. This makes sense, as the flow losses would be greatest when the ring gaps are as far apart as possible, so for a given pressure difference there would be less flow through the gaps in the rings.

The speed of rotation of piston rings has been measured experimentally. Shaw and Nussdorfer (2) examined the phenomenon on a large engine and found that, at 1000 rpm engine speed, the piston rings were “observed to rotate as rapidly as 1 rpm”. Jung and Jin reported in more detail: on the engine they used, at 4000 rpm and 2 bar bmep, the rings rotated at 0.6 rpm in opposite directions to each other, with the second ring initially oscillating between two positions before finally beginning to rotate continuously.

At higher load, the top ring didn’t rotate, and the second ring rotated at speeds from 0.5 to 3 rpm. On the same engine and at lower engine speed, the top ring simply moved to a given angular position and then remained stationary at 2 bar load. With an increase to 4 bar, there was a change in top ring position but still no continuous rotation. The second ring was also observed to be stationary at this speed in some tests, some of which found the ring gaps aligned – the condition where oil consumption is highest.

It is clear that the piston rings in any engine lead a mysterious life where, depending on the load and speed conditions that apply, they might rotate continually, oscillate between certain positions or remain stationary. Oil consumption is found to vary with ring position and, where the rings rotate periodically, the rate of oil consumption is a function of the rotation period of the rings. Other than pinning the piston rings to prevent rotation (which is commonly done in two-stroke engines), there is little we can do to influence ring position or speed of rotation. It is clear that whatever position the rings are in during the engine build will not be maintained during service.

References

1. Jung, S., and Jin, J., “Monitoring of Rotational Movements of Two Piston Rings in a Cylinder Using Radioisotopes”, Journal of the Korean Nuclear Society; vol 31(4); ISSN 0372-7327, August 1999

2. Shaw, M., and Nussdorfer, T., “A Visual and Photographic Study of Cylinder Lubrication”, NACA Technical Report no 850, 1946

Written by Wayne Ward"

From here https://www.highpowermedia.com/Archive/rotation-of-piston-rings

It seems more variable than certainty with most studies aimed at much larger dimensions. Whether it relates to small engines like ours isn't clear. Just things like angular displacement of rods and linear piston speed would seem to bring about huge variations in influence to the rings and their interactions with cross hatch hone geometry.
Also, when a used engine is taken apart after more mileage, the bores are normally worn slightly oval in front to back orientation. I can't visualise the rings rotating easily with that condition taken into account.

Interesting subject though, but has anyone got examples of this phenomenon directly related to our engine size?

[Laverda Dave]

I think I've possibly opened a can of worms with this thread 😱!
I couldn't believe the location of the ring gaps when I took the barrels off hence why I made a note. Given it now appears the rings actually move in service (every day is a school day as they say) maybe they move more during the running in process when the bores are actually round and the rings can move freely as opposed to when the engine is worn and the bores go oval therefore maybe preventing the rings turning?

Thanks for the advice, considering the milage in the engine I'm going to refit the existing rings and gap at 120deg. By all accounts they will move anyway!

I'll post some more progress reports under my Z400J thread.

Great forum as always👍

[robvangulik]

There is lots to be found on the net about this subject, but almost all studies are run with (in our eyes) low revs. But as I wrote, I have never found the rings in the same orientation I mounted them in!