Do piston rings move around on the piston when running?

[creia]

Something I have always wondered about,,,
When we install piston rings on a piston we are told to stagger/set the ring gaps at 120degrees apart from each other. When the engine has been in use for awhile (maybe a LONG while) are they still in that same position or do they move around over time? Here is where I am going with this...
If I remove a piston/ring assembly from an engine, then later reinstall this same assembly, is it important that the relative ring gap positions are in the exact same position as when I removed it? That is to say have the rings "seated" in this position, and changing the position will cause the rings to no longer be seated? (compression loss, oil passage, etc.)
Thanks for your guidance.
Michael

[RoyM]

The rings are constantly moving so it is not a concern as long as the gaps are staggered to prevent blowby. They probably are aligned at times during running but not long enough to worry about. 2 strokes are a diffent matter.

[bgsengine]

what Roy Said.. Yes rings will move around - due to the cross-hatch in the cylinder bore.. (part of the reason for it) - rings (other than pinned 2-stroke rings) will & should move around (which evens out wear a little bit) .. if you ever take apart an engine and notice the bore is "wiped clean" as in no cross hatch, and you may also notice that the rings in that engine may have inconsistent thicknesses - you might realize why they should move around (and why the cross hatch in the bore needs to be there) because rings stopped moving, and eventually they wore in more on one side (usually on the thrust face of the piston - the side that is pushing into the cylinder wall due to angle of connecting rod when coming up on compression.. opposite side the skirt may show wear from being the thrust face on the power stroke... 2-strokes - almost all of them- have ports machined into the cylinder, so they must have locating pins in the piston so the rings do not move (and ends catch on a port) and those, almost always rings are more worn in one spot than on the other side... if you are re-using the old rings, (which I don't recommend, really, unless they are very low hours) I'd check them for consistency in their thickness all around and if they seem relatively close all the way around , they should be fine for re-use assuming all else is good.. but if you see them with a lot of variation (Not comparing rings to each other, but just the thickness of the ring in one spot relative to the other spots on the ring) you may wanna consider a new ring set (or plan on a rebuild, etc) Gaps are generally staggered to minimize blow-by and maximize oil control... they don't need to be EXACTLY 120 degrees apart (just that 3 rings at 120 degrees makes 360 degrees, right?) - a 4 ring set might be 90 degree staggered.. (and, generally a 3 or 4-piece oil control ring, you stagger the gaps on those as well - typically ring gap is 180 degrees from expander gap then the rail gaps 1" or so to the left and right of ring gap... then the other 2 rings 120 degrees from oil ring gap.. Some manufacturers recommend not having ring gaps line up with piston pin or flats on the skirt, also..)

[creia]

Thank you BOTH gentlemen for your replies- much appreciated.
I'm restoring a 1969 Briggs 3HP horizontal-shaft "KoolBore". This engine is unusual and atypical for me in that the bore has minimal wear (only .0005"-.001" depending on where you measure) and the original cross hatching is about 50% visible compared to how a new engine bore/hone looks. (Most of the engines I take into inventory are worn out and look like they went down with the Titanic! ) The ring gap with the (used) ring is .023" and with a new ring it is .018". I'm kind of on the fence on this one- either just put the original piston/ring assembly back in or install new rings and have the bore honed...
Michael
Sidebar comment; Brian- With the way you always explain things (detailed, yet very easy to understand) you have the makings of being an excellent instructor.
I have been the recipient of MANY of your explanations to my questions for which I am most grateful!

[bgsengine]

Well, .023 vs .018 new, I would think would represent rather little wear.. (Normally to judge wear I just look at the oil control ring on those, and see how wide the "flats" on the ring look compared to new ones, which might look to be about 1/32 - 1/64 inch wide.. I never measured them, just "eyeballed" them..) if all else seems to be within tolerances (not over reject size) being a koolbore, if you don't have a set of new rings handy, I'd have no hesitation at re-using old rings.. I'd never hone a koolbore (aluminum bore) - aluminum tends to "ball up" in typical honing stones and you risk digging deep grooves - If you can still see the crosshatch in a koolbore, I'd call it in pretty good shape.. aluminum bore, if it is past reject limits, only real thing to do would send it out to a machine shop to have it bored (NOT with any kind of boring stones - ONLY with a milling machine type cutter, followed by light honing using stones specifically for aluminum) In other words, unless you have high quality, perfectly dressed honing bar stones specifically designed for aluminum bore, I'd not even think about honing... and given your ring end gaps, which don't seem too terrible, if your oil control ring seems to be relatively in good shape, I'd probably say re-use... (of course assuming your rings don't have too much variation in thicknesses around their circumference)

And as for my detailed explanations - It simply is a matter of learning and understanding the why and how something works, then those basics can be applied largely to pretty much anything else of a similar design.. .. I have always thought, if you can't explain the why and how of something, you really don't know as much as you think you do...

[KE4AVB]

You definitely don't want to be using a flex hone.

Lisle make a good ridge hone setup for doing this. You do want the 280 grit stones for aluminum. You definitely don't want to use the 180 grit stones. Lisle does have stones especially for aluminum for cylinder under 2.5" but the 280 grit was what Briggs recommended years ago when I first got my hones. Now most of honing has been on 5 hp and up. Actually haven't honed an engine in the last 5 yrs. Most have been pass the reject level. Otherwords couldn't stay under .020 over.

Either way the Lisle 15000 or 16000 hones are fairly expensive themselves and you will need extra stone sets as they only come with a 180 grit set. I got both here with extra stones and racks but they seen the light of day in five years.

And you would need a 1/2" low variable speed high toque drill as you will need to stay under 300 rpms as any faster will gall the aluminum. Actually slower as the cylinder diameter drops. Plus a good honing oil.

Here is a source that would give an idea of the costs of the Lisle hones.
https://www.enginehones.com/lislehones.html

Ridge hones eliminate ovaling and and tapering. Flex hone only amplify these two problems. You are well with specs for the compression rings. What I have seen on several of these kool bore is soft spot wear and tapering. One thing to note too is that when honing you may ran across a cylinder casting defect such as an air pocket. I had few cylinder that worried me as the taper would not hone out until the last few thousandths of the over sizing.

[creia]

Thanks guys- I appreciate the continued interest and replies to me thread.
Re: Boring and honing aluminum bores- I am very fortunate to have an "old school" machine shop within a hour's drive from me. They have been in the same location since the late 1960s and specialize in small engines. They have all the correct tools and knowledge and they do ALL of my re-bores and honing. Like I said, having an old engine in this (internal) condition is foreign to me. In 15 years of restoring these engines (My son and I have done about 30), this is only the second engine that was in such good (internal) condition. Every single engine we have ever worked on has needed (at least) "Chrome rings" or a full O/S rebore. Now, the EXTERNAL (cosmetic) condition on this engine- that is a different story. While this 52 year old engine apparently had very little use , it must have been left out in the weather for whatever equipment is was installed on (Think: "Going down with the Titanic" analogy that I mentioned earlier).
Michael

[Skywatcher]

Greetings All

On the subject of rigid hones, I bought a used Lisle rigid hone from a friend who's dad was a motorcycle mechanic back in the 1960's and 1970's. The hone doesn't have any instructions with it, so I'm assuming they got separated from the tool long ago. The only reference to any numbers that I can find on the hone are CHJ-809 stamped into the main body of the tool and 240-2/21 on a price label stuck to both the box and the tool. Does anyone know if there are any downloadable instructions for this hone. Thanks and all the best,

Sky

[Walt 2002]

My experience is it is not practical to resize a Kool Bore engine. They have a special bore coating which is removed, require a special piston and rings with very problematic results. Suggest scraping it and rebuilding an I/C engine.
Walt Conner