I just took apart a twincam motor for a buddy of mine because it was using too much oil & found all the ring gaps lined straight up & down with each other.
I didn't build the motor (117") but a buddy of mine did & he was good a mechanic as you could find anywhere (he died last year of a heart attack). He had been building hot-rod Harley's for years (& raced a pro-mod bike for a number of years) & I trusted his ability 100%.
This is the second time I've seen this.
~John
Piston rings rotate. If they didn't 2 stroke pistons wouldn't have a pin in the ring grove to keep their rings from rotating.
Quote from: jmorton10 on May 13, 2019, 08:39:11 AM
I just took apart a twincam motor for a buddy of mine because it was using too much oil & found all the ring gaps lined straight up & down with each other.
I didn't build the motor (117") but a buddy of mine did & he was good a mechanic as you could find anywhere (he died last year of a heart attack). He had been building hot-rod Harley's for years (& raced a pro-mod bike for a number of years) & I trusted his ability 100%.
This is the second time I've seen this.
~John
Found the same thing this winter on my rear piston. 100% that they were not put in that way 25,000 miles ago.
I do know that rings rotate around the piston over time. To get all three to align is bound to happen at sometime statistically speaking but I'm guessing that most people would not notice as I would think that sooner or later the alignment would change.
Following up on guppymech's remark about 2 cycle pistons being pinned, I've often wondered why 4 stroke motor pistons are not pinned. Most likely an accountant's cost/benefit analysis :potstir:
PS - I'm an accountant by trade :emsad:
2 stroke rings are pinned to stop the end gaps from catching the ports.
Two strokes are pinned to keep the gap from snagging in a port.
Was there a groove in the cylinder stopping them from moving?
Beat me to it...
Quote from: remington007 on May 13, 2019, 09:46:53 AM
2 stroke rings are pinned to stop the end gaps from catching the ports.
Quote from: Barrett on May 13, 2019, 09:47:32 AM
Two strokes are pinned to keep the gap from snagging in a port.
Was there a groove in the cylinder stopping them from moving?
Beat me to it...
So true - totally forgot about that.... Been a long time since I worked on and raced 2 strokes.
John, it happened to me in one barrel when I built my 120. It happened on the dyno with no prior running and I KNOW where the rings were on assembly. When I was building it I measured and recorded everything including the Cylinders at each inch of depth and at each 45 degrees around the circumference and then made a picture blueprint of them. The front barrel was absolutely perfect in all ways. The rear had a very slight out of round and very slight taper but still well withing spec. Just noticeable enough to get a measurement. It was similar through the full length of the cylinder.
When I checked the blueprint after the ring alignment was discovered, it turned out the rings had lined up directly in line with the loosest part of the egg shape in that barrel. A light whisker hone and back together with no re occurrence. Have you checked your barrels for true?
Here's a conversation about ring alignment that includes my experience. I will try to load a pic too.
https://harleytechtalk.com/htt/index.php?topic=102736.msg1212575#msg1212575
They are Axtell barrels & both are a touch out of round, but well within acceptable specs.
The leakdown was around 3% in both cylinders.
~John
The cross-hatch, and the degree of cross-hatch is what makes rings rotate. The steeper the angle the more the movement.
The next time you remove the cylinders, they will more than likely be staggered, and if they're still lined up, play the lottery. :wink:
John
Quote from: jsachs1 on May 13, 2019, 02:01:06 PM
The cross-hatch, and the degree of cross-hatch is what makes rings rotate. The steeper the angle the more the movement.
The next time you remove the cylinders, they will more than likely be staggered, and if they're still lined up, play the lottery. :wink:
John
That's what Hastings told me John, they will keep moving. :up:
Did they line up on the loose or tight part of the out of round? Mine were exactly on the largest dimensional part of the out of round of the cylinder if you know what I mean. I have been wondering if it was coincidence or the cause. We ran it on the dyno for quite a while troubleshooting it and it didn't correct itself. We redid valve seals and checked the guides using a Nub tool. CCP was 205 / 205. It was pushing oil into the rear intake port so we knew where it was. When described to Tom Reiser (Tman) he called it as rings lined up and he was right. Bottom line is it didn't correct itself in more than an hour of dyno time hot and cold.
It seems to me that once lined up, it would be harder for them to rotate back to normal as the gases keep pushing through that opening, restricting the rotation.
What about on intake and exhaust strokes where there is no compression, they'd still move.
Rings move. It's not uncommon for a leak down test to fail on an aircraft on one cyl or any other engine for that matter. First step is go fly or drive the bitch an hour and retest. They usually pass then. HD owners are too quick with the wrench sometimes but if they remain lined up, you can blame it on an egg shaped cyl. Is there such a thing as a round air cooled HD cyl?
Ron
Maybe when they're still in the torque plates, and have been honed. After that all bets are off. :doh:
Maybe I'm just simple minded, but so what if the gaps are lined up? Even if they're all 90deg to each other, there is still a "hole" through the rings for air or oil to pass through. Separating the gaps don't make the hole go away, maybe make it a little harder to snake through. Or not. Are you blaminng the oil consumption on this, or just commenting on the observation?
-JW
When they line up you can actually get a jet of oil into the combustion chamber when the piston drops on the intake stroke. Negative pressure in the cylinder combined with positive crankcase pressure from the piston pumping up the crankcase pressure on the way down shoots a shot of crank oil (flinging of the flywheels under the piston) and is drawn through the straight through route left by the ring alignment. 3000 rpm is 1500 tiny shots per minute. It doesn't take long to pass a decent volume of oil.
Not steep enough cross hatch.
It can happen at the very highest levels of racing and cross hatch is not always the cause. It's a freak occurrence. What were the gaps?
I neither refute or endorse the information in the link below. But it does give some interesting (and highly opinionated) perspective on rings and gaps.
http://www.diagnosticengineers.org/journal_%20articles/Ring%20Gaps%20vs%20Knowledge%20Gaps.php
-JW
Quote from: JW113 on May 21, 2019, 10:50:00 AM
I neither refute or endorse the information in the link below. But it does give some interesting (and highly opinionated) perspective on rings and gaps.
http://www.diagnosticengineers.org/journal_%20articles/Ring%20Gaps%20vs%20Knowledge%20Gaps.php
-JW
Interesting!
86
Quote from: remington007 on May 13, 2019, 09:46:53 AM
2 stroke rings are pinned to stop the end gaps from catching the ports.
YEP
I agree with the basis of the argument but not the 1/16" ring gap over 4 rings totaling a 1/4" open gap. In fact it remains as a 1/16" gap on top of another, on top of another and another ....... which is ultimately a 1/16" gap. I don't think I have that wrong. :nix:
Yeah, that math don't quite make sense to me either. Or I just ain't smart enough to see the logic in it. But the thing that I didn't quite follow was the part about the alignment of the ring gaps. One one hand, he says that staggering them during the engine building is "poppycock". But then also says that compression and oil control is a crucial aspect of the serpentine airway path through the ring gaps. So if that is true, then why is not staggering the ring gaps a critical concern? Yes, we can't control it after assembly, since the rings obviously spin during operation. Buy why promote the ill effects at first time start up?
Those darn Brits!
:SM:
(no disresptect!)
-JW
I just put a set of rings on my Dad's 106" (4.5" stroke). The bottom ring is dimpled. The dimple goes down and must be in the wrist pin gap. It's supposed to keep it from rotating out of that gap. I'm pretty sure this is unique to pistons which have rings covering the wrist pin gap, so probably not 100% applicable to this conversation.
Zach
Quote from: Boe Cole on May 13, 2019, 09:29:39 AM
I've often wondered why 4 stroke motor pistons are not pinned.
Quote from: biggzed on May 22, 2019, 09:20:05 AM
I just put a set of rings on my Dad's 106" (4.5" stroke). The bottom ring is dimpled. The dimple goes down and must be in the wrist pin gap. It's supposed to keep it from rotating out of that gap. I'm pretty sure this is unique to pistons which have rings covering the wrist pin gap, so probably not 100% applicable to this conversation.
Zach
Quote from: Boe Cole on May 13, 2019, 09:29:39 AM
I've often wondered why 4 stroke motor pistons are not pinned.
That's an interesting feature I was not aware of. Thx
http://blog.wiseco.com/ring-grooves
QuoteThe cross-hatch in the cylinder walls induces rotation of the rings.
:up: Great article.
Quote from: Admiral Akbar on May 22, 2019, 10:09:45 AM
http://blog.wiseco.com/ring-grooves
QuoteThe cross-hatch in the cylinder walls induces rotation of the rings.
yep good Read!
When I worked in the Radial Aircraft Engine shop the clearance between the rings and the lands was a major aspect of our rebuilds. Since new pistons were virtually unobtanium we had to do things differently. The boss would true up the ring grooves on the pistons we had. He would order rings from a special supplier we used that were just oversize in circumference and in thickness. We would grind them in a surface grinder to fit in the lands at the low end of the allowed tolerance. Then we would file them to get the correct ring gap.
kk