Piston ring gap position during assembling



7 replies to this topic
  • Hurky

Posted July 28, 2012 - 08:39 AM

#1

Hi everybody, I just bought an new cylinder big bore kit from Cylinder Works for my Yamaha WR450F 2008 and have some doubts about ring positioning on the piston cause I've found different and contradicting instructions out there I want to clarify and discuss with you...

To begin with this Big Bore come with an Vertex piston with only one top ring, beside the group of the typical spacer with two side rails and the original Yamaha piston uses two top rings and most of the instructions from piston manufacturers out there are referring to pistons with two top rings.

Here I have compiled some instructions to have it more graphical:

Yamaha WR450F:
Posted Image
As shown, they avoid the wrist pin axis and also the "longitudinal" axis as well, positioning the second ring at 180º from the first



Wiseco:
Posted Image
I suppose that by "Piston Pin Center Line" they mean the longitudinal axis (because of the Engine Front arrow), I so they prefere to put the top ring pointing to the intake and the second to the exhaust, the next oil rail only a few degrees apart, spacer at 120º and next rail also at 120º



Duncan Racing:
Posted Image
I don't like this "flavor" at all, first and second gap are too close together... The scrapers are positioned at least 20mm apart from the expander gap.


Now some instructions I found for one compression ring pistons:


Vertex:
Posted Image
I like this positioning, first ring pointing to the intake just in the longitudinal axis, and the spacer just opposite at 180º, with it's side rails 25º of it's gap.



Honda CRF450R:
Posted Image
I think I like these most if they didn't position the first gap just pointing to the exhaust were most of the heat will be, but positioning the first side rail just at 180º from the compression ring gap seems logical to me, and then the spacer at 90º and the lower rail at 90º again being in line with the top ring gap.



Well, being the piston actually a Vertex I should simply use they instructions and starting this post may be stupid but I like to investigate things and understand why so many different flavors exist. This fact says me it cannot be that important or critical, I just want to know what other people thing and do.

Some fundamental questions:
- It the gap direction important (exhaust, intake, wrist pin...) or are the degrees the only real thing ?
- Will the rings actually move if the motor is running ?
- If they move, will they do it all the time or only during break-in ? Because I actually cannot understand why it is so important to break in correctly the rings so that they "match" the cylinder surface and seal well if the rings constantly are moving to a different mating surface area in the bore... Maybe it's more a thing of the ring taper than the actual bore wall ?


Thanks for reading this far and more thanks if you contribute the the thread.

Regards !

  • 1987CR250R

Posted July 28, 2012 - 01:54 PM

#2

I would stick with the OE recommendations as they know the most about the engine. The only thing that really changes with pistons is possibly the ring gap which largely based on how far the ring is from the crown.

  • Jayrod1318

Posted July 28, 2012 - 02:08 PM

#3

The top rings rotate anyway while the engine is running. Fact

I think the theory is you don't want them to start out with the ring ends lined up cause it could cause blow by and stick the ring(s).?

Also don't &%$#@! foot your break in, piston rings are positive sealing as such they need forced into the cylinder wall to seat.

Let it warm up and go romp on it for a half hour. "break-in completed"

Edited by Jayrod1318, July 28, 2012 - 02:28 PM.


  • BDubb106

Posted July 28, 2012 - 02:25 PM

#4

The top rings rotate anyway while the engine is running. Fact

I think the theory is you dont want them to start out with the ends lined up cause it could cause blow by and stick the ring(s).


I never understood this... my ring gaps are never in the same position as they were installed. Never had any running issues so I figured it was normal. Glad to know it is.

I always offset mine exactly to the Vertex recommendations in the above illustration.

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  • KJ790

Posted July 28, 2012 - 02:27 PM

#5

You thinking way too hard about this. The rings often rotate when the engine is running, so it doesn't really matter how you put them in a four stroke. I would recommend not having all of the ring gaps lined up to start with. Just stagger the gaps by about 120 degrees each.

  • Hurky

Posted July 30, 2012 - 08:09 AM

#6

Thank you for your tipps, I'll opt for the CRF flavor as I like the 180º offset of each gap, I think this could be a little better than the Vertex being the gaps of both rails pretty close together...


Greetz :)

  • Frepps

Posted June 14, 2014 - 11:20 PM

#7

Just got the same kit for my wr450 2006. I was surprised to see only one ring. Are there no disadvantages in omitting the second top ring?

  • grayracer513

Posted June 16, 2014 - 11:29 AM

#8

  • Ring gap placement should follow the recommendation of the engine manufacturer first, piston manufacturer second.
  • Rings don't so much rotate in operation, they "wander".  Some rotate, some just shift in random directions.  There is no specific, predictable force that dictates the direction, speed, degree, or frequency of such migration, it's just random.  Nothing really prevents it, is the most certain thing you can say about it.
  • When oil rings rotate or shift, they nearly always do so as a set, with the gaps between the upper and lower rail staying in their same positions relative to each other. 

They should be staggered as instructed on assembly, however, nevertheless.  One thing to watch out for as the cylinder is lowered over the ring set is the ends of the oil ring rails in particular catching at the cutout area of the bottom of the sleeve and being dragged out of the groove and rolled over.  Watch for this as the cylinder is installed and prevent it by pushing the ring ends in as they bear.

 

Omitting the second compression ring is a race engine only drag reduction tactic.  It reduces the expected sealing life of the ring set as a whole, and since the rings are part of what stabilizes a slipper skirt piston, it can reduce piston life.  I never recommend it for people using their bikes more than 50% for recreational purposes because the service life of the piston assembly is reduced by doing so.






 
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