The Science of the Second Ring

Do tight pistons make your motor perform better ? I've always found the opposite to be the case. However by the time the pistons are loose, I have probably tuned the motor better.

so for the standard hastings ring package should the 1st and 2nd rig gaps be the same or run a larger 2nd ring gap? for average riding and rarely approaching 6500RPM.

2nd ring gap 30% bigger than top ring gap comes well-recommended and works in my non-Norton 650.

Triton Thrasher said:

Yeah but I don’t know what he did!

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https://www.motorsportweek.com/joeblogsf1/id/00169

His work at Norton consisted of mainly DOHC squished head and short stroke Manx engine.

xbacksideslider said:

Yes, and that system requires a process of running, take down, reading wear patterns, and re-profiling the piston’s shape, it’s barrel and roll , to match the conflicting expansion rates of the cylinder (at different points up and down and radially) and the piston (again at different points up down and radially), followed yreassembly for another round of takedown and reprofiling

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No - there was no re-profiling of the piston, no checking of wear patterns or anything of that sort. The pistons were simply coated with graphite and installed with .0025 to .003" clearance to the graphite. When the pistons heated up during break in the interference areas of graphite burnish away to the perfect clearance with no scuffing. The photo of the piston below was taken after 100 miles of breakin when an oil leak developed and needed fixing.

Its an ideal situation that has not been available until recently. If you haven't tried it then you don't know what you're missing. You don't need to redesign or re-profile the piston by machining because the graphite does that for you automatically. Once the interference areas are worn away the graphite stays put - the clearances are ideal and the life of the piston is approximately doubled.

Yes, I get it. Did not mean to imply that you did that. I did mean to say, though, that that process is in the history of the development of the shapes of pistons generally and in the proprietary shapes for particular pistons as offered by the major piston manufacturers for particular engines.

xbacksideslider said:

Yes, I get it. Did not mean to imply that you did that. I did mean to say, though, that that process is in the history of the development of the shapes of pistons generally and in the proprietary shapes for particular pistons as offered by the major piston manufacturers for particular engines.

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Yes the piston manufactures make their refinements which we appreciate. I mention the graphite coating for those who are concerned about skirt clearance, rocking and keeping the rings perpendicular to the bore. The graphite allows tighter clearances without scuffing. Rocking is pretty much gone when the pistons are warm and the skirt clearance is near .001"

xbacksideslider said:

....and that allows a thinner ring cross section/less surface area presented to the cylinder wall, and that means less friction so lighter ring pressures still seal.

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No it doesn't. Provided piston rings are considered solids moving radially and up/down in their slots (as observed by Jim), friction law of physics demand that friction force remains constant and independent of friction area. Friction varies with back pressure of course.

-Knut

Thank you Knut. Would it be correct then to say that a thinner ring, by its rear face's lesser area, experiences less gas pressure on that face and that thereby transmits less pressure by way of its outer ring face to the cylinder wall?

There shouldn't be any ring flutter with thin 1mm top rings (its a problem with thicker rings). I'm willing to test for changes in blowby with gapless rings - but still trying to find the gauge I need to read the sump at WOT and high RPM.

The rings in 73mm Honda Fireblade pistons are thin. The operating circumstances are far more extreme than a Commando, but the piston designs are several decades apart. In two strokes, the change from thick to thin rings happened in the early 70s. I like the thinking that has gone into Jim Schmidt's long rods and short pistons.