Using oversized piston rings

Far from it, two questions from two people, two answers...

J.

 

Obviously it would be suboptimal according to manufacturers' standards.
I don't see that anybody has offered standard knowledge or experience
with the long-term effects, so I offer the partial results of a thought
experiment.

The rings would bear unevenly on the insides of the cylinders, and with
a greater-than-standard total force. There would be a couple of high
points on the circumference of each ring. I would expect those high
points to fall into troughs that would eventually be worn into the
cylinder walls as the rings stop rotating in the piston grooves. Each
cylinder would develop an oblong cross section. I would expect problems
with ring and cylinder lubrication after the rings stop rotating.

 

Be careful about how much you play with your brain. I've heard you'll
go blind ;-)

And some rings don't rotate at all. Two-stroke rings have pins to
prevent the ends from rotating out into the exhaust port and snagging.

Apparently all of this commotion about fitting oversized rings---which
I'd never heard of before---is to optimize the ring gap to reduce
compression leaks to the smallest possible amount.

But ring manufacturers advise engine rebuilders to stagger the end gaps
120 degrees apart on a 3-ring piston to provide a tortuous path for
what minuscule amount of hot gas does slip through the tiny end gap.
With three gaps, the gasses will have to go 2/3rds of the way around
the piston to get to the crankcase...

And then consider the time aspect when an engine is operating at, say,
12K RPM.

It completes 200 revolutions in a single second. A complete revolution
takes place in 0.005 seconds. But the amount of time that a piston ring
is "seeing" a large pressure, starting at around 900 PSI and declining
to maybe 20 psi is about 90 to 120 degrees of revolution. And even this
event occurs only once every *fourth* stroke.

I think we are talking about how much hot gas can leak past a ring gap
in a bit over a thousandth of a second, folks. I wonder how much of
this precious combustion gas we can afford to lose?

Are we gonna run out of combustion gasses and have to go to electric
motors because of the horrendous losses? Will we have to go to fuel
cells or a hydrogen technology just because of end gap blowby?

It staggers the imagination! And now my vision's getting blurry! I
think I'm going blind, Matt!

Maybe this whole concept is like penis enlargement. Obviously,
everybody in the world (except me) needs a bigger penis.

The performance enthusiast's penis has wheels attached. This dedicated
amateur performance tuner decides that he needs a better diagnostic
tool than a compression tester, so he runs out and buys a *leak down
tester* and he pressures his cylinder and watches to see how much
static leak down he's getting.

Once he sees that he's losing his "purity of essence" through the ring
end gaps, he tears the engine down and installs over-sized rings so he
can reduce his end gap and save the essential combustion gasses.

I'll bet the static leak down fan is thrilled to death by how "tight"
his engine is!

But you don't make any power when the piston isn't moving, so the
tightest engine in the world is about as useless as a nun on Thursday.

 

I did it because I got sold the wrong size ring and couldn't be arsed to
go back, get them changed, etc etc.

And I was a teenager, like I said.

Anyway, it worked for me and worked for the other poster here who's done
it, so end of story, really.

 

The OP was talking about oversized rings on standard pistons, same as "R.
Pierce Butler". This could be done by filing the ends to get the correct
gap. I wouldn't do it, but I won't say it wouldn't work either.

Les

 

0.0196 rounds off to 0.020 in. Difference between a 0.5 mm over and
0.03 inch over is only 0.01 inch so it's not that much. Difference in
radius for 0.03 in. over is only 0.094 in. For say a 4 inch dia.
piston, that is only 2% change, so I doubt spring tension was an issue.

I somehow feel something else was going on. Did V-8 installer ensure he
had correct ring gap after filing and correct ring fit in groove, same
ring width as original?

 

Just from an engineering point of view, a .5 mm (about .020) oversize
piston ring is ground "round" at that diameter.

If it is used in a smaller bore (on a smaller piston), it is no longer
circular even though you've ground down the ends to have the correct end
gap.

I know it's not much out of circularity, but the "hoop stress" that a
piston ring imparts is a function of how circular, and consequently, how
even is that stress distributed.

But, if someone has done it and it has worked... hey, what do I know?

CP

 

Nobody on this thread has offered experience regarding the long-term
effects of using oversize rings.

 

Somebody who would use oversized rings to reduce the leakage past the
end gaps probably wouldn't care about the long-term effects, they'd be
chasing the immediate mouse milk.

You have to milk a lot of mice to get a drop of milk...

 

Who cares?

 

That is quite true. Most people do things by the book.

My father had a strange air cooled engine that he needed rings for. There
were no piston rings available at the time so he grabbed a book of the
various automotive piston rings available and found that he could use piston
rings from a Packard. They fit so he used them.

pierce

 

probably not anybody who expects to be kicking the bucket soon

 

You're not very bright, really, are you?

 

Hey, did you get an insurance payment for that messed-up CB175 that got
stolen?

 

What messed-up CB175?

If you mean my bloody nice, clean, sparkling and sweet-running bike,
then yes. More than I'd paid for it, too )

 

I mean the one that had wrong-size piston rings.

 

<snip>


<loses patience>

Do **** off, you pathetic, inadequate, unprincipled, nasty, stupid,
vindictive, childish little by-product of an inbred village idiot's
wank.

 

that's what you believe you are

 

Oh, shit! I didn't realise that TOG's 175s had a habit of getting
stolen... Oh well, it's in the hands of the insurers now.

Why didn't you turn up at the pub Friday night, anyway? You had
enough instruction on ways to get home!

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD, DT175MX-MIA "You Porsche. Me pass!" DoD #484 JKLO#003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

Got the wrong date.....