Using oversized piston rings

Would it cause problems to take .50 oversized rings and grind the end-gap
for use on standard pistons? i.e. is the interior diameter going to be too
large, and/or would there be too much spring tension?

 

Um, I've actually done this, and it worked.

 

The problem with oversized rings as opposed to oversized pistons is the
increased ring area exposed to combustion pressure. Not *necessarily* bad,
but definitely not something the engineers anticipated...

 

Were the pistons new? How big was the bore? For how long did it work?

 

..050 is quite a bit. I knew a fellow that put .030 oversize in a 350 V8 and
he said it was the hardest starting SOB he ever encountered. After about
3000 miles then it started to loosen up a bit but he also added that he would
never do it again. He sold the car so the engine's longevity was never
determined.

I think that in an aluminum block, .050 oversize in a standard bore would be
a very bad thing.

pierce

 

1. Do you have any relatives named "Nomen Nescio" by any chance?

2. What kind of motorbike or engine are you talking about anyway>

3. Are you talking about 1/2 an inch or half a millimeter oversize?

4. Is the engine a 2-stroke or a 4-stroke? An oversize ring might tend
to snag in 2-stroke ports worse than the right sized ring.

5. How large is the bore of the piston? Large bore pistons around 100mm
in diameter probably would probably tolerate the oversized ring better
than smaller bore pistons around 50mm.

6. Radial spring tension isn't the major compression sealing force on a
ring. It's the hot combustion gasses between the piston and the ring
that push the ring against the cylinder wall. Will there be enough
space behind the ring to allow the passage of gasses?

7. You could carefully measure the cylinder bore with an internal
micrometer, somehow measure the diameter of the ring groove (a vernier
caliper jaw won't fit in there), and calculate whether there would be
adequate space left behind the ring.

8. Is there some particular reason why you want to risk destroying an
engine in order to use an oversized ring with a standard piston?

9. Has Abby ever had any purebred Abbysinian kittens? I want one!

 

This is in a 1980 CB750, so it's a 4-stroke with 62mm dia. pistons. The
rings in question were .50mm (2 sizes) over, which I ground to the proper
end-gap for my standard-bore cylinders & pistons. I had seen postings of
this being done one size over to get a better than out-of-the-box end-gap.
I just didn't know if going 2 sizes over was going too far. I had no *good*
reason - just I had the rings on-hand and got in a hurry.

Sorry, kittens are not in the cards I'm afraid.

 

Standard pistons, standard bores, and it worked fine - the bike got
nicked a few months later, mind. It was a Honda CB175cc twin.

But it didn't use oil or smoke.

A complete bodge, I agree. I was a teenager then. But it worked.

 

So half a millimeter is 0.0197 and half of that is 0.00985 inches that
the inside of the rings will have to move toward the piston on each
side.

I wonder if plastigauge is made that thick?

It seems that if you could lay a strip of plastic behind a ring, insert
it into the cylinder past the chamfer using two screwdrivers so you
could get the piston in without overly-compressing the ring, and then
removing the plastigauge, you could see if the plastigauge crushed when
the ring was evenly compressed all the way around.

But that's still a *cold* test. How much is the ring going to expand
when it gets hit by 900-degree F combustion gasses at about 900 pounds
of pressure?

Mechanics are given an end gap number so they will know if the ring is
going to have enough room to grow with heat.

What does Plano, TX look like these days? Plano, CA looks like a soggy
cow pasture as I look out across the settling basins. There are Canada
Geese out there every morning among the cattle.

It seems like some cowardly Texans vamoosed out of Plano, TX in 1861,
and headed for the California gold fields instead of joining up with
the Confederacy. But they got here and looked around and saw how nice
the soil was by the river, so they stayed and called it Plano.

 

New rings on a old piston is always less than an ideal situation, as the
rings will never seat fully in the ringlands..

J.

 

That statement is interesting, because it goes against accepted practice
on high performance two-strokes for the last 30+ years.

In other words, for two-strokes at least, it's cobblers.

 

Since when?

pierce

 

Nope, in high performance Two-stroke we used to hone the cylinder for
the next ring/piston size. These days you do not hone, merely measure
the wear and fit a proper sized piston with rings..

(Aircooled 100cc go-kart engines at aprox 18000 rpm.)

J.

 

Since dawn of the internal combustion engine...

J.

 

I can see you might for an 18,000rpm race engine, but for road engines,
you simply re-ring. This is in some manufacturers' service advice, too.

 

It will always work, my statement was that the solution is less than
ideal, as you will get more blowby compared to fitting a new piston with
new rings.

J.

 

My statement was:
"New rings on a old piston is always less than an ideal situation"
With emphasis on "less than ideal" as in increased blowby compared to
using a new piston with new rings.

J.

 

Hair-splitting. In practice, it makes sod-all difference.

 

Do I detect some frantic justification and dissembling?

 

i would if they're straight-end rings.
i usually figure about .003" of gap per inch of diameter on water cooled
engines.
good luck, sammm