Hi , i am looking for some opinions on the YS 120 four stroke piston versus the OS 120 piston. The reason i'm asking is because i have modified an OS 120 SP engine . It now spins a 16 x 10 prop at 11,600 rpm. However when initially tested i ran a 15 x 8 prop and it over revved causing ring flutter and a cracked piston. Now i need to replace the piston , ring and liner. I have checked the YS 120 piston dimensions and it is the right diameter and within 1 mm of the correct compression height via a phone conversation with LHS. What i would please like to know is experienced engine users/rebuilders thoughts on the two pistons.Is the YS piston sttronger than the OS piston? Or should i stick with the OS piston? I have not eyeballed the YS piston yet but from what i am told it is a billet piston , is this correct ? The OS piston is a cast , lightened piece. The stock piston has failed in my opinion due to ring flutter , but i am not ruling out fatigue or lack of strenght because of the increased cylinder pressure. I am reticent to use the stock OS piston as it has failed quickly with the increased power. Any thoughts or alternatives would be appreciated. Thanks in advance ,Leon.

I believe the underside of the YS piston is partly filled in to enhance crankcase pressure for supercharging, this would not be a good thing on your engine.

The Y-S piston is made rather unusual from the way the OS is. The Y-S piston has a flat bottum and the lightened part is on the outside of the skirt. There is only a narrow slot for the rod underneath.

If you run that much RPM with the Y-S piston, you are going to have ring flutter too. While the piston may not break, anytime you have ring flutter, there is a loss of power.

You could make a different length rod to allow for the deck height difference.

Good luck,
Jim

what mods did you make to the motor that seams like allot of increase from the stock motor mine turns a 17x6 apc at 9500 or so I would expect that a 16 10 would be about 9000 to 9300



let me know

Why do you think you had ring flutter? I'm not doubting you, I just wouldn't know what would indicate the ring was fluttering.

To bad someone would'nt make forged pistons as they do for cars, bikes, snowmachines,etc, but Isuppose it would be cost prohibitive. Just keep it below flutter speed and it will be OK.

I would think you'd get valve float long before any ring problems developed. I wonder if valve float caused your cracked piston problem? (Piston hitting valve).

BTW - What is ring flutter??

The only time I heard of ring flutter it was nondestructive, it just caused high oil consumption. How did the piston fail?

I know on big stuff like cars and boats if you run into spark knock or detention it can cause rings to break, just a thought. If thats the case then you may be over compressed or too much nitro, good luck

I too think that ring flutter is not what was being experienced but it was valve float.

Ring flutter is usually some combination of stroke (resultant piston speed) and rpm. This also has other variables such as the cross section of the ring and the resultant harmonic frequency of the ring. When everything matches, you have flutter and resultant lose of power and increased oil consumption.

It baffles me as to how one would be so positive about the presence of flutter?

Jim

I'd guess detonation, personally. Certainly not ring flutter.
Valve float would be seen as a power loss, non-destructive unless piston contacts valves,
which is extremely rare except on very high lift engines with very little piston/valve clearance.
(The tell-tale sign there is that you'll also have bent valves, most of the time)

Detonation, on the other hand, can certainly destroy the top of a piston, all by itself.

1mm is a big difference on a 350 chevy, much less a 1.20 OS! I know with balsa on wings you can deal with 1mm tolerances but on a precision engine you will not get it running right if at all and then it may cause damage if it does run.

I hope you folks know what I meant earlier, anyways I'll stick to my detonation theory on why rings break, but most anything can happen when start re-engineering things

From my old hot rod days.

Valve float, this is when the valve fails to fully close before the next cycle. Since it usually only takes a very small amount to cause a significant loss of power it limits further RPM increases. It rarely causes damage.

Ring flutter, this is caused by several things but my experience was my first high performance rebuild, I either failed to check the fit of the rings inside the groove or did so improperly. The piston ring groove had widened past tolerances. This caused the rings to move excessively inside the piston. The extra flexure cause the rings to crack, poor compression and poor power. Rarely causes further piston damage. At least it didn't in my case. This can also happen when the engine over heats causing the groove to widen. Overreving can cause this though it is rare. If from overreving the piston can crack the piston but it is rare.

Rod stretch, I suspect this is the problem. The rod stretches from overeving. If it stretches enouth the rings contact the wear ridge at the top of the cylinder. This causes the rings to crack and often causes the piston to crack as well. Fairly common with overeved engines. Rods should be checked that they have not permanently stretched.