Nova let go today :(

[Blaise Recca]

Looks like lack of lubrication, too much heat in the combustion chamber (evidenced by the caster cooking under the piston) and too much cooling from the colder water. The reason it might have happened this time was the accumulation of conditions to contribute to the perfect storm. It seems it got too tight…. metal to metal and the weakest link broke. You might try a better oil package (make sure it’s enough and not too much) and watch head temps more closely when the water gets cooler.

And now off to open my GOODIES! Merry Christmas!

 

[Marty Davis]

Here is a re-cap


The scoring was caused by a retainer that let go AFTER the piston broke, this is a known.

The oil is not the problem, Every one of my engines gets a light coating of oil dye over them while im breaking a engine in, the color fades as i start to lean the motor down, after breakin.

iv ran this exact mix of fuel for 3 years, the is the only failure iv had, in 3 years.

top of the piston is clean and has no burn marks or detonation.

Ron had a idea Ill be testing in the morning, the sleeve is out of round is the idea, and the odd pattern of breakage leads me to believe this might be correct.

specs of white looking material under the piston is that powdered piston material I was speaking of.

Please expand on the Oil Dye and how you use it.

 

[F.Orlic]

Any chance the rod got sloppy and the skirt hit the counterbalance ABDC?
I'd look for marks on the counterbalance...

ps: I've heard Motul is a good oil, but I'd ditch the castor!

Don't think so bud, the rod was fine, bearing's are fine (novarossi ceramic's) and no marks on the cranks counterbalance. if a rod got lose enough to hit the crank or the back plate, The plug would have been punched, and this plug survived to run again.

I don't think Ill ever know what really went down, but I can say this, for all the damage that was done, I never hurt any part other then the p/s when it let go, wrist pin's got to be trashed just because.

With few more pics we could put the sequence together. Few pics of piston and few of the sleeve and rod would shed some light how it happened. Your pic is to small to make definite conclusion. I've noticed few things but wihout better pic there is no point in discussing them.

For nova to break like that something was very wrong. I have seen many pistons all chewed up, broken rods, cranks , bearings and the pistons ALWAYS survived.

 

[Marty Davis]

Any chance the rod got sloppy and the skirt hit the counterbalance ABDC?
I'd look for marks on the counterbalance...

ps: I've heard Motul is a good oil, but I'd ditch the castor!

Don't think so bud, the rod was fine, bearing's are fine (novarossi ceramic's) and no marks on the cranks counterbalance. if a rod got lose enough to hit the crank or the back plate, The plug would have been punched, and this plug survived to run again.

I don't think Ill ever know what really went down, but I can say this, for all the damage that was done, I never hurt any part other then the p/s when it let go, wrist pin's got to be trashed just because.

With few more pics we could put the sequence together. Few pics of piston and few of the sleeve and rod would shed some light how it happened. Your pic is to small to make definite conclusion. I've noticed few things but wihout better pic there is no point in discussing them.

For nova to break like that something was very wrong. I have seen many pistons all chewed up, broken rods, cranks , bearings and the pistons ALWAYS survived.

Franco:

I agree with you 100%. My failure of the piston breaking on a .21 is my first. I have seen this happen on a .67 but NEVER on a .21 (the failure of the .67 was a wrist pin boss failure).

I believe that maybe my problem was a MUCH too tight fit liner/piston. I did that because of the poor piston material in a batch of pistons - they were way to soft with probably too little silicon content.

 

[BB Mods's]

Marty, Ill send you a PM.

 

[BB Mods's]

Sleeve had 3 notable high spots in it right near the compression band, 2 measured .002 and the other was .003 protruding into the cylinder, right where the piston failed.

Ron was right, sleeve is out of round.

 

[jetpack]

Putoline Kart Tech 2 two stroke racing oil, castor/synthetic, for gasoline, methanol and nitromethane racing fuels. (oil Mix 4% - 15%)

Kart Tech 2 is the ultimate race proven synthetic ester & castor based lubricant. Its specially selected additives make it extremely suitable to lubricate today's high revving 2-stroke Kart engines. Kart Tech 2 is a premium synthetic ester & castor based racing oil that contains advanced additives providing improved film strength and load carrying properties.

Composition: Synthetic ester & castor based

 

Colour: brown, Application:

Pre-Mix only

Suitable for use with high-octane racing fuels

Suitable for model glow and diesel engines

Do not mix with other oils

Can be mixed with Methanol, Nitromethane as well as conventional fuel

Attention! Mix very thoroughly! When mixed use within 1 day

I know oil has been ruled out as the culprit, but it was interesting to see the final note on this kart oil.

 

[Mike Hughes]

Attention! Mix very thoroughly! When mixed use within 1 day

You would think that has to do with it seperating. I would hope it was not because it breaks down. Kind of leaves you guessing.

Mike

 

[jetpack]

Nitromethane is a compound with few enemies...

Here's two examples of what could occur:

Nitro penetrates the oil compound with oxygen (proven) and when given enough time to saturate...enables the oil to burn, tipping the scales of its capacity to lubricate.

Take the short picture of fuel-foaming problems up to the carb, now imagine micro-foaming of the oil within the engine due to the time allowed added oxygen it now contains.

A separation between the fuel and the oil cannot be trusted as the indicator, as the two have combined past simple mixing.

Cloudy fuel can be formed by more than just the methanol that has combined with water. Nitro can cloud a fuel by attacking its oil with oxygen.

I think I just got my geek on.

Oxygen Rich Oil = Bad Monkey!

 

[jetpack]

PS: Perhaps the out-of-round condition (or its buddy, the hot spot) of the cylinder allowed enough by-pass combustion to reach below the piston and allowed a back-fire inside the lower case to occur. One glowing hot chip that is able to reach into the crankcase is all it takes.

Rare in two-stroke engines, but boy can it stop a piston dead in its tracks with nothing but a welcoming row of intake ports for the skirt to escape out of.

Fragments suspended in a millisecond of time inside the boost runners until the crown of the piston continues down, exhales them back into the cylinder, then back up straight out the exhaust as it makes its final journey, leaving only the bigger bites in the crankcase left for people to wonder.

 

[Jerry Wyss]

I think the breakage is secondary,, the black on the underside of the piston is singing the primary cause.

Looks like you had FIRE in the Wong Ho,, and why anybody would use such an obscure

lube such as that makes me wonder about you too.

I would have to call this one "pilot error",, it usually is,,

 

[Ron Olson]

Sleeve had 3 notable high spots in it right near the compression band, 2 measured .002 and the other was .003 protruding into the cylinder, right where the piston failed.

Ron was right, sleeve is out of round.

Yeah, I was wondering about that after talking to you in the chatroom the other night.

Just so the others know, I had a NR .21 that did just that. My son and I were getting his boat dialed in for a race and there was a storm brewing. We said those imfamous last words, "One more run". The engine ran fine all day until the last run when as soon as I fired it up and went to launch it, the con rod broke pretty much in half. That was the obvious part after I pulled the back cover off then when I took off the head, that's when I found the warped/out-of-round sleeve. Mine was much worse though as I couldn't get the piston out or the sleeve out of the case.

 

[jetpack]

Barometric pressure changes how an engine "sees" nitro.

A big drop in air pressure due to an approaching storm might have been a contributor?

Running almost pure nitro, with an "old" mix of fuel with a drop in air pressure (change in temp usually comes with that) changed the compression + air/fuel mix also, adding to the breakage formula?

A tri-oval pattern can often develop during the manufacturing process and only continues to get worse if the machine is not stopped and re-dressed, and can usually be found at the entrance of bores before the machine picks up full tool pressure.

A side note on pre-mix:

Citco Oil used to produce a specific two-stroke outboard oil which contained a higher amount of Ether than common industry percentages and when mixed and run fresh had noticably earlier ignition timing than a later-day run on the same small batch.

I mention this only to stress how pre-mix oil can change in time and have a noticable effect on a race tuned motor.

All of what I've been mentioning is just food for thought, I still don't have a clear answer in my conclusions as to what happened with the pineappled piston. I'm still learning and keeping my ear to other people's thoughts too. Sometimes it's pretty difficult to get a dead piston to talk to find out what really did happen, with so many variables in the picture.

A rare occurance such as this has not plotted any trend yet, and because there is no trend established to look at - it just makes it even harder to guess.

 

[BB Mods's]

The black under the piston is dye, how many times do I have to say it.. Its dye, not carbon, not burned, and not the cause. I started this thread looking for a answer that I could not find, the sleeve is out of round.. Ron solved that one for me, my fuel mix is not/should not be in question. Ill say it 1 more time for those that have not read back.

IV RAN THIS EXACT SAME FUEL MIX FORMULA FOR 3 YEARS THROUGH 14 MOTORS, THIS IS MY 1 AND ONLY FAILURE.

The color you see is NORMAL for my engines while there being broken in. The dye is diagnostic in nature, Same fuel for 21's to 101's, no deviation in formula, 2 well known SAW drivers run this EXACT SAME MIX with no failures on there part.

Here is my process.

step 1, Lap P/S in at home with WD-40

step 2, Run 3/4 gallon through motor fairly rich, I don't have to go super rich because half the break in process was done at home

step 3, after 3/4 gallon pull motor down, replace bearings and rod, c clips and wrist pin, take my break in crank out and install normal crankshaft

step 4, After 1/2 gallon of fuel, pull head button/sleeve as 1 unit out and check for my caster seal in the head, once I can confirm iv got a total seal, snatch the piston and rod and check my under side color, if all is golden, put the motor back together, and go run the holy *&c% out of it

The only question that's left is how my sleeve got high spots in it, I know for sure during the lapping process the sleeve and piston wore in correctly, noting any non shiny/overly shiny spots on the sleeves wall. I guess its possible I dropped the sleeve, or dropped something on it, I don't store my sleeves any special way, just stick them and there rods in a 4" x 4" bin inside my rigger stand, I keep about 12 sets of ready to go p/s sets in there, guess I need to go check them for dimples.

 

[jetpack]

Factory grinding could have produced a heated, lobed stress relief within the sleeve material that could have passed inspection and lapping, and later showed up as out-of-round as the engine reached and maintained race temp?

A three-lobed grinding pattern is probably the most common problem when centerless grinding - especially when working with smaller wheels such as this. Maybe a poor finish/size was detected during grinding, process was stopped and corrected, but the heat damage stayed hidden within the material until the engine started to season.

Tech Paper on Lobe Problem

Seasoned sleeves that are still round are precious commodities at times, because they allow tear-downs that do not upset them. This is one reason why the chance of moving the sleeve when pulling down a motor runs risk, and minimal tear downs are always strived for, not implying that excessive tear-down is the case.

The oil talk on my part was just to expand on just that, and not really aimed at being the failure and guess should have been kept more clear as a side-line topic. The tri-oval issue clears up the failure, but knowing how it reached that point after lapping and inspection is still open in my mind. It's a puzzle I cant put down.

 

[Jerry Wyss]

Here is my process.
step 1, Lap P/S in at home with WD-40

step 2, Run 3/4 gallon through motor fairly rich, I don't have to go super rich because half the break in process was done at home

step 3, after 3/4 gallon pull motor down, replace bearings and rod, c clips and wrist pin, take my break in crank out and install normal crankshaft

step 4, After 1/2 gallon of fuel, pull head button/sleeve as 1 unit out and check for my caster seal in the head, once I can confirm iv got a total seal, snatch the piston and rod and check my under side color, if all is golden, put the motor back together, and go run the holy *&c% out of it

J.C.,, I'm outa here.

 

[steve wood]

Here is my process.
step 1, Lap P/S in at home with WD-40

step 2, Run 3/4 gallon through motor fairly rich, I don't have to go super rich because half the break in process was done at home

step 3, after 3/4 gallon pull motor down, replace bearings and rod, c clips and wrist pin, take my break in crank out and install normal crankshaft

step 4, After 1/2 gallon of fuel, pull head button/sleeve as 1 unit out and check for my caster seal in the head, once I can confirm iv got a total seal, snatch the piston and rod and check my under side color, if all is golden, put the motor back together, and go run the holy *&c% out of it

J.C.,, I'm outa here.

yeah!

 

[BB Mods's]

Here is my process.
step 1, Lap P/S in at home with WD-40

step 2, Run 3/4 gallon through motor fairly rich, I don't have to go super rich because half the break in process was done at home

step 3, after 3/4 gallon pull motor down, replace bearings and rod, c clips and wrist pin, take my break in crank out and install normal crankshaft

step 4, After 1/2 gallon of fuel, pull head button/sleeve as 1 unit out and check for my caster seal in the head, once I can confirm iv got a total seal, snatch the piston and rod and check my under side color, if all is golden, put the motor back together, and go run the holy *&c% out of it

J.C.,, I'm outa here.

You see a issue with my method? Fastest A mono SAW driver uses this method on 21 N/R engines...

 

[Speed Jr.]

You see a issue with my method? Fastest A mono SAW driver uses this method on 21 N/R engines...

Kinda overkill is what I think there hinting at. Completely unnecessary IMHO, but hey whatever "Floats your boat" :lol:

 

[Blaise Recca]

So that’s what I should be doing...... I've just been running the heck out of my 21 CMB on Technology 50% from the get go and I can't get them to fail yet. I got to change what I'm doing this year! I'll grenade one this year.... I swear.