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Old 03-13-2013, 08:29 PM   #1
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Default Possible "Piston-Rock???"

Can anyone explain the purpose of the pistons skirts hanging out of the sleeve at BDC? I'm a registered drag racer, and I have been building race engines for more that 30 years. In a real race engine, pistons hanging out the bottom of the sleeve is not a good thing and causes piston-rock. Several pistons have broken in a few of my little nitro engines and I'm wondering if piston-rock (due to several mm of the piston hanging out of the sleeve at BDC) is a factor. What do you guys think? I'm thinking of machining the piston so that there is no part of the piston that extends beyond the sleeve. Has anyone ever did this?
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Old 03-13-2013, 09:52 PM   #2
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Good question but are you sure its piston rock causing you issues. Seems to me that since there are no rings the piston would not move all that much especially at TDC. Where are your pistons breaking at?
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Old 03-13-2013, 11:56 PM   #3
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The skirt is there so that at TDC it covers the exhaust port, otherwise the engine looses all the incoming mixture out the pipe. Are you setting the piston at TDC BEFORE you take out the backplate? A lot of engnies have a groove on the backplate because otherwise the skirt would hit it.
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Old 03-14-2013, 12:00 AM   #4
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Good question but are you sure its piston rock causing you issues. Seems to me that since there are no rings the piston would not move all that much especially at TDC. Where are your pistons breaking at?
They are breaking at the pin. I just machined one so that it lines up with the sleeve at BDC and took the car out and ran it a little. Revs quicker and accelerates quicker. Don't know the long term effects yet, but so far seems to be a go. I'm not sure it's piston-rock. That is just something I'm wondering. Thanks for the response.
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Old 03-14-2013, 12:11 AM   #5
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The skirt is there so that at TDC it covers the exhaust port, otherwise the engine looses all the incoming mixture out the pipe. Are you setting the piston at TDC BEFORE you take out the backplate? A lot of engnies have a groove on the backplate because otherwise the skirt would hit it.
Okay, now that you pointed that out, I can see why the extended skirt is there. I got lucky and didn't take enough off the piston to uncover the exhaust port. I just can't figure out why the piston are breaking. All in the same spot. Detonation maybe? Running 30% Byron's with a #4 turbo plug at 7000 feet.
Thanks.
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Old 03-14-2013, 12:26 AM   #6
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as long as the exhaust port stays covered at tdc it will be ok for awhile but remember it is a 2 stroke and the skirts get flexed into the ports under pumping compression on the down stroke all be it minimal because of the small displacements we are dealing with it is the cause of the majority of piston wear besides the liquid sand paper we run in them and the average piston speed at 40000 rpm is almost 4200 feet per second on a 16mm stroke just something to think about
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Old 03-14-2013, 02:50 AM   #7
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You mentioned the engines are running much better than stock after modifications, several things are happening.

More performance does mean more forces. The piston has already to deal with a lot of G-forces and every extra bit of performance is dubbeling the load. A thin part like the skirt is the weakest part of a piston. Just for fun calculate the speed of the crankpin turning arround at top RPM, it can go up to 100 km/u, The piston is accalerating from 0 to 100 km/u in an half stroke and slows down to 0 in the other half stroke. The G-force on a few grams piston can go up to several kilograms.....

Then there is flex in the crankshaft, with more RPM the bending of the krankshaft is getting larger and it is possible that the skirt is touching the backplate or the piston is even touching the head, if that is the problem you should see a mark in the backplate and/or head.

And a last thing is heat. The skirt has to deal with a lot of heat and heat is affecting the strength of materials. It becomes weak and can deform or snap off due the high forces. In this case some extra cooling with better lubrication can do the trick.

Some things to look at.....
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Old 03-14-2013, 06:49 AM   #8
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They are breaking at the pin. I just machined one so that it lines up with the sleeve at BDC and took the car out and ran it a little. Revs quicker and accelerates quicker. Don't know the long term effects yet, but so far seems to be a go. I'm not sure it's piston-rock. That is just something I'm wondering. Thanks for the response.
As a fellow machinist and engine builder (Subaru in my avatar is 589 to the wheels on E85!!) I wonder the same things! these engine mfg have been doing this for a long time with that being said I am sure there is a good reason for the skirt being so much longer. Personally i havent figured it out yet and will eventually get to it, right now I am concentrating on porting and port timing. In my 1:1 experience its not typically the high RPM's that kill an engine(unless you have oiling issues) its midrange torque that will send a rod through the block. Detonating while your in that range will break parts. once you get passed peak torque, momentum becomes your friend.

I have not actually looked at the relationship to the long skirt to the exhaust port but it seems to me if you were to bring the piston to TDC and then scribe the piston skirt from the exhaust port opening that would be the shortest that the skirt could possibly be. If that port opens up to the crankcase you would end up with exhaust waste in the crankcase which would in turn end up back in the CC. This would eventually turn everything inside the engine into a black nasty mess! You would also loose crankcase pressure. Keep in mind that the exhaust system sends pulses backwards.

Roelof , The crank is not going to flex it is Hardened steel. It is supported by one huge bearing and a smaller one meaning that the area supported by those bearings will not move unless the bearings move. So you are left with a very short amount of the crank sticking out past that point and connecting to the rod. Hardened steel will break before it flex's thats why they harden them! Only way the piston will hit the head is if the rod is either stretched or the bushing is worn out. Not sure which engine is being used but a billet 6061 or 7075 piston more than likely will not break but can deform slightly under stress but a hypereutectic piston will break and snap off as they have more silicon in them giving lighter weight and better friction coefficiency . more food for thought!

How about a pic?

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Old 03-14-2013, 07:20 AM   #9
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You are overevving your engine. Ive had the exact same problem. The piston from the pin down basically comes apart. The engine is too lean on the top, but being maksed by too rich of a bottom end.
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Old 03-14-2013, 08:38 AM   #10
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Roelof , The crank is not going to flex it is Hardened steel. It is supported by one huge bearing and a smaller one meaning that the area supported by those bearings will not move unless the bearings move.
You would be amazed about the flex. Indeed some play etc is also hapening. I can tell you that an engine compleetly builded up with fresh bearings and almost no play on the rod with a 0.35mm head clearance at full rpm the piston of our engine is hitting the head. Making 40.000 or 45.000 rpm can make a difference of hitting the head and/or backplate or not.
With wrong hardened crankshafts we have seen that they will snap within an half our, hardened in the right way is keeping some flex damping the peak forces. We also had made conrods from a very strong piston material, because it had no flex it shattered in pieces in 15 minutes.

Flex is always needed, maybe you can not see it but it is there!
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Old 03-14-2013, 09:38 AM   #11
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You would be amazed about the flex. Indeed some play etc is also hapening. I can tell you that an engine compleetly builded up with fresh bearings and almost no play on the rod with a 0.35mm head clearance at full rpm the piston of our engine is hitting the head. Making 40.000 or 45.000 rpm can make a difference of hitting the head and/or backplate or not.
With wrong hardened crankshafts we have seen that they will snap within an half our, hardened in the right way is keeping some flex damping the peak forces. We also had made conrods from a very strong piston material, because it had no flex it shattered in pieces in 15 minutes.

Flex is always needed, maybe you can not see it but it is there!
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Old 03-14-2013, 10:18 AM   #12
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Originally Posted by Roelof View Post
You would be amazed about the flex. Indeed some play etc is also hapening. I can tell you that an engine compleetly builded up with fresh bearings and almost no play on the rod with a 0.35mm head clearance at full rpm the piston of our engine is hitting the head. Making 40.000 or 45.000 rpm can make a difference of hitting the head and/or backplate or not.
With wrong hardened crankshafts we have seen that they will snap within an half our, hardened in the right way is keeping some flex damping the peak forces. We also had made conrods from a very strong piston material, because it had no flex it shattered in pieces in 15 minutes.

Flex is always needed, maybe you can not see it but it is there!
Maybe you are confused on what flex is?? I dont dis agree with the fact that you need a certain amount of flex but not on a crank, you will get more play from the bearings than you will from the crank. Exactly where do you think the flex will come from? think about it, your not going to get any flex between the bearings ( if you did the crank would bind)meaning it can only happen on the largest area of the crank! secondly with a properly hardened crank at most you would see maybe .0002" and once again thats not enough to do any damage. now on top of that you have to think of what would cause the flex and that would be the force of the piston and rod moving the crank and both of those two parts are made of soft aluminum with bronze bushings! both of which will bend/flex before the crank ever would. As stated before i have been building real engines for most of my adult(I am 42 now) life ranging from 700hp methanol rotary engines to twin 100mm turbo charged 2500hp v8's!! Granted i dont have that kind of experience with these engines , the principle is still the same metal is metal! If you are breaking cranks i tend to believe it is more of a frequency issue than flex,40k RPM's is alot no matter what scale!

I am just giving my 20+ years of machine shop/engine building experience, i could be wrong as i have been before. But i dont see how the laws of physics change with nitro engines


Z, i think over revving and too lean is right and probably what caused the damage. Hoes is probably correct.
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Old 03-14-2013, 11:26 AM   #13
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Originally Posted by Roelof View Post
You would be amazed about the flex. Indeed some play etc is also hapening. I can tell you that an engine compleetly builded up with fresh bearings and almost no play on the rod with a 0.35mm head clearance at full rpm the piston of our engine is hitting the head. Making 40.000 or 45.000 rpm can make a difference of hitting the head and/or backplate or not.
With wrong hardened crankshafts we have seen that they will snap within an half our, hardened in the right way is keeping some flex damping the peak forces. We also had made conrods from a very strong piston material, because it had no flex it shattered in pieces in 15 minutes.

Flex is always needed, maybe you can not see it but it is there!
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Quote:
Originally Posted by MantisWorx View Post
Maybe you are confused on what flex is?? I dont dis agree with the fact that you need a certain amount of flex but not on a crank, you will get more play from the bearings than you will from the crank. Exactly where do you think the flex will come from? think about it, your not going to get any flex between the bearings ( if you did the crank would bind)meaning it can only happen on the largest area of the crank! secondly with a properly hardened crank at most you would see maybe .0002" and once again thats not enough to do any damage. now on top of that you have to think of what would cause the flex and that would be the force of the piston and rod moving the crank and both of those two parts are made of soft aluminum with bronze bushings! both of which will bend/flex before the crank ever would. As stated before i have been building real engines for most of my adult(I am 42 now) life ranging from 700hp methanol rotary engines to twin 100mm turbo charged 2500hp v8's!! Granted i dont have that kind of experience with these engines , the principle is still the same metal is metal! If you are breaking cranks i tend to believe it is more of a frequency issue than flex,40k RPM's is alot no matter what scale!

I am just giving my 20+ years of machine shop/engine building experience, i could be wrong as i have been before. But i dont see how the laws of physics change with nitro engines


Z, i think over revving and too lean is right and probably what caused the damage. Hoes is probably correct.
Marcus,

Roelof is correct, there is movement inside the engine. I'm not saying you're incorrect on some of your points but I DO have many years experience with designing, building, and tuning these small 2 strokes we use for our R/C cars.

With a top on road engine spinning at approx. 42-43000+ RPM there is deflection in the crank pin, piston pin, piston skirt, and rod at a minimum. There are also frequency movements in the bearings and crank body. Even the carb venturi resonates when the engine reaches peak power.

4 stroke engines like you are familiar with building have much lower piston and crank speeds even at full power/torque. Cylinder pressures are also lower due to the lower compression ratios typically run.

With the 2 stroke interference fit, schneurle port design we use the piston skirt is a smaller diameter than the top of the piston at the sealing area. Cutting the skirt, even if the exhaust port remains cover at TDC will result in a crankcase pressure loss due to venting around the smaller diameter skirt through the exhaust port. There is piston rock, but not enough to break the skirt or cause it to bind in the sleeve. The longer skirt also helps to maintain crankcase pressure.

Something else to ponder. The 2 stroke has no harmonic balancer or counter acting power pulses to nullify the shock induced into the reciprocating assembly at each combustion pulse. The counter weight on the crank is NOT the equal weight of the piston, rod, and pin/keepers. In most engines it is between 55 and 65% of the mass.

Long answer short, I would not recommend cutting the skirt shorter on the piston. I would also not recommend overly aggressive porting on existing engines for more performance. Too much material removed can reduce charge flow and cranking pressure which will net you less power and lower fuel mileage than if you left the engine stock. Not too mention the risk of damaging the chrome liner if you try changing port timing.
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Old 03-14-2013, 12:16 PM   #14
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Movement or flex? I just find it hard to believe that the crank "flexes" more than it simply have movement from bearing play and what not. Flex in my mind is the bending of the crank which I just can't see happening in such a short distance , does that make sense ? Just trying to get the right terminology!

The amount of movement Roelof mentions is about the same amount as the bushing clearances from the top and bottom of the rod.

Just so there is no confusion I am not arguing the points just trying to verify them in my head! So take no offense to what I am saying.

Rick ,A lot of my experience stems from rotary engines which are not really four stroke !
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Old 03-14-2013, 01:39 PM   #15
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Movement or flex? I just find it hard to believe that the crank "flexes" more than it simply have movement from bearing play and what not. Flex in my mind is the bending of the crank which I just can't see happening in such a short distance , does that make sense ? Just trying to get the right terminology!

The amount of movement Roelof mentions is about the same amount as the bushing clearances from the top and bottom of the rod.

Just so there is no confusion I am not arguing the points just trying to verify them in my head! So take no offense to what I am saying.

Rick ,A lot of my experience stems from rotary engines which are not really four stroke !
The crank also flexes in addition to the radial play in the bearings/bushings. That's why the crank pin is surface hardened and not thru hardened. Thru hardened cranks snap due to the bending load imparted from the piston assembly.

I would guess that the engines you base most of your experience on are multi rotor. Very smooth in operation with the counterbalancing effect. Think of our little 2 strokes as a whack-a-mole game every rotation of the crank, that illustrates the action better than anything else I can think of.
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