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Why is filling in the crank with silicone a good thing?

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Why is filling in the crank with silicone a good thing?

Old 04-11-2014, 02:48 PM
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Default Why is filling in the crank with silicone a good thing?

Why was this done in the first place? I was browsing ebay, when I noticed
a recent NovaRossi that looked similar to a model airplane crank without any
silicone.

It seems more likely filling the crank with silicone would allow crankcase pressure
to leak out the front bearing?



Bill M.

Last edited by wmazz; 04-18-2014 at 09:20 PM. Reason: defined "it"
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Old 04-11-2014, 03:02 PM
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It lowers crankcase volume. I'll let you look up what that does , but it has nothing to do with the front seal of the engine.

Also, Novarossi probably doesn't do it to all their engines to decrease production time and costs. Having hundreds of cranks sitting around waiting for the silicone to "setup" is costly.
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Old 04-11-2014, 03:14 PM
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my favorite shirt I made in high school 40 years ago was
STUFF
YOUR
CRANK
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Old 04-11-2014, 05:01 PM
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Originally Posted by wmazz View Post
Why was this done in the first place? I was browsing ebay, when I noticed
a recent NovaRossi that looked similar to a model airplane crank without any
silicone.

It seems more like it would allow more pressure to leak out the front bearing?



Bill M.
The crank is bored 'till the front for weight reasons and then it get's a drop of silicone to retain volume
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Old 04-11-2014, 05:27 PM
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Originally Posted by 30Tooth View Post
The crank is bored 'till the front for weight reasons and then it get's a drop of silicone to retain volume
On other model engines, crankshaft mass is not as important. Why then,
do they bore out the front of the crank?

And if it is to increase crankcase compression ratio, doesn't that mean
you just decreased the amount of fuel-air mixture in the crankcase?


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Old 04-11-2014, 07:55 PM
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It replaces the mass that is taken from the crank when the crank is drilled. The crank is drilled to save/balance weight. A light crank yeilds a much quicker "spool up" due to less rotating mass. In order to replace the area that has been drilled out, silicone ramping is used due to the weight savings it provides, plus it accelerates the air/fuel mixture into the crank case. The main drawback is when the crank gets lightened too much, tuning on the bottom end becomes erratic and there is often a "flat spot" felt in the upper rpm range.
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Old 04-12-2014, 03:25 AM
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Because we want to buy an engine with the most bullsh*** technology

Yes, the silicone is lighter and does replace the heavier drilled out metal, but the drilled out metal is from the center of the crakshaft what regarding simple physics has less influence on rotating mass.

Yes, it does lower the crankcase volume what gives a higher pressure when the fuel is tranferred to the combustion chamber, that does give more bottom power and a better follow of the mixture at higher RPM. On the other hand most people loosing the silicone do not notice a difference or only "feel" a small difference.

Some believe the angle of the ramp does give a better flow. Yes, when the mixture is pushed in.... but the mixture is sucked in where such a ramp on the largest distance does not give much improvement.
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Old 04-12-2014, 07:22 PM
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Model airplane engines have been using reversion dams inside the crank for a long time.
When I see something that was intentionally built into an engine and filled with silicone,
it makes me wonder why?

Originally Posted by Roelof View Post
Yes, the silicone is lighter and does replace the heavier drilled out metal, but the drilled out metal is from the
center of the crankshaft what regarding simple physics has less influence on rotating mass.
Exactly, the rotating mass of the outer portions of the clutch and flywheel are more
important than the center of the crankshaft.

Originally Posted by Roelof View Post
Some believe the angle of the ramp does give a better flow. Yes, when the mixture is pushed in.... but the mixture
is sucked in where such a ramp on the largest distance does not give much improvement.
Atmospheric pressure always pushes in the mixture, but what I think most people don't realize is it takes
at least 2 or 3 revolutions for that fuel-air mixture to reach the cylinder.


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Attached Thumbnails Why is filling in the crank with silicone a good thing?-filled-crank.jpg   Why is filling in the crank with silicone a good thing?-bonitocrankshaft2.jpg  

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Old 04-12-2014, 08:55 PM
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You have to remember that these engines are spinning 30,000 + RPM so saving that little bit of weight makes a HUGE difference in how fast that engine can spin and how quickly that engine can get there... I don't know about the crank volume or anything like that. I'm just pulling this from the same reason they lighten full size racing engines.
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Old 04-13-2014, 03:06 AM
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The physics on rotating mass are different compared witht the other moving parts inside an engine. Running on a constant RPM the rotating mass has no influence but the piston and rod are still moving from 0 to 100km/h and back to 0 again per stroke. 1 gram from the piston will do more than 1 gram of the crankshaft. Calculating with the G-forces can make a 7 gram piston/rod combination up to more than 100kg.

Lightweight has become a selling item but meanwhile the use of a steel flywheel is more used....

True that with these high revs a resonance can be builded up, 36.000 rpm is stll 600 Hz. But does it need to be filtered or can it help to get more power? A resonance in the exhaust does help with the performance so why not at the intake?
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Old 04-13-2014, 11:46 AM
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Originally Posted by Roelof View Post
True that with these high revs a resonance can be builded up, 36.000 rpm is stll 600 Hz.
But does it need to be filtered or can it help to get more power? A resonance in the exhaust
does help with the performance so why not at the intake?
I estimate the speed of sound in the intake duct is around 350m/s. If that is true, the
intake and carb ducting may be too short, unless you include the length of the air filter
tube. It seems possible that air filter tube length may make a contribution at high rpm.
If the intake is similar to the exhaust, then the diameter of the air filter tube will play
an important role in how effective it is.

BTW, I could see the argument that stuffing may increase fuel economy, but it would still
have drawbacks for many engines. Engines that use mechanical labyrinth seals normally
have at least 3 groves, not just one, but it also possible these engines only need 2 grooves
to be effective.


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Old 04-13-2014, 12:38 PM
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Under operating conditions this area of the crank is a useless void of air and fuel trapped by the predominant flow into the crank case. Voids can generate turbulence and effect flow. Silicone eliminates the void and offers an alternative to more expensive and time-consuming machine work.

The resonance in the pipe provides a positive pressure wave that helps keep the new air-fuel charge in the combustion chamber. The window in the crank is designed to shut off the intake when pressure rises in the crank case on combustion and piston drop. Although this does cause a resonance in intake vacuum, excessive resonance in the intake would cause erratic flow when all you want is continuous vacuum to draw in air and fuel. Ideally, resonance would be eliminated on the intake side to optimize flow and fuel atomization.
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Old 04-14-2014, 06:14 PM
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Isn't the extra metal in the crank there as more of a counter balance? Harmonic vibrations in an engine can destroy it really quickly. Not sure how much one piston generates but in bigger engines it's detrimental to the life span of he engine.
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Old 04-15-2014, 01:48 AM
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Balance in an 1 cilinder engine is always a difficult thing.....
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Old 04-15-2014, 11:31 AM
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Originally Posted by Roelof View Post
Balance in an 1 cilinder engine is always a difficult thing.....
Boy howdy
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