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Author Topic: external oil filter  (Read 4768 times)


  • Guest
Re: external oil filter
« Reply #30 on: June 21, 2008, 02:35:23 am »

. Even if you do that, during the first couple of minutes the crankpin and the conrod with the soft white metal bushing between the two will be crashing onto each other. Lots and lots of wear.
Just a quick question to underscore my own ignorance.  This idea that when a bike has been setting, maybe overnight or even longer, or when the oil is changed, then all the oil drains from the inner bearing surfaces - is that true?  I would have thought there would be a residual film of oil remaining there until new oil takes over.


  • Grand Gearhead
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Re: external oil filter
« Reply #31 on: June 21, 2008, 10:29:23 am »
a copper/tin/zink variety as that is the normal choice [and there is a lot of motorcycle history in this] for a big end full floating bearing.  They almost always funtion in a hydrostatic mode.     

You are right oldsalt.  I've never seen a RE connecting rod bush, but somebody who has says it looks just like bronze.  And it has rounded edges almost like a donut.

And copper/tin/zink are the alloy components for both brass and bronze.  Probably the best properties of both.  And a good hydrostatic match for a mild steel crankpin.  Also can survive minor oil contamination.

The Bullet oil pump is a very low pressure system which also supports the above.  The common automotive shell type bearings (babbit/white metal) requires a high pressure oil system.
Diagonally parked in a parallel universe.

Fortiter Et Fideliter


  • Bulleteer
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Re: external oil filter
« Reply #32 on: June 21, 2008, 12:27:45 pm »
  Realizing  there is a definate difference between sliding surfaces that operate in a hydroststic and hydrodynamic mode. ...
... We know for sure that the bearing operates somewhere between 'mixed' and 'full film'.   

The Bullet iron engine big end bearing is a fluid bearing in form of a floating bushing arrangement and oil is supplied via an orifice in the crankpin. This makes it a fluid bearing of the hydrostatic type. The fact that the oil pressure of in the Bullet is low does not change the mode oil is supplied to the bearing: Directional flow along a pump generated pressure gradient. Bear in mind that the rotating crankshaft is to be considered part of the pump due to the accelerating oil column in the flywheel bore.
The existence of an oil feed pump and the rotating crank and, for completeness, the subatmospheric pressure in the crankcase must result in a pressure difference between the oil tank and crankcase. If your Bullet is not equipped with a working overpressure release valve, all circulating oil flows along this gradient  through the big end bearing.
The placement of the big end bearing in line with the oil flow defines the bearing as a hydrostatic fluid bearing. The steepness of he driving pressure gradient is not relevant. The only thing what makes a bearing a hydrostatic fluid bearing is that the pressure gradient (also known as "hydrostatic" gradient, duh) is utilized for oil supply.
Hydrodynamic fluid bearings do not have a directional oil supply. Fluid layer thickness is maintained by the bearing movement itself. For obvious reasons, you won't find an open hydrodynamic fluid bearing in a dry sump engine.

So now we got this squared away:
Classification of a fluid bearing as a hydrostatic or hydrodynamic fluid bearing is strictly based on the mode of fluid supply.
Hydrostatic fluid bearings utilize a hydrostatic gradient for oil supply and because it is rather difficult to create a physical hydrostatic gradient in an engine application, pumps are generally used to create the functional equivalent.
Hydrodynamic fluid bearings do not utilize a hydrostatic gradient or its functional equivalent but rely on hydrodynamics alone for replenishment of the fluid layer.

Unfortunately, the plot thickens a bit when we consider how the fluid layer separating the surfaces is maintained during normal operation of a fluid bearing. At that level, inside the operating bearing, fluid bearings of either type operate on hydrodynamic principles. Yes, that's right. Hydrostatic and hydrodynamic bearings both operate hydrodynamically under normal conditions.
There is a hydrostatic effect which is detrimental when it occurs and that is when channels of fluid flow occur and disrupt the hydrodynamic condition. Fluid bearings never operate in "hydrostatic mode". They operate in hydrodynamic mode during normal operation, in mixed and/or boundary mode during startup and when operated beyond design specification. Finally they may operate with full contact under dry conditions which is bad and for which additives are contained in engine oil to save things if this occurs transiently.

I have not yet had to look at my big end bearing so I do not know what it looks like. All I can say is that the Bullet Workshop Manual page 30 says that it is a white metal floating bushing.

I wish everybody an uninterrupted oil flow supported by a stable hydrostatic equivalent pressure gradient and a fully hydrodynamic condition in their big end at all times.


« Last Edit: June 21, 2008, 12:32:46 pm by Peter »


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Re: external oil filter
« Reply #33 on: June 21, 2008, 09:13:23 pm »

Thank you sincerely for going to the trouble to continue my education.  Your concise account of what goes on at the big end was enlightening and welcome.  It always makes mechanical things more interesting when the phenomenon of their funtion is understood.  In any case, the fact that the lower end of a RE is basicly unchanged since the 50s makes me like it even more. 
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