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Author Topic: 535 bore break-in  (Read 4449 times)


  • Bulleteer
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Re: 535 bore break-in
« Reply #15 on: August 23, 2007, 05:53:06 pm »

I have not measured oil pressure at the quill myself. Others have: about 50-60PSI (no PRV) when cold. Dropping to verrry low when warmed up.
Problem is that the pressure in that location is not the pressure acting at the big end journal oil feed which is dependent on the centripetal acceleration the oil column in the crankarm bore experiences and is conveniently RPM dependent. Devilishly difficult to measure. But one thing for sure: If it's air instead of oil in the crankarm bore, the effect disappears and your bearing goes  :'( The centripetal force is absent in the center of the rotating mass which is where the crankshaft bore (quill bolt!) happens to be. So no more suction from the accelerating oil in the crankarm when it is filled with air. The darn thing will not purge itself nor prime itself. In comes the oil pump which in the Bullet pumps every 8 revolutions (If I remember correctly) resulting in spikes of pressure. If you manage to get a pressure reading, it is because your measurement apparatus is properly damped to indicate something close to the pressure averaged over time.
The oil in the big end bearing constantly flows out into the crankcase and has to be replenished 1:1 or the bearing crashes. The bearing functions as a flow resistor so for any directional flow to occur, there must be a pressure gradient, however small because of: Flow x Resistance = Pressure (assumed an ideal unchanging liquid, same viscosity oil is good enough) Otherwise the oil gets squeezed out in both directions and the bearing crashes.
Apparently, when the Bullet engine is warmed up, measurable oil pressure in the crankshaft goes down rather dramatically because the resistance of the bearing goes down when warm. However, pressure will build up the moment flow through the big end slows down for whatever reason. Here is where a failed PRV comes in: It will prevent the vigorous purging which would inevitably occur in the normal condition. One could say that the original engineers walked a rather fine line and should have provided some excess oil pumping action to keep oil pressure up at all times and not balance oil needs of the bearing so closely with the pump capacity. But: The genius, the genius! The oil feed pump runs with minimal load in a warmed up engine and the correct oil! No parasitic power losses and decreased wear of the worm drive etc.!
I can already hear the devil's advocate: what about the inevitable wear of the big end bearing causing oil starvation by virtue of lowering the flow resistance? A new Bullet engine has the oil pump to nicely match the big end needs and after a few K miles it's no longer good enough? Programmed failure that would be! Ha!
The answer is that floating bushing bearings do not wear. No they don't. No No No They Don't. They operate without wear or fail within a short time. The genius, the genius!

Check this if you want some in depth plain bearing stuff:


So, plain bearings are critically pressure dependent to insure oil film thickness via directional flow at all times - not necessarily from the pressure in the supply system but rather on the effective pressure at the journal feed.
And roller bearings? They just need enough oil splashed at them. They also have little flow resistance (forget pressure) so HiVol pumps are probably ok but I'm not sure that they are needed when I'm thinking of my two strokes running fine on 50:1 premix.

Just some musings  :)