Originally Posted by
Lou Crane
Recycled,
The oil holes in the rod ends, as I understand it, are not to let the oil out.
Well in model engines, I disagree. The oil holes simply allow passageway and in most engines it is artificially pumped into them but here we have to rely upon naturally existing pressures and the greatest pressure or wicking will be from the crank web end.
With a single overhung crankshaft as compared to a double one the difference is that there is only one crank web and therefore only one oil slinger and pressurised crank web joint, so the oil should be flowing from a high pressure end to a low pressure one - or from the centrifugal slinger to the static backplate.
Providing an oil hole along a pressurized oil gallery will allow passage, let that pressure drop and bleed away.
Plus the fact that most oil holes in model two strokes that are drilled into the lower part of the big end eye are simply eclipsed more than half of there rotation by the very close fitting crankcase anyway.
Originally Posted by
Lou Crane
Recycled,
Oil delivery is important. I can't place the reference just now, but I recall a discussion of why rear rotary valve engines almost always have the air/fuel inlet path off-center from the shaft center. It may have been that the rod, sweeping across the chamber, obstructs the incoming charge less when the path is offset while the valve is open. In that case, if it were sweeping 'open' past the valve 'window' when open, it may even enhance delivery of fresh charge for the next ignition up top...
Rear rotary valved engines have the inlet path off centre as to better aid the oil supply to the crank pin (the most critical part of the engine then gets the coolest, freshest oil supply possible) and (if there are crank web cut outs present) to directly oil the main bearing.
To sum up, oil relief holes or slots have great purpose when the pressure is presented from both ends of a doubly supported crank pin but only provide an early exit when one end is not supported.
Cheers.