ET: Engine (Twin) Oil, is there a definitive answer?

[Peter Holmes]

Having just discovered a front cylinder big end bearing failure, I thought I would pose the question, is this failure entirely of my own doing.
The engine was rebuilt approx 60k miles ago by a very reputable engine builder, particularly well known for excellent flywheel assemblies.
To the best of my knowledge everything else was as it should be, ignition timing, valve timing, good carburation, clean oil tank, frequent oil
changes, paper filter, always primed before use after any lay off, etc. etc.

So now the oil, for many years I have been using Motul 300V Ester Core Oil, originally 20-50w, and latterly 20-60w, and some people throw
up their hands in horror, and in light of recent events they might be well justified in doing so.

But the main point of discussion always seems to be cam and cam follower wear, and this is the curious thing, after the same amount of miles (60k)
the cams and followers show no appreciable wear whatsoever, in fact nothing else in the engine shows any signs of wear attributable to bad lubrication,
the front piston and barrel has suffered, but that is only due to the debris that the big end threw out.

So my conclusion could be that Motul 300V 20-60w is a good oil to use, and reading the information on the Motul website certainly seems to suggest this,
but then we come to roller big ends, in my case an INA bearing, a motorcycling friend of mine, ex Isle of Man TT racer and engineer says it is the worst
oil that I could possibly use, as the Vincent engine was not designed to run on it, and I think the rationale for his thinking is that on startup, the oil is so
slippery that the big end rollers momentarily skid instead of roll as intended.

So now the unanswerable question, if I had been using Castrol classic 20-50 multigrade or Castrol Classic 40 grade would I have avoided my current
problem, and more importantly, once my engine back up and running, what oil should I be using?

 

[Vincent Brake]

Miller or Kendal does it for me.

 

[Peter Holmes]

Which grade etc.

 

[brian gains]

'.....is that on startup, the oil is so
slippery that the big end rollers momentarily skid instead of roll as intended.'

when I was a callow youth and thought Hardly Ablesons were the be all and end all in classic motorcycling this was often cited as the reason not to put synthetic oil particularly Motul, despite it's otherwise high regard, in the motor.

 

[Bazlerker]

I’m a luddite..Pennzoil 50w racing oil in the summer. I ascribe to the theory that synthetics are too slippery for Vincent big ends.

 

[Simon Dinsdale]

On a big end assembly on a twin the rear cylinder bearing is the first to get oil and the front cylinder is further away from the pump. So did both cylinder big ends fail or just the front? Was there a blockage in the middle of the oilways in the pin? If skidding of the rollers was the problem then would both front & rear big end bearings suffer the same problem?

To answer the question, I use Miller 20w50 mineral classic oil.

 

[Peter Holmes]

Only the front cylinder big end failed, apparently the rear is fine, the report that came back to me is there were no blockages, all oilways clear. I also considered that if the skidding theory was correct they both should have gone, but perhaps things are not that clear cut, I have been using this oil for years, and 60k miles, then suddenly the magnetic sump plug picked up loads of metal, without any other discernible symptoms, would not skidding rollers have presented problems long before they have.

 

[oexing]

Skidding rollers - OWT I say. When actually skidding, there would not be any wear ?? Poor oil supply to the front cylinder - no, you´d have a seized piston there long before the big end bearing would go ! Roller and ball bearings are very sensitive to grit in the oil. That is why filters or an effective sump are so important in preventing bearing failures. Defects start with very fine surface imperfections and mini craters and progress in the long run into total destruction. Also hardness specifications might be a factor when one conrod was allright and only the other failed.
I don´t know much about those crank pins and their oil hole orientations when designed for a twin. I position my holes at 90 degrees to the rear , past TDC so to say. Effect is you get a level of oil in the drilled crankpin that supplies oil evenly into both bearings, no matter which is closer to the oil entering the crankpin from one end. And yes, oil holes not in line with rollers but both sides where the cage ends float in the conrod.

Vic

Ducati 860 conrod 35 000 km:

Vic`s special:

 

[Vincent Brake]

Pictures Peter??

 

[greg brillus]

I've had a few cranks apart that either showed wear on the pin and the rod liner........a loss in the oil delivery for whatever reason is an obvious culprit........another issue I've seen is on some big ends, the rollers are quite a bit larger in diameter than the originals.......some say 3/16" in diameter or so........because of this, the liner that presses into the rod large eye bore is very thin.......I've seen these either crack very badly and/or a slither of metal cracks away at the edge........in one case this actually locked the crank up all together. Often the new big end assembly might come with the liners actually slightly oversize........obviously so you can hone the rod eye larger to true it up, but if this was not done, then the liner will be an overly tight fit in the rod.......this could cause too much stress on the liner and cause it to crack.......perhaps.......the sides of the rod/liner should have small grooves ground into them to allow the spent oil to drain away.......some rod to side plate clearance is too tight and very little side play.......obviously it will be difficult for any oil to pass if this is the case. I have one in the shop now with a total failure of the pin and rod liners.......the oil delivery holes in the pin were pointing straight up........my guess is this is wrong as the load is max at that point..........Lots of things to consider with the crank/rods.