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Big end journal alignment

Matty

Well Known and Active Forum Website User
VOC Member
The big end journal for a twin has two oil holes drilled along it into an annular groove which is filled with oil from the mainshaft. The oil then is fed to each big end.

Where should these holes be positioned (say by clock figures) with respect to the position of the crank at top dead centre when the crankshaft is assembled please - or does it really matter?
 

Matty

Well Known and Active Forum Website User
VOC Member
Thanks - the works drawing data seems to be much the same as information now kindly told to me by Alpha Bearings today, who said 3.00 oclock (90 degrees) from the vertical would be OK.

Presumably this would mean that the oil would be fairly evenly distributed from the two holes in the journal instead of being flung mostly out of a hole on the outside of the journal if it was at 12 oclock.

I'm not really sure however if it would make much difference - unless someone can explain why in terms that an electronic engineer might understand !!!
 

roy the mechanic

Well Known and Active Forum Website User
VOC Member
i have an alternative theory on this- with the oil appearing at 3 o'clock the work has already occured. a bit like bolting the stable door once the animal has escaped! i think,maybebetter, to have the oil appear before t.d.c, so that the oil is ready to help during the power stroke, at the time of maximum load. ford essex(3.0 litrev6) engines have the oil after t.d.c, they will not survive in a "hot "engine unless cross drilled to give oil as above. any one got some thing more? come on guys, this one is important.
 

Robert Watson

Well Known and Active Forum Website User
VOC Member
About 10 feet back from the clock, and in answer to the question you will ask next, --- around half past two
 

passenger0_0

Well Known and Active Forum Website User
VOC Member
With reference to Roy the Mechanics comments plain and rolling elemt bearings are two different beasts. Plain bearing motors use an oil film to form a dynamic wedge to prevent sliding metallic contact whilst a rolling element bearing has constant rolling mechanical contact and only requires lubrication for the cage and possibly for point of contact elastic deformation.
I have no knowledge of Ford Essex engines but know that it is difficult for oil to be forced against centrifugal force in a high speed engine. Vincents suck the oil into the crank through a low pressure created by the oil being flung from the big-end - this is why the oil feed quill is a loose fit in the crank since you dont want a good seal here It is likely that the positioning of the oil hole in a Vincet big-end pin is important not with consideration to the direction of load force but rather to prevent the oil from being flung out before it has been distibuted throughout the load bearing surfaces. Interesting discussion point for the forum and be interesting to hear other peoples point of view....... Cheers David
 
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Matty

Well Known and Active Forum Website User
VOC Member
Thanks again - on a twin there are of course 4 oil holes in the crank journal, two for each big end which are 180 degrees spaced. So in fact there is one oil hole at say 3 oclock and one at 9 oclock for each big end, if I position the journal as suggested. For a roller bearing I still can not see why it makes much difference where the holes are, as long as there is a good supply of oil to the bearing - after all a two stroke has no pressurised oil system and relies on the very small (50:1) amount of oil mixed with the petrol for roller bearing lubrication and these seem to be pretty reliable !!

On my Comet there is only one oil hole (I have old journals) but I do not know where it has been positioned because I did not assemble the flywheels, not having a lathe any more - I think at 12 o clock when I changed it in 2009 for one I bought from VOC spares.

When I last rebuilt the crank myself in 1957 (with Alpha Bearings) I did not know about any of this and just pressed it all back together. The bike has been used continuously since then on 20/50 oil with no problems -up to 2009 and even then it was not in a very bad state.

Thanks again.

Matty
 

Howard

Well Known and Active Forum Website User
VOC Member
Here's another take.

Perhaps the oil holes are not positioned at 3 o'clock (or 9 o'clock viewed from the south) because of oil flow, I can't see that it makes any difference when the oil reaches a rolling element bearing, as long as it reaches it in an adequate supply. I accept the theory about equal distribution (earlier) may have some credibility.

Perhaps they should be positioned where they cause the least weakening of the crankpin. Along the neutral axis at the point of maximum load? Please don't ask me to even guess where that might be on a twin, but I suspect 3 o'clock on a Comet would be about right.

I haven't given this a huge amount of thought, so be kind with your comments.

H
 
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