Big end journal alignment

[Tom Gaynor]

In both theory and practice, the pairs of holes ought to be different sizes. The nearest pair to the pump should be smaller. If the two pairs of holes are the same size, more oil will flow through the first pair because there is less resistance to flow - it doesn't have so far to go. I know this to be true both by calculation and observation (drilling oil wells involves a lot of hydraulic calculation, and distributing oil evenly to the rocker bearings on a Rudge radial involves a lot of experiment). I have been told that this is often a cause of the remote big-end being the one to fail but have no personal experience of this. Nor do I know how one might calculate the difference, although experiment with an oil can might give an idea.

Apropos the 3 o-clock positioning, I can understand this on a plain bearing crank, because around TDC and BDC the shell is bearing hard on the crankpin, and no oil could escape. I think it matters far less on a roller big-end, because all that is necessary is that there IS oil in there. There's no need to maintain an oil film the way there is with a plain big-end.

(FWIW, I was amazed to discover that 60 bhp (rear wheel) Manx short-strokes with plain-bearing cranks use the standard Norton oil pump, which must generate the same immense pressure as a Vincent pump, ho, ho. The only difference(which can be seen below the bevel cover) is the fitting of a restrictor to reduce the oil fed to the cam box in favour of the crank.)

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.

 

[Howard]

Tom, I take your point on the hole sizes, but I think there'll be so little difference in head loss between them, the difference in hole size would be minimal. As long as the worst case is adequate, the other hole will provide slightly too much oil (if you can have too much oil anywhere on a Vin, except running down the outside ).

H

 

[clevtrev]

Hi Trev,
Viewed from where?
Cheers, John

Looking at the timing side, with the pin at top.

 

[clevtrev]

Ref, the four holes in the pin. This is not a Vincent pin.
Roy , the oil is introduced just after the load, ready for the next pass, and you have to remember that the pump turns 15 times between each pulse, so the oil flow is only then, so that`s fouteen turns of the pin with no oil flow, perhaps.
Anyway have a look at a pin that`s been well used , and note everything, position of hole relative to the crank, and where the wear is on the pin. Then start looking for answers, don`t hold your breath.
Tom, I`ve found most clapped out pins with the supply side gone first. Ask Simon what he has found.

 

[ksbundy]

I would tend to think along the same lines as Howard, in that I would not want the holes in the heavily loaded section. On my H model triumph oil is squirted onto the flywheels at the rate of about one pump every 7 miles. indicating that not a lot of oil is needed to lubricate roller bigends.
KS

 

[roy the mechanic]

i'm reading from instruction sheet no 5 a dated september 1964, so i figure this must be from harpers, it starts by saying fit the big-end pin into the drive side flywheel "press in the new pin with the oil holes positioned between 2 degrees and 3 mins o'clock, and pullup the crankpin nut very tightly". no mention of which side to look from! have i missed something or are these instructions a load of "wossnames?

 

[Howard]

Yes Roy, a load of ballcocks.

Were Harpers based in Korea. that looks simillar to my TV instruction leaflet?

I do like the obvious engineering term "very tightly". "Very tightly" seemed to be much tighter when I was younger and stronger.

H

 

[Matty]

Thanks all - I will try to draw some sort of conclusion.

First the Comet big-end journal :-

Big end oil supply seems to be very simple in this case because there is only one "gallery" drilled through the crankpin with one drilling in the bearing journal to it. The oil therefore has only one way to go and I should think the best location for it may be at 12 oclock to gain maximum advantage from the "suction" created by centrifugal force.

For a twin, the situation is more complicated and both the crankpins we have (wrecked one and new one ) have two galleries drilled through them at 180 degree spacing. There is then a drilling to each gallery from each big end journal position - hence four holes in total.
So with the same pump as the Comet, the oil can go to four different places, implying that there is enough flow to not lose too much oil through the two oil feeds for the big end nearest the feed through the mainshaft before supplying the other big end !!

I still think that in general roller bearing lubrication will be OK provided that there is a good supply of oil and feel that the angular position of the oil holes for the journal should not be critical - after all many other machines just rely on drip feed or splash for a roller big end.

The Rapide that I am working on totally destroyed the rollers for the front cylinder and in fact none were found, just grey metallic dust in the engine, though the con-rod was intact but with the outer big end bearing ring pretty chewed up. The other big end looked fine !!
With all this rubbish in the engine it has been quite a job to clean it all up and the crankcases have had to be put into a de-greaser and then ( don't tell wives) into the dishwasher.

However there was some blockage in the mainshaft/flywheel oilway which could have restricted the supply to the big ends - though this could have been effect rather than cause.

 

[Matty]

Sorry didn't quite finish the tale, but the crankshaft has in fact been re-assembled with a new crankpin and set of bearings. The oil holes have been set at 3 oclock and 9 oclock with respect to TDC and we hope all will now be OK.

Thanks again for the advice

Alan M (Matty)

 

[clevtrev]

I still think that in general roller bearing lubrication will be OK provided that there is a good supply of oil and feel that the angular position of the oil holes for the journal should not be critical - after all many other machines just rely on drip feed or splash for a roller big end.

The Rapide that I am working on totally destroyed the rollers for the front cylinder and in fact none were found, just grey metallic dust in the engine, though the con-rod was intact but with the outer big end bearing ring pretty chewed up. The other big end looked fine !!
With all this rubbish in the engine it has been quite a job to clean it all up and the crankcases have had to be put into a de-greaser and then ( don't tell wives) into the dishwasher.

However there was some blockage in the mainshaft/flywheel oilway which could have restricted the supply to the big ends - though this could have been effect rather than cause.

Just remember all that blockage material, had only seconds before, been in the filter.
Also a Comet would get lots more oil than a twin, being as it generally runs faster, RPM wise.