Big End Oil Groove

[Zetomagneto]

Austin did it the way they did because they knew it would work, time has proved them to be correct.
Oil not only lubricates, it cools as well, a good flow through of oil is essential, if the oil is held in the bearing it will overheat and the bearing will run.

The holes in the rod will allow a through flow every revolution of the crank resulting in a fresh charge of cooler oil.Side clearance is also important.Woodrow suggest up to 1/16” .

Re big end bearing loading, the biggest load is on the cap side on the exhaust stroke so I wouldn’t modify the cap in any way. Obviously use new bolts on reassembly

[Ruairidh Dunford]

I consider the oil holes essential in a standard A7 rod, they allow the old oil out and new oil to enter keeping the bearing cooler and better lubricated.

I know quite a few who don’t use them however and suffer no ill effects.

My experience with the oil trough is that very few commercial white metal shops use them, they all seem to prefer a plain finish these days - with oil holes drilled.

Clearly there is more than one way to successfully approach it.

[Colin Reed]

HI Zeto,
The side clearance on rods produces a more than adequate oil flow, if the holes are so important for flow they would still be used today.
Also can you explain how the cap takes all the load on the exhaust stroke.

Colin 
NZ

[Bob Culver]

Hi Colin

Centrifugal force at revs is very great and is simply calculated. At the top and bottom of stroke for an instant the con rod and piston assembly experiences the same force as if it was part of the crankpin. This is increased about 25% at the top of stroke and reduced at the bottom due to the short con rod swinging over the outside of a curve or the inside respectively. On the exhaust stroke there is little gas pressure to oppose the piston rod assembly and the upward force is a maximum. It is hard to believe but at just  reasonable revs on a long stroke engine on the full throttle firing stroke the net peak down load is about the same as the peak upload on the exh stroke.  At racing revs  the upward force on exh stroke considerably exceeds even the full explosion downforce of low revs.  On older cars particularly a lot of running is at lowish revs and the shaft of the rod concentrates the load on middle of the bearing. So the top half  fatigues first. It is the greater upward thrust of two middle cyls esp at closed throttle which bows and contributes to breakage of raced cranks. (it pays to slow from 60mph in nuetral!)

[Chris KC]

Steve, maybe I'm misreading it but that XL150 drawing is showing the groove on the split line between rod and cap, not in the 'TDC' position.

This is one of those areas where I suggest it is wiser to follow established practice than hang your hat on a theory - fact is there aren't a lot of splash feed engines around these days and much of the original thinking (assuming it was correct to start with) is lost in the mists of time.

Bob, in answer to your earlier point, I've never used a shell big end with a groove in it (centre mains have a fat groove around the circumference centrally). Though my engine is pressure fed through the crank I'm sure others use them with splash feed. And I agree the 1 or 2 thou running clearance at the edges of the bearing should be ample to allow exit of oil (at least, if the incoming oil is under pressure - though I imagine it departs equally quickly when splash-fed while rotating at 50 or 100 times per second).

[Zetomagneto]

Hi Colin, this discussion relates to splash fed big ends, modern engines are pressure lubricated, the oil pressure ensures the oil passes through the bearing so no need for a hole in the rod.
You will find some modern rods have a cutout in the sides.

Side clearance on modern engines is calculated by the manufacturer based on bearing area and bearing clearance , the side clearance is such that once the oil has passed through the bearing it can quickly escape.
Oil pressure is generated by bearing running clearance, not by having small side clearance.
You will know modern engines have very tight tolerances which is why they use such thin oil.

It is well known that big end bearings are highly loaded on the exhaust stroke, there is no resistance for the piston , the only thing stopping the piston hitting the head is the big end bearing.

[Hedd_Jones]

Just to give you some perspective from other forms of engine. Not only do steam engines close the exhaust before the piston reaches the end. They also open the admission. This cushions the piston at the end if its stroke. Remember most are double acting.

You can mess with the valve. Reducing the exhaust lap, or reducing the 'lead'. This gives a free er running engine. More revy. But doing so means your more likely to have little end trouble.

Mine has too much exhaust lap.

[Ian Williams]

Fascinating discussion. All I can add is that we have no evidence of oil holes causing big end failure in the 300 odd thousand sevens Austin built, and that I have run a splash fed crank at high rpm for many years with no holes! As R said seem there are two schools of thought and both appear to work, so unless someone wants to set a number of engines up on a test bed and run them to destruction it appears somewhat academic. However I think we would all agree that running a full width a groove is illustrated in the OP is probably not a good idea.

[Mark McKibbin]

Not wanting to be a pedant but the Austin engine is not splash fed, a Model T Ford is splash fed. I'm not sure what you should call it but it's not splash, Squirt? It would be interesting to know what the pressure is at the big end at speed.

[Tony Press]

Unfortunately 'splash fed' has been in use for many years and I don't remember coming to a better name in previous discussions.

Possibly 'Jet Fed' might be a better description, seeing as that is how it is !