Big End Oil Groove

[Chris KC]

The pressure at the big end will be much the same as any other engine - it is generated by the relative movement of the rod and pin and has nothing to do with supply pressure.

[Zetomagneto]

That's called hydrodynamic pressure and is dependent on very small running clearance.

[Bob Culver]

I have not worked on 3 brg engines and questions about them are not rhetorical. Did they retain the rod drillings? Were the original shells provided drilled? Did the original shells have trenches?
Incidentally modern shells for all cars may not have obvious relieved areas but the clearance adjacent the parting line is generous. A quite generous minimum side clearance is usually specified.
 
Edge apparently cribbed the Seven engine from the FN motor bike with the same arrangement bearings and lubrication. Did early FN versions have the conrod drillings?

If Woodrow suggests up to .060 side clearance that seems excessive. Any factor which caused the rod to move back and forth could cause an elusive knock
 
Local pressure distribution varies with the  film which is influenced by oil supply. The leading edge of the trench often shows slight distress, evidence of either close contact with the crank before film builds, or the effect of entry dirt on a thin film. Film pressure on the trailling side must be higher than in a flooded bearing without trench

Many grapple with the concept of very different de/accelerations at top and bottom of stroke. Most easily understood if you imagine a 3 inch stroke Seven with a conrod just say 1 5/8 inches long. For the crank turning 90 to 270 deg the piston would barely move, but from 270 through 360/0 to 90 again the piston would execute near the whole 3 inch stroke. The de/accel involved going over the top obviously hugely increased. With the 6 inch conrod of Seven more than half as much again as at bottom. Hence the high upward load on rods and crank. (And the vibration of a four because the down pistons do not counter the up)

[Ruairidh Dunford]

Three bearing shell rods are drilled, as is the shell and the surface is completely smooth with no troughs.

The side clearance is significant compared to most A7 white metalled rods.

[Bob Culver]

Thanks Ruairidh

The bearings being smaller, offset, and higher c.r and more power, heavier car, loads greater than earlier cars.

One day when really old and nothing else to do I will draw up a list of all my questions not answered!

Keep On

Bob C

[Dave Mann]

Jet feed is quite correct Tony, whilst I think the Ford T was dipper.

[Chris KC]

I hadn't really registered until recently how big the drillings are which run between the 1/4" 'splash feed' entry hole in the crank web and the 3/16" exit at the crank pin - 3/8" dia on a 2-brg up to 1/2" or thereabouts on a 3-brg. They are perhaps cavernous enough to look upon as a 'reservoir' which is topped up by the jets. I wonder if 'dribble feed' might be an even more accurate term...?

Interesting then to compare the Nippy crank where the (tiny) 'feed' hole was drilled through directly to the big end at (as others have pointed out) a precarious angle.

[Steve Bryant]

After considering all of the great replies regarding this issue, I've come to the conclusion that the best option for me in this case is to retain the chamfered edges where the cap joins the rod and drill the two holes but not groove the rod.

Your thoughts would be welcomed.

The engine will be dynamically balanced, and the oil pump  modified to deliver more volume just so you know the  status of the conditions  the rods will be working under.
Cheers,
Stephen

[Ian Williams]

"Interesting then to compare the Nippy crank where the (tiny) 'feed' hole was drilled through directly to the big end at (as others have pointed out) a precarious angle."

This is the only Production A7 crank to commonly have big end failures, I have encounter Nippy crank where the already tiny drillings were reduced even further by deposits from non detergent oil.

[Colin Reed]

Hi Steve.
One other consideration is the Type of Lubricant that you will be using .

Colin 
NZ