Leaky crank

[dobba99]

Just stripped a client's 750 crank, unknown mileage but on -0.010 big end journals. Photos are of the timing side crank flange faces, note the leakage path (that is the grey area between the two outermost bolt holes) that the oil (and whatever was being carried with the oil) has scoured into the faces.


The drive side was just as bad. scraped and blued the drive side to get rid of the high spots and the get the faces mating tightly together.
How many of our cranks will show this kind of defect if stripped? Along with the oil squirter holes in the con rods constantly teeming off the high pressure oil in the big ends its a wonder they work as well as they do.

 

[kommando]

BSA B25's with the bolt on flywheels also had oil leaks at the joint but at least by blanking off the sludge trap hole it could be fixed.

CRANKSHAFT ASSEMBLY, B25 WITH MODIFIED OIL FLOW
NOS CRANKSHAFT ASSEMBLY
USE STD SHELL BEARINGS
FITS IN ALL B25s & TR25Ws FROM 1967-69


THIS NOS ASSEMBLY HAS HAD THE OIL FLOW ROUTE MODIFIED TO ENSURE HIGH PRESSURE OIL FEED TO THE BIG END SHELL BEARING
THE FLOW ROUTE TO THE SLUDGE PLUG HAS BEEN BLOCKED OFF AS SIGNIFICANT OIL PRESSURE WAS LOST THERE.

 

[Stephen Hill]

Speculation on my part, but the minor amount of leakage probably makes no practical difference to the lubrication system.

 

[acotrel]

Speculation on my part, but the minor amount of leakage probably makes no practical difference to the lubrication system.

What oil pressure should a Commando engine have when running, and what is required to float the big-end bearings? - I suggest you are correct. The leakage past the gears in the oil pump might be a bigger problem. Triumph 650 motors have plunger oil pumps. Gear pumps were probably designed for roller-bearing big ends. 'Everything which glitters is not gold'. I suggest that in the Norton factory, twin cylinder road bikes were probably a secondary consideration, however I really liked their race orientation.

 

[dobba99]

Speculation on my part, but the minor amount of leakage probably makes no practical difference to the lubrication system.

On the contrary, I have just spent a few minutes using the free calculating tools available online and say we have 200grams of oil in the crank that is revolving at 4k rpm. At the 70mm furthest point from the crank center in the flywheel void our 200 grams of oil is now exterting 2403 Newtons = 540lbf This 'minor amount of leakage' is now squirting out both sides of the flywheel, nicely robbing the big ends of its supply of oil, and at the same time scouring away at the cheek and flywheel faces, making the situation worse.

 

[kommando]

Gear pumps were probably designed for roller-bearing big ends.

No, gear pumps are positive displacement pumps as are plunger pumps but have the advantage of constant flow and not pulsed like the plunger. Positive displacement pumps work best when pushing against a resistance so as the resistance increases the pressure will increase. So the exact opposite of your wild guess. A centrifugal pump such as in the bottom of a washing machine would be the worst pump to use in an engine, as they meet resistance on the outlet the flow decreases and will drop to zero, their advantage over positive displacement pumps is on the input side where the centrifugal pump performs well and is self priming, this is the weak point of the gear pump which does not self prime and why its defeated by air in the feen pipe.

 

[dobba99]

No, gear pumps are positive displacement pumps as are plunger pumps but have the advantage of constant flow and not pulsed like the plunger. Positive displacement pumps work best when pushing against a resistance so as the resistance increases the pressure will increase. So the exact opposite of your wild guess. A centrifugal pump such as in the bottom of a washing machine would be the worst pump to use in an engine, as they meet resistance on the outlet the flow decreases and will drop to zero, their advantage over positive displacement pumps is on the input side where the centrifugal pump performs well and is self priming, this is the weak point of the gear pump which does not self prime and why its defeated by air in the feen pipe.

Not true either, The vast majority of centrifugal pumps (when pumping fluid) will need to be primed somehow. Either by gravity or by a priming line, Have a look at a fire engines set up, If the fire service has to draw from open water (instead of a hydrant or the onboard tank) they have a vacuum priming pump (a rotary vane pump i seem to remember) that draws the water up to prime the main pump. In my first job (as a labourer at the local boat yard), the first task of the day was to pump out the dry dock, close both inlet and outlet valves to the centrifugal drain pump, the fill said pump with water using the vent valve connection and a hose pipe. close vent, start pump then open the inlet and outlet valves and proceed to drain the dry dock.
You are correct stating a centrifugal pumps flow will decrease with outlet flow restriction. Given a big enough pump and the motor driving it, if the outlet flow gets restricted down to nothing, the pumps temperature will increase very rapidly.
In our engines the pump has an easy time coping with the gravity fed delivery from the oil tank. Its when a restriction is put in the feed line that other problems start. Yes it is a positive displacement pump, but it is quite small, often not in the best of condition and struggles to pull open a non return valve that someone has fitted in the feed line.
Your washing machine pump sits at the bottom of the washer and is automatically primed by gravity. When the washer is full of water the pump is already primed for action.

 

[kommando]

I use a centrifugal pump to transfer heating fuel with a 5m increase in height, 1m before the pump and 4m after and it works well. The first time I ran it I injected some oil into the rotor cavity to lube it until the fuel came through which was a few seconds.

 

[dobba99]

I use a centrifugal pump to transfer heating fuel with a 5m increase in height, 1m before the pump and 4m after and it works well. The first time I ran it I injected some oil into the rotor cavity to lube it until the fuel came through which was a few seconds.

Are you sure its a centrifugal pump? Easily spotted with its snail shell like casing. To the untrained eye a rotary vane pump looks similar but is a totally different beastie. These are often sold as fuel transfer pumps.

 

[kommando]

No definitely a cheap Chinese centrifugal pump, one failed and I was able to look at the internals.

 

[mdt-son]

On the contrary, I have just spent a few minutes using the free calculating tools available online and say we have 200grams of oil in the crank that is revolving at 4k rpm. At the 70mm furthest point from the crank center in the flywheel void our 200 grams of oil is now exterting 2403 Newtons = 540lbf This 'minor amount of leakage' is now squirting out both sides of the flywheel, nicely robbing the big ends of its supply of oil, and at the same time scouring away at the cheek and flywheel faces, making the situation worse.

It's an impressive figure (I checked the figure and got 2456 N). If we knew the dispersed area, we could estimate the pressure acting radially at the joint. Let's assume the oil wets a projected area of 40x100 mm. Then, the pressure at 4K rpm will be a whopping 61.4 bar .
This example shows that the centrifugal action far outweighs the pump's outlet pressure (about 5-6 bar). Yes, there will be a leak between cheek flanges and the flywheel, and more so due to the crank's deformation at revs, which tries to open a crevice at the furthest point from the crank center.
This is shown at your crankshaft as "clean area", so it fits with theory. I think you need to buy one of AN's billet crankshafts to mitigate this flaw!

- Knut

 

[maaseyracer]

i wonder if something like 518 loctite worth it here to maintain oil pressure. It is going to leak regardless of how you set it up if it is not sealed. Hylomar would probably ok, but if your petcock was ever left on and fuel drained into your sump it would be toast.