Oil Pressure

Bernhard said:

IMO pressure is also dependent to some extent on how worn your big end shells are, if you see what I mean.

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The pressure within the high pressure zone is dependant on the flow into and the clearances that flow is forced out of, so as your engine wears the clearances increase and the pressure drops as the resistance to outward flow drops.

I agree that that the pressure is equal everywhere in a static system - like the air in a tire. But in a dynamic system (fluid/air moving under pressure) it has to pass through passages, turn corners, bleed into/through bearings/fittings/etc. Friction is involved as is every change of direction as far as flow/pressure. I swear we did some sort of experiment back in Physics class/hydraulics about this but I don't recall any specifics

However, I readily admit I have never checked an engine for this. Most engines do not have any way to easily attach a pressure gauge to anyplace other than the main oil gallery anyway (except the Norton which has no way to connect it TO the main gallery!) So it may be as you say, the pressure is the same throughout the system.

I've just been able to spend some more time on the bike, and I was wrong about the conical seal. I have a spare seal for comparison, and there are witness marks on the original from the timing cover, so it looks like that's all OK.
I did discover that the oil pump retaining nuts needed tightening a little, although no obvious signs of leakage on the gasket. I've now replaced the gasket and torqued the nuts correctly, with loctite. Could that have caused oil pressure loss?
I checked the oil pressure release valve. That is set correctly, as per Mick Hemming's dvd, with 3 shims. The piston seems loose in the body, but maybe that's how it should be. Could there be leakage there?
I can't think of anything else, so maybe I'll just put it all back together with new seals, and see if there is a magical improvement.

MexicoMike said:

This thread got me to thinking...(which I have often found to be dangerous, confusing and or non-productive).


However...

Engine oil pressure senders/sensors are located by engine manufacturers in the main oil gallery at or immediately following the oil pump, where pressure is the highest in the system. BUT, the valve train, basically the END of the oil plumbing where pressure is lowest, is the source point for the typical Norton oil pressure gauge setup.

Oil pressure senders located anywhere other than the main oil gallery are not reading engine oil pressure as it is commonly understood/calibrated. The standard "10 PSI per 1000 RPM" applies at the oil pump/in the main gallery, not at the valve train.

What I'm suggesting is that readings on a pressure gauge plumbed into the Norton head/valve train is not 'accurate' oil pressure and it may cause consternation over perceived low oil pressure where no such problem exists.

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Perfect. This is exactly the thought I attempted to provoke.

I bought my bike (almost) running, 12k miles, looked well kept.
Head gasket leaked.
Compression test was ok.
Ran good.
I was a bit tight for funds, and didn’t want to open the bottom end needlessly.
First thing I bought was the OldBritts oil pressure gage (with the braided top end oil line kit) Oil pressure seemed in line with what I read here.
After reboring the cyl, new pistons, I enjoyed 20,000 miles of good service.
The hot oil pressure was consistently 27 psi at 2500-3500 rpm. And just 2-3 lbs. at hot idle. Cold was 40 psi at 1500-2000.
The trend was stable for a loooong time. Fell off a bit near the end (I decided to tear into the bottom end to find where the metal chunks came from, main bearing failure)
I had the crank ground, fitted new rod bearings, new A/N oil pump, Jim Comstock freshened up the head, etc.
Still 27 psi at cruise....

So, OPRV operates at 40, and while the oil pressure is stable, it is lower than many posters report here.

Gage calibration could be an issue.

Plumbing design is what also could be at play here.

Only way to tell is to test the gage, and re-plumb to the oil galley adjacent the oil pump.

JMWO

Equally interesting is that most of the responders to this thread have fitted oil pressure gauges to their bikes, and therefore know a) that their bike has oil pressure, b) the patterns of pressure that their bike follows at different RPM's and engine temperatures.

The most important thing to me is that the bike has some oil pressure at all engine speed and temperatures. The exact values are highly debated and likely not as important.

To me, I could not imagine operating a pressure lubricated engine (especially one that could strand you 100's of miles from home with an expensive repair bill) without some sort of oil pressure monitoring system, be it a light or a gauge.

I personally like the fact that my gauge is plumbed to the furthest reach of the oil system - to me, it shows that oil is getting there, and under pressure.

I don't think this is a game of who has the most or who's follows the established pattern of values. This is a game of watching for anomalies and then taking action on them before serious engine damage occurs.

Oil pressure is a proxy, as long as you have pressure then it stands a good chance the oil film thickness in the big ends is not being breeched. The pressure within the oil film is in MPa.

The megapascal is a x1000000 multiple of the pascal unit which is the SI unit for pressure. 1 megapascal equals 1,000,000 pascals. Primarily used for higher range pressure measurement due to its larger value (e.g. 1 MPa = 10 bar)

Typical oil film pressures in a big end are in the order of 10 to 100 MPa which is 100 bar to 1000 bar.

You can have good oil pressure but an oil useless at keeping oil film thickness so oil pressure is only part of the equation.

Distribution of hydrodynamic oil film pressure in big end bearing of V type connecting

The reason I posted re the possible pressure difference was because people DO post about Norton pressures and worry about whether the reading they are seeing is sufficient. Since it is taken at the valve gear, I don't think it is an accurate indicator of engine oil pressure. Therefore, USING it as something to worry about in reference to a standard OP number - say 10 PSI per 1000 RPM might be an exercise in needless worrying.

OTOH, I worked on some marine engines that when at operating temps, the idle OP was under 5 PSI which was perfectly normal! From fast idle and above, pressure rose appropriately within the 10psi/1000 RPM guideline that was included in the service manual. I once worked on an E-type jag that showed NO oil pressure it idle. Owner said it have been that way for years yet showed no signs of distress.

MexicoMike said:

The reason I posted re the possible pressure difference was because people DO post about Norton pressures and worry about whether the reading they are seeing is sufficient. Since it is taken at the valve gear, I don't think it is an accurate indicator of engine oil pressure. Therefore, USING it as something to worry about in reference to a standard OP number - say 10 PSI per 1000 RPM might be an exercise in needless worrying.

OTOH, I worked on some marine engines that when at operating temps, the idle OP was under 5 PSI which was perfectly normal! From fast idle and above, pressure rose appropriately within the 10psi/1000 RPM guideline that was included in the service manual. I once worked on an E-type jag that showed NO oil pressure it idle. Owner said it have been that way for years yet showed no signs of distress.

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The E type has a much bigger oil pump than a Norton, and like everything else that obeys Newton's law, the faster its spun the harder it works. (Subject to the oil viscosity, of couse)

None of the OP gauges commonly available for the Commando would pass muster on the international space station, but all function well enough to signal trouble.

The best (relative) OP I have experienced was with a new pump, a one piece Falicon crank, Carrillo rods, a SRM OPRV and .0009" shell to journal clearance.

Best.

MexicoMike said:

I agree that that the pressure is equal everywhere in a static system - like the air in a tire. But in a dynamic system (fluid/air moving under pressure) it has to pass through passages, turn corners, bleed into/through bearings/fittings/etc. Friction is involved as is every change of direction as far as flow/pressure. I swear we did some sort of experiment back in Physics class/hydraulics about this but I don't recall any specifics

However, I readily admit I have never checked an engine for this. Most engines do not have any way to easily attach a pressure gauge to anyplace other than the main oil gallery anyway (except the Norton which has no way to connect it TO the main gallery!) So it may be as you say, the pressure is the same throughout the system.

Click to expand...

On the subject of pressure drop in a pipe over distance, I performed some tests a few years back showing how pipe length affects both flow rate from the pipe and pressure drop along the pipe. Thought this information might help clarify some points discussed in this thread regarding the location of an oil pressure gauge in a flowing system. None of the parameters explored here were intended to mimic or address anything related to a Norton motorcycle engine. Nevertheless thought the results had merit as they provide fundamental understanding of pressure and flow in a pipe.

The test rig employed consisted of the following components.

• A metal tank fit with a quick connect inlet that can be pressurized with regulated air pressure
• A valve fit at the tank outlet
• A 0.040” (1 mm) diameter discharge orifice
• Various lengths of 3/16” OD copper line that was interposed between the tank exit and the discharge orifice
• Pressure gauges fit at the air regulator, tank outlet and near the terminal end of the copper line.

Tests were conducted by filling the tank with Amsoil Formula 4-Stroke 0W-40 motor oil and fitting an air line with pressure regulator and gauge to the top of the tank to achieve the desired pressure in the system. The valve was then fully opened which exposed an interior valve area larger than the interior area of the copper tube, thus the valve was not a flow restriction in the system. With the system under full flow, oil pressure was recorded at the tank exit and just ahead of the discharge orifice. Oil flow rate was measured by collecting oil at timed intervals in graduated beakers at the discharge orifice.

In all cases when in the static mode, i.e., with the exit to the system capped off, the air pressure recorded at the regulator, the oil pressure recorded at the tank exit and near the end of the copper tube just upstream of the discharge orifice was identical regardless of the length of copper tubing interposed between the tank and discharge orifice. In contrast, in the dynamic mode, i.e., when oil was being discharged from the system, the moment flow commenced a significant drop in oil pressure was recorded on the gauge near the discharge orifice.

Tests were performed at pressures ranging from ~ 5 to 20 psi, applied to the system via the air pressure regulator attached at the tank inlet. Pressure and flow rate results in the dynamic mode were recorded and are presented graphically below. The results show clearly how pressure and flow rate decrease with increasing pipe length. No revelations here since anybody that has ever used long and short garden hoses has likely made similar qualitative observations. In conclusion, circling back to the subject of Nortons, as others have indicated in this thread, there would be a considerable difference in oil pressure measured directly at the pump output vs at the end of the small plastic line feeding the cylinder head.





There is one more plot related to this endeavor that is not presented here, and that is essentially the same experiment reported here, i.e., oil expelled via a single 0.040” dia discharge orifice, was repeated but with an udder having 3 x 0.040” discharge orifices. As one would expect, the flow rates increased approximately 3X those shown here.

One other interesting aside to this undertaking was realizing that if you have an actual 10 psi pressure acting on motor oil having a viscosity of ~ 20 CSt (the viscosity of 50 weight motor oil at 100C) and being discharged via a 0.040” dia orifice, it will shoot a vertical stream ~ 10 ft in the air where air drag tends to fragment the stream into a spray. That’s an impressive jet of oil coming out of a tiny little 1 mm orifice that is under not very much pressure!

snowdr said:

I've just been able to spend some more time on the bike, and I was wrong about the conical seal. I have a spare seal for comparison, and there are witness marks on the original from the timing cover, so it looks like that's all OK.
I did discover that the oil pump retaining nuts needed tightening a little, although no obvious signs of leakage on the gasket. I've now replaced the gasket and torqued the nuts correctly, with loctite. Could that have caused oil pressure loss?
I checked the oil pressure release valve. That is set correctly, as per Mick Hemming's dvd, with 3 shims. The piston seems loose in the body, but maybe that's how it should be. Could there be leakage there?
I can't think of anything else, so maybe I'll just put it all back together with new seals, and see if there is a magical improvement.

Click to expand...

You're travelling along a route many of us have travelled, and with good reason. No need to tear down a sound engine. But ignorance isn't bliss, and the evidence you have collected by now suggests that your big ends are indeed wiped out. There's one thing I'd try first if it was me, a different oil pump, but don't ignore the evidence.

SR

You either know or you don't know. You are riding a bike, not a car, engine goes bang you may well find yourself in real trouble real fast. Working on a Commando is a bother for some things but bite the bullet and at least pull the top end so you can wiggle the
rods. Pull it apart fully isnt that expensive just time and effort. I'd pull it apart.