Smiths etc, mechanical oil pressure gauge

[daverclasper]

Hi. Wondering how accurate these generally are, and if not, how much maybe are they likely to/could be out?.

Also, my plastic feed tube has sections of no oil. Does this affect the reading?. 

Any help great, thanks.

Dave

Edited November 24, 2018 by daverclasper

[Guest]

Hello Dave,

In fine fettle Smiths gauges (if that is what you have) are generally spot-on provided it has been treated properly.

I would assume that the oil gaps are being replaced by air, which means it is either trapped or getting in somewhere which may also suggest oil is getting out, quite often at the gauge end. The pipe-to-gauge should have a small rubber oil seal - some are leather which prevents seepage.

What does the plastic pipe look like when the engine is running - is it full of oil is the air still there. If the latter the reading will be incorrect IMHO.

Regards.

Richard.

[Clive]

They are not a precision gauge, but generally pretty good. You can check by trying others and swapping with other people. But they are most useful for observing any changes in pressure behaviour...

And remember preesure is not everything, good flow is more important (ie using a thick oil to maintain pressure in a worn engine is not the answer, an engine rebuild is...)

The pipes get air bubbles, not unusual as the bore is small and they can't move. I guess you could try bleeding the pipe, but it won't affect the reading. The pressure will compress the bubbles to the oil pressure, so no issues. 

[Anglefire]

Mine is full of air (Well, not full, but pockets of air as Dave says)

I can't see the air making much difference TBH - though happy to be proven wrong, if it means its reading low 😉

[Guest]

My rationale is that you can compress a gas (air) to a certain extent quite easily , whereas a fluid (oil) is not so easy - certainly not under the pressure from an engine.

The difference between the two mediums may be minimal, but there will be a difference, depending on the amount of air in the tube. We need a science boffin...……… Clive  !!!!

When I placed an oil pressure gauge in the Vitesse, I went for the s/s pipe option and as such I'm not sighted if it has air within or not - I may well be in the same situation as Dave and Mark.

One point worth mentioning again, you need to ensure you have the pipe-to-gauge seal in situ.

Regards.

Richard. 

[johny]

I reckon the air pockets will affect changes in pressure to cause slight delays but for practically stable pressure as we generally have in our engines it will make no difference. The air compresses as the oil pressure rises and reaches a point where it just transmits that pressure onto the gauge. The same applies to the tubing which might expand slightly but also reaches a stable state.....

[daverclasper]

Hi. and thanks.

It's a jauger  gauge. Have pressure of 25 idle/near 60 at 2000 revs, even on very hot days, so that seems good.

It does rattle on start up, even later in the day a bit sometimes (have a Mann filter, tilted downwards).

As it has apparently good pressure, then maybe, ignore, as some very knowledgeable/experienced folk have suggested, or could one or two  bearings be suffering.

No facilities to remove engine and work on it, So could maybe? remove Sump (Vitesse) in situ and replace shells, though prefer not to if it's likely not a issue.

thanks, Dave       

[Nick Jones]

Air may act to slightly damp/delay the reading but won’t affect the steady-state reading.

If there is no air in there it probably means your shoe is about to fill with oil as there is a small leak at the gauge end allowing it to self-bleed.  Guess how I know that......

Accuracy..... simple mechanical gauge and fairly old and unknown history, so who knows without comparing it to a calibrated one. Typically within +/- 5psi if not damaged though.

Nick

[Nick Jones]

Bearing rattle suggests some wear to big end bearings and quite often slightly ovalled bearings. They’ll go on a good long time like this, just keeps the revs right down until the oil light goes out.

Nick

[Guest]

Dave, 

I think removing the sump from a Vitesse, in situ, is a bugger of a job IIRC from previous postings when such a mention occurs.

It can be done, but certainly not easy and a real faff to do.

Regards.

Richard.

[dougbgt6]

Like Richard I replaced the oil gauge plastic pipe with an armoured version so I can't see the air bubbles, which is a relief, although I know they're there. But what occurs to me is it's always the same oil, it doesn't move. Until I replaced the pipe the oil in the tube was around 40 years old, I suppose we should drain the tube at oil change time? Or is that a bit OCD?

I've taken the sump off my GT6 to change the bearings (also a bugger of a job!) I can still remember how wet the grass was. And it was a complete waste of time, if the crank's worn the bearings will be worn and new bearings will rapidly be in the same condition as the old. 

Doug

[johny]

Oil pressure does sound good Dave and goes against the idea that you have worn bearings. My pressure was lower with warning light as the revs dropped at high engine temps in traffic so I changed bearings and pump in situ (Vitesse) - not nice but doable and very successful. However probably the greatest improvement was due, when cold, to choke adjustment and use as I have it set so that I pull it out enough to start but the choke cam opens the throttle butterflies very little so preventing revs high enough to cause rattle....

[daverclasper]

Thanks everyone. I did wonder Johny, about the choke, as car starts well and maybe I could give a bit more cam gap so it will start with lower revs.

Dave 

[johny]

yes it takes a bit of experimenting, both in the choke mechanism adjustment and also how much to operate it for any given temperature of the engine, but I think its worth it as I cant imagine the rattling does the bearings/crank any good! In fact Ive not thought about it before but wonder if it would be worth fitting one of those Kenlowe engine heater systems to avoid using the choke at all.....

[Pete Lewis]

Rattle or not there is only one triumph specification across the whole range and thats 40 to 60psi at 2000rpm 

That is what you are seeing so alls good  sleep well

And Smiths and Jaeger  are one and the same just the dial has a different name printed on it 

Pete

[Clive]

2 hours ago, classiclife said:

My rationale is that you can compress a gas (air) to a certain extent quite easily , whereas a fluid (oil) is not so easy - certainly not under the pressure from an engine.

The difference between the two mediums may be minimal, but there will be a difference, depending on the amount of air in the tube. We need a science boffin...……… Clive  !!!!

When I placed an oil pressure gauge in the Vitesse, I went for the s/s pipe option and as such I'm not sighted if it has air within or not - I may well be in the same situation as Dave and Mark.

One point worth mentioning again, you need to ensure you have the pipe-to-gauge seal in situ.

Regards.

Richard. 

As others have said, pressure in  a "sealed" system is the same throughout. The air will indeed slightly damp the changes in pressure, but in reality I very much doubt it will be noticable, the actual volume of air is so tiny. The air will have been trapped when the gauge was fitted, and just won't really move much....

[NonMember]

2 hours ago, dougbgt6 said:

But what occurs to me is it's always the same oil, it doesn't move. Until I replaced the pipe the oil in the tube was around 40 years old, I suppose we should drain the tube at oil change time? Or is that a bit OCD?

Definitely OCD. The oil in that tube may be old, because it never moves, but by exactly the same reasoning it has spent those 40 years tucked safely inside that tube where it won't have suffered. And even if it had, it won't be coming out of the tube to anywhere it would matter.

Several others have already stepped in on Richard's call for a science boffin so I'll just add my agreement with them.

[mpbarrett]

When you start the car and the pressure rises the bubble should get smaller as the air is compressed. In fact if you knew the pipe diameter and measured the change in bubble size you could calculate the change in pressure.... Just need a camera to monitor it and some image processing to do the calculations..... 

Mike

[Pete Lewis]

With years of air and oil in the pipe you could bleed  at  the gauge union but you are most unlikely to get oil in the bourden tube in the gauge it will remain full of  ....air 

And having to calibrate all our test gauges it reads what ever you put in , air or oil  you get the same readings with either 

Pete

[thescrapman]

You could blow the pipe out and then syringe some nice clean oil into the pipe at oil change time, you could then use it to monitor the oils condition as it will blacken up over time as oil is pumped up the pipe and back out again when the engine is turned off,

[daverclasper]

Thanks everyone. Will leave the tube alone, rather than disturb any end fittings, I think.

Dave

[Pete Lewis]

think thats a sound idea 

Pete

[dave.vitesse]

As said the Smith gauge is not a precision instrument but is fit for purpose as it's accuracy is well within the operating oil pressure range of an engine. It is often forgotten that changes in both temperature and humidity will alter the calibration of any gauge and speedo. So it could be spot on one day but not on another due to changes in the environment.  

Most used in classic cars are up to 10% accurate. If the engine has low oil pressure, say 30 lb then the maximum the gauge could be out is 3 lb. Therefore it would still tell you the engine has a problem.

Dave   

[Anglefire]

Mosty gauges are "indication" only but are good for changes - once you get to know the car, you can work out what is normal!

[dougbgt6]

Mark,

Spot on! They're only there to tell you if something's changed, if something's different from yesterday. My temperature gauge sits at half way, another compatible gauge and sensor might show the temperature at 3/4, it's only a concern if it starts to change.

Doug