Oil pressure woes

[Guest Ken G]

Just before I start expensive work on the lower engine, I would like reassurance, or otherwise, that what I think is happening is possible or likely.

Symptoms.

a) Low oil pressure at low revs, and audible objecting noises from the engine when the pressure drops below perhaps 15 psi. (They are the same noises that the engine makes when first started before the oil pressure comes up). The instruction book says the pressure ?should remain steady at between 50 and 60 psi when the car is running at anything over about 15 mph. Below that speed it may drop slightly, but should never go below 20 psi even when engine is idling?. Well, it does drop below 20 psi at low revs., even actually driving as opposed to idling. This is with 20/50 oil, which is thicker than the SAE 30 originally intended.

Oil pump starvation whenever the oil would tend to surge away from the back left of the pan where the pump is mounted, that is, coming to rest, on downslopes and at speed on left-hand bends. This is with the static oil level well above the mark on the dipstick.

When I first received the car from my cousin, oil pressure was maintained properly, so this problem has developed. Last time I had the pan off, I examined the oil pump (mounted to the floor of the pan) and it didn?t seem worn. I have had the pressure release valve out several times, and cannot see anything wrong with it; it certainly controls the pressure to the correct 60 psi at high revs.

My provisional theory is that there is a serious leak, probably from one of the main or big-end bearings. At speed, the pump can pass enough oil to maintain pressure, but in so doing it lowers the level of the oil in the pan, because the oil cannot drain back under gravity fast enough. Thus, with a lower level in the pan than intended, the pump is starved whenever the oil sloshes away from it. At low revs. the pump cannot deliver enough oil to maintain pressure.

Does this make sense? The (Babbit/white metal) bearings were renewed by my cousin only perhaps 1500 miles ago, so it is unlikely that they are all seriously worn. I am guessing therefore that one of them has failed disastrously. If so, can anyone suggest a simple test after the pan is removed to see whether oil is leaking much more out of one bearing than the others, before major dismantling?

And finally for the present, can anyone recommend a firm in the San Francisco Bay Area capable of remaking Babbit bearings?

Ken G, 1925 Rover 16/50 (San Francisco)

[Guest BillP]

Hi Ken,

When running oil level as you say well above the mark you risk whipping and aerating the oil with the rotating parts which would effectively reduce its apparent viscosity. I think I'd also run straight 30 weight as a further test; there's something about multi-vis oils in very old engines I'm leery of but incompetent of to discuss.

Do you have an oil filter you can change?

If you have a gear-type oil pump, check clearances between the teeth and the case with a feeler gauge. I guess you'd look for 3 or 4 thou but you should look it up. You can make flow with wide clearance but still lose pressure.

While you have the pan off, check the pump pickup screen for debris that would reduce inflow. Also check for loose fittings on the pickup. While you're laying there enjoying the view and getting dripped on, you might as well see if you can rattle the rods back and forth; probably can't but a loose one would save further detective work.

If the main caps are easily removed, take them off one at a time and inspect for scoring or damage. Plastigage is supposed to be good for checking bearing clearance, read the package. Same with rods. This is all for checking with the engine the car; I usually do this using instruments with the engine out, but trying to save you effort. Work safe when underneath.

Recent engine rebuild isn't necessarily a guarantee of a serviceable product, everyone has a story. If you need babbitt, Blackhawk is not far from you north of Pleasanton, maybe call the curator there or their technical guy and ask if they could refer you to someone. Good luck

[Guest Ken G]

BillP, Many thanks for your comments/suggestions.

When I first ran the car after doing top-of-engine work (new exhaust valves), I used 30-weight oil, and saw the beginnings of this problem. I then changed to 20/50, and the problem remained and has got worse over the few hundred miles I have driven, so I don't think it's associated with the oil itself. Besides, any explanation has to account for the pump starvation as well as the loss of pressure at low revs. and I can't see how oil viscosity would be relevant to that. You didn't address the starvation. I think it can only mean that when the engine is running the level in the pan is lower than intended, so we need an explanation for the level to drop seriously. I suppose it could be an obstruction in some return path, but I cannot imagine where; the oil does not back up in the valve gear, visible under covers at the top of the engine.

I might add that there is a significant disincentive to changing oil casually; the capacity of engine, clutch and gearbox (all common) is six US gallons! By the way, judging from the cam-shaft well, the oil is reasonably clean.

There is no oil filter apart from two cages of chicken-wire (what a friend called bird-catchers). I installed magnets on the insides of the two drain plugs, but not having had them off since, I don't know whether they have caught anything.

The pump is mounted solidly to the flat floor of the pan with the pickup merely a hole in the side (no screen). It is indeed just two gears, and I had it apart. Wear seemed negligible, and as I say, the problem has progressively got worse, much faster than the pump could have worn since I looked. There is nowhere to look up the intended clearances, but yes, I think clearances were and should be just perceptible and no more.

There is a splash-plate, so with luck the crank-shaft should not hit the oil surface. Statically at least, the oil level, while well above the mark on the (original) dipstick, does not completely fill the pan, i.e. is not up to the splash-plate. Against that, at the most downstream part of the valve gear I do get a little clean yellow foam, like whipped butter. But then, when the pump is starved it tries to pump air, and that would probably give foam.

Having had the pan off twice, lying on my back (and putting it back single-handed the first time ... never again), I am ready for professional assistance, so I have arranged to go and talk to my tame mechanic, semi-retired and probably old enough to have worked on Babbitt bearings, who has expressed interest in getting his hands on the car! They at least have a car hoist, which makes things much easier.

I will report back in due course.

Ken G, 1925 Rover 16/50 (San Francisco)

[Guest imported_oldmitchell]

Ken, if you have poured babbit bearings there should be some sort of provision (like shims) to decrease bearing clearance. These engines typically wear in rather quickly and need shims removed to maintain proper bearing clearances.

[Guest Ken G]

Oldmitchell (but you are only as old as you feel!),

Thank you for that comment, which I shall certainly bear in mind. If and when I get round to checking bearing clearance, it will presumably become quite clear whether or not there is provision for such an adjustment.

The mechanic who put in some hours a week or so ago, tracked down a knock (using a stethoscope; I must get one), and found that the vertical shaft driving the oil pump, cam-shaft and magneto appeared to be riding up and down. He made the necessary adjustment, greatly reducing the knock, but not, I think eliminating it; perhaps now it's something else. I don't understand why that should have had any effect on oil pressure, but it has, in that the pressure is maintained down to lower revs and perhaps more significantly, now the engine doesn't make objecting noises for the first two or three seconds after starting, before the oil gauge comes up. That is certainly progress.

However, the pump starvation remains whenever the oil would tend to slosh away from the pump. If the mechanic's ears are correctly tuned, it sounds (literally) as if there is nothing seriously wrong with the bearings, or at least, nothing audible. I have another theory for why oil might be retained somewhere away from the pan and hence the oil level might be lower than it should be when the engine is running. It is possible that the return from the gear-box to the pan is obstructed, so the level in the gear-box is rising. It will involve significant dismantling of the shift mechanism (which is all tied up with the handbrake) to be able to see into the top of the gear-box and thereby check whether the level is changing, but I suppose I have to do it. At least it is easier than removing the pan to look at the bearings. I will report further information as and when I have it!

On a topic that may or may not be related, does anyone know whether on engines of the 1920s valve guides were generally made of bronze? I would have expected the guides to be made of a significantly softer material that the valve stems. I have noted that the stems of two of the brand-new exhaust valves that I installed perhaps a 1000 miles ago show signs of wear. The guides, which were replaced perhaps 2000 miles ago, not by me, look to be steel, not bronze. There appears to be no provision for lubrication other than splash, and the exhaust valve stems were dry. When installed the clearances were small but the valves moved quite freely, so I don't think the wear is because the valves were too loose or tight.

Ken G, 1925 Rover 16/50 (San Francisco)

[Jay Wolf]

The owners/operator manual for my 1916 Elgin said to check main bearing clearance.

Grab on to the crankshaft if you can move it up and down the bearings are too loose, remove one or more shims from under the main bearing caps, If you can not crank the engine with the hand crank the bearings are too tight, put shims under main bearing caps, recheck at 200 mile intervals

That is on a splashier system with 3 main shell babbit bearings.

[HarryJ]

What kind of auomobile, and what year are we talking about? Plastigage and careful observation should give you an idea about the bearings. Oil pumps wear on the end (thrust) surface, allowing for pressure leaks between the end of the gear and the housing surface. You should also check for a bubble in the oil pressure guage line. I recently acquired an 1921 Studebaker Light 6. I had an oil pressure "problem", according to the guage. Some time it would register sometimes not, Turns out the oil had been thinned and there was a air bubble in the oil pressure guage line.

[Guest imported_oldmitchell]

The valve guides are probably going to be cast iron.

[Guest Ken G]

Harry J,

It's a 1925 Rover 16/50, the only one in North America. Last time I had the oil pan off, I dismantled the pump, which is mounted on the floor of the pan, to see whether it showed signs of wear, and concluded that wear was negligible. I think you would need a lot of wear before the leakage from that cause compared with the inherent leakage between the teeth of the gears that form the pump. However, I have since realized that since the pump output is up the middle of the hollow drive shaft, which has splines to engage with one of the gears, wear of the splines might lead to leakage and hence loss of pressure at low revs. When the opportunity arises, I shall look at the pump again. By the way, I am confident that the gauge is telling the truth, in that when it dips the engine makes objecting noises. The instruction book warns that if the gauge moves randomly or wildly (which it does not), you should bleed the tube that feeds it.

Yes, I know about Plastigage, and there too, at the next opportunity I should probably try to assess the bearing clearances.

Oldmitchell.

Whatever the valve guides were originally, they have been replaced, and I would imagine that the new ones were turned on a lathe. I doubt therefore that they are cast iron (the block and head are cast iron, of course).

Ken G, 1925 Rover 16/50 (San Francisco)

[Guest JT]

Ken, cast iron valve guides are the most commonly used guides and probably what you have.