Oil Pressure

[Bruce Fenstermacher]

I've got a question for the collective.

While it's not about a 170 or a C-145 it's about my C-85 which has the same exact pressure relief system as our 170s C-145. In fact the same part numbers.

Here is my problem that I'm having with the C-85 that I just overhauled for my Cub. I'm using straight 50W oil. When the oil temperature reaches 190 degrees, which is within limits, the oil pressure is just at the minimum 10 psi., at best, at an idle. Otherwise at higher RPM or cool oil the oil pressure is in normal range.

I'd like to see the idle pressure higher at an idle with warm oil.

I'm wondering if I shim the relief spring will it raise the overall pressure range thus raising the idle pressure or will that just raise the high pressure end?

I'm in the process of changing bypass springs and other trouble shooting methods as outlined in the service manual and have yet to even check to see it the pressure gage itself is correct.

For the short term we are just keeping the oil pressure in limits with RPM and having fun flying the Cub with all that power.

[3958v]

Bruce I just went through this last week in my 170. One shim increased both idle and high rpm oil pressure. Not saying it will be the same on your cub though. Hope to see you Saturday afternoon. Bill K

[Bruce Fenstermacher]

Thanks Bill that's the info I'm looking for.

So far today I used 4 thin washers which pegged the oil pressure at over 80 psi with cold 50w at an idle. Woops to much.

Took half the washers out and the pressure acted as if there was no shims installed with a max pressure just over 40.

Can't believe that one or two thin washers would make that drastic of a difference. Perhaps with the first attempt the washers weren't seated, acting as if there was more shims than actually installed.

At this point I was just wondering if I was barking up the right tree.

[Bill Hart]

Bruce,

This may not be what you want to hear but my O-200 , covers the C-85 also, overhaul manual says that oil pressure at idle should be minimum 10 psi. and cruising 30-60 psi.

When I overhauled my O-200 I spoke with someone at Continental and was told that there should not be any shims on the pressure relief spring. The intent of the pressure relief is that during start up when the oil is cold the valve will offset and allow the cold thick oil to bypass. When the oil reaches operating temperatures the valve should in theory be closed.

Now I know everyone and their brother puts washers on that spring and says that it increases the pressure but is it increasing the pressure or just covering a week spring.

All of that being said I think that you pressure is within what the manufacturer calls out and that adding shims won’t do for you what you are looking for.

[Bruce Fenstermacher]

Well Bill since we are getting technical.

I said in my first post it made 10 psi at best. Meaning it often appears to be lower. It's difficult to tell on the Cub gage because there isn't much movement between 0 and 10 psi.

Now TECHNICALLY (yes I have the same overhaul manual) I shouldn't even be using 50w oil according to the table on page 43. 40w is the thickest oil to be used over 40 degrees F.

With 40w oil at 190 degrees the pressure is 0 psi or perhaps just above it at idle. Definitly not 10 psi.

While I haven't installed a new spring I have changed the spring to one of the other 5 or so I have and have had the same results.

[bradbrady]

Bruce,
Guages are Just that, a referance to what is happening, Have you checked your guage to a true referance? If the guage is running correct (or a little over) at power I wouldn't worry to much about it. Every C-85 I have seen runs under the green arc at idle. (after flight) And most C-145's, O-200's,O-300's Ect. (unless someone screwed with the spring or washered it up) (You get my drift.) And this is dependent on the type of oil, ect. When you say 10 LBS at best! Iv'e seen C-85's that show no oil presure at idle and have run over TBO! With no problems! Iv'e also seen crank shafts with engines that held good oil presure look like rat tail files! Who's to KNOW?
brad

[Bruce Fenstermacher]

Brad I'd heard that low oil pressure was typical of the C-85 at idle and also stories of engine running fine with no oil pressure at idle. Doesn't make me feel better. If it was just under the green arc I'd be happy. When it's pointing darn near at 0 I just have to do something.

Hopefully I'll be checking the gage today with another one o rule that out.

The point of this thread was to find out how the pressure in both both the C-85 and our 170s C-145 would act if the spring was shimmed.

So far Bill K. and by reference Bill H and Brad I think, have indicated that by stiffening the spring pressure the entire pressure range from idle to high RPM will be raised.

[johneeb]

Bruce,

It seems to me that any self respecting spring would not be relieving at 10 lbs and really should not relieve until what ever pressure it is ment to maintain. So if shimming the spring raises the presure at idle the spring is weak.

[Dougie]

I had a problem very much like your describing with my O-300. I hooked up a test gauge (stewart warner) and it was immediatly apparent that I had been chasing a dying oil pressure gauge, purchased an acceptable A/C gauge, installed it, removed the washers, and have had good oil pressure since.
Good luck
Doug

[cessna170bdriver]

I'm wondering if I shim the relief spring will it raise the overall pressure range thus raising the idle pressure or will that just raise the high pressure end?

Bruce,

I think johneeb hit the nail on the head. The operative word is "relief". Below the "set pressure" the valve should be closed, period. Shimming the spring shortens the installed length of the spring, which in turn increases the force it puts on the plunger, which in turn raises the pressure required to lift the plunger off of its seat, in effect only limiting the HIGH end of the pressure range. At pressures below the "set pressure", it wouldn't matter if the pressure relief port were simply plugged. I'm not sure what was going on when you got 80 psi at idle with the 4 washers installed

One possible cause of low oil pressure at idle is that your relief valve itself may be leaking (rough seat, or trash in the seat). If that's the case, you're bypassing some of the oil pump capacity back to the sump, and not getting enough flow through the engine itself to generate enough back pressure to register on the gauge. That still doesn't explain how you got 80 at idle with the 4 washers.

I have very little time behind C85's, but from what others have said, low idle oil pressure may be normal. Without a higher displacement pump, or tighter tolerances in the engine, I don't think there's a lot you can do.

[Bill Hart]

Bruce,

How did the gauge work out?

[Bill Hart]

With 40w oil at 190 degrees the pressure is 0 psi or perhaps just above it at idle. Definitly not 10 psi.

You could try Pep Boy's 90w gear lube

[Bruce Fenstermacher]

While it was not a fruitful day as low oil pressure goes it was interesting non the less as I've learned more than I probably care to know about oil relief springs and small Continentals.

Today after talking with old car mechanics one wise old guy mentioned that when shimming the spring I needed to make sure that the spring was not bottoming out and not allowing the relief to open.

So I took another -8 accessory case I have up to the airport where my assortment of springs was located. I removed the set up I had in my engine and installed it into this extra accessory case and from the inside pushed in on the piston to open the valve. Worked fine.

I then installed a stock configuration I had from my old A-65. keep in mind all these springs and pistons are the same part number. The stock configuration worked with about the same force as my shimmed configuration but wait. The piston in the stock configuration opened much farther that the shimmed version and it was more than the difference of the shims.

Well after careful inspection I found that someone else had put a thin washer in the nut. To this point I'd been adding washers to the inside of the piston.

What I discovered is that adding a thin washer in the nut under the spring shortens the piston travel in the nut about 1/4 of an inch which reduces the amount the valve can open by that much as well.

So Miles how did I get over 80 psi with just 4 thin washers in the piston under the spring. 4 thin washers plus the thin washer in the nut at the other end is enough to jam the valve close and not let it open. Mystery solved.

I then had the bright idea of using a Maule fabric tester to push open the valve with various setups and compare the pressure required. Those not familiar with the Maule tester it's like a spring scale that you push rather than pull.

So I spent the next 2 hours with a Maule fabric tester measuring the resistance of various setups of springs and washers and compared them against each other.

First there are 2 different springs for the small Continentals to include our C-145. Part number #21352 is the original spring starting with the A-65 which was superseded by #637083 which is called for for the C-75 to C-145. #21352 is longer but lighter in gauge than #637083.

With no shims the #21352 needed about 25 lb. on the Maule tester to open. I have 2 of these and they where the same. The #637083 required about 30 lb. on the Maule gauge. I've run both of these springs in my C-85 with the #637083 producing more pressure at RPM but the same low end pressure. Presumably because the valve is closed in either case a the low end.

A friend had a new spring for an OI360. We tested that with the Maule tester and it required 50 lb. to open.

Based on that finding I stretched the #637083 so that it took 40 lb. to open which was a compromise from what it the spring was and the IO360 spring.

This produced higher oil pressure at RPM than any setup I'd had yet but did not significantly increase the low RPM pressure. But the gauge now reads what seems to be a more healthy bottom red line of 10 psi.

So here are my conclusions which I feel hold true for any small Continental to include our 170s C-145.

First. Shimming the oil relief spring IS NOT an authorized procedure for a Continental engine. It is for a Lycoming how ever and so it seems a common practice.

Second. If you are going to shim your pressure relief spring DO NOT put the washers under in the nut under the spring. Doing so really limits the relief piston travel and I don't think it would take more that a thick and a thin washer to force the piston closed not allowing any relief. Instead put washers in the piston under the spring.

Third. To this point increasing the spring pressure either by stretching or with shims has not had the desired effect of razing the oil pressure at low RPM with 50W oil at operating temperature of 190 degrees.

When I have more time hopefully but maybe not before our convention I might temporarily put in the 50 lb. spring my friend has. This I think will at least put to rest whether spring tension can raise the low RPM oil pressure. I also plan to buy a new #637083 spring and compare against the springs I already have with the Muale tester. I'll let you all now how I progress.

[GAHorn]

At the risk of being a johnny-come-lately.... (I've been putting a new metal roof on the house and finishing my annual inspection, and haven't had much time to visit the forums the last day or so)...
Bruce, the relief plunger, if healthy, is not designed to affect idling oil pressure except with congealed oil. Unless the plunger face/seat is damaged altering the spring tensions shouldn't make any noticeable difference at idle.
I believe it will be helpful to re-consider all that's necessary to provide oil pressure. First there must be a good pump that has low wear and good clearances/tolerances. The oil pump gear shafts to housings and cover-to-gear clearances will be important in order to provide a good basis for oil pressure at the beginning of the system. Those specs are usually found in the ovhl manual. Without a pump within design specifications, oil pressure source will not be up to spec. A good pump will provide a large reserve capacity that should test the relief plunger on a cold engine operating at higher rpm, (the real reason for the plunger), and will provide plenty of reserve at hot idle. (But keep in mind that at idle, only oil flow, not high pressure, to all moving parts is necessary for lubrication and cooling. That's why TCM is happy with only 10 psi at idle.)
Next we must have good rotating-parts clearances. In other words, the bearings/shafts, conn-rod bushings, cam-surfaces/journals, etc. must all be within tolerance. Too much clearance in those areas will allow too much spillage back to the sump and pressure will not come up....and this will be manifested at all rpm ranges across the spectrum.
The last place oil pressure has a chance to be captured is at that relief plunger. (The relief plunger face/seat we've already discussed, this would ordinarily only come into play at idle with congealed oil or at high rpm with a tight engine...but the spring itself has a specification vs. length which should be met. The spring for the C145/O300, PN 631706 should be wire of .041 ga., and when compressed to 1.56" in length should have a minimum pressure of 5.75 lbs for a used spring, 6.06-6.31 lbs for new. This is not a difficult measurement to take. Using a scale, simply stand the spring on the scale and with a ruler or caliper beside it, compress it down to the specified length and read the scale. A common fish-scale can also be used with a wire thru the center of the coils tied to a cotter spread to capture the lower coil. The wire can be passed up thru a hole in any convenient object such as a board or table-top, and by pulling up on the fisherman's scale until the spring is compressed to the specified length, the scae can be read.
If you've recently overhauled your engine, it's possible that internal pump clearances and/or cam bearing-surfaces-to-case/journal clearances were overlooked, and if excessive, would lead to low oil pressures especially at idle. Any case perforations , mismatch, leaking seals/gaskets or irregularities might also give an opportunity for the pump to pick up air and entrain it within the oil galleries. Air is compressible and will result in low pressure indications, but if your cruising pressures are good, that's an unlikely event.
I'd suggest you measure the plunger spring length/strength per the manual, inspect the seat/face of the plunger/case, and then quit looking at that for a solution. (I don't personally like shims in the plunger at all. I believe if the spring/plunger/case is good and up to spec, then no shims are necessary anyway since it's primary job is to prevent over-pressure....not regulate basic pressure.)
An accurate pressure gauge is the first thing to confirm before doing anything to the system...but you already know that...
I don't recall any plugs in the smaller crankshafts, but it's been a long time since I worked on one... if any are missing in the engine it would certainly have an adverse effect...

[Bruce Fenstermacher]

.... The spring for the C145/O300, PN 631706 should be wire of .041 ga., and when compressed to 1.56" in length should have a minimum pressure of 5.75 lbs for a used spring, 6.06-6.31 lbs for new.

George where did you get that part number? I just rechecked my parts manual for the c-145 and it is the #637083. I've found no other cross reference except to the part #21352 that it superseded.

And as always I'm curious where the heck you got the description for that spring?

George as for the engine assembly I was pretty careful but I have to say could have been more careful in the oil pump area.

I have done a calibration of my 60 year old Cub gauge and found it to be fairly accurate with another 2 gauges I have. Some of the people that have reviewed my work and looked at each of the gauges feel that indeed the engine is making minimum pressure of 10 psi.

The difference between 0 and 10 on the gauge is just not that much and having paid for the engine makes me uncomfortable. I guess I need to install a separate more accurate oil gauge and monitor that. I'm just trying all the easy fixes before I do what I think I probably need to do and that is remove the accessory case and review the oil pump.

Interestingly if the engine oil is just 15 degrees cooler at 175 degrees the idle pressure is nearly 20 psi. FYI the 190 degrees the engine runs at is well within the 225 degrees max that is allowed.