Oil Pressure ?

[jatkins]

In cruise , @ 2450 RPM , and oil temp @ approx 150f, my oil pressure is at about 60-65 psi ?
This is the 2nd gauge.
I am running Shell W-80 oil.

I have reviewed the O-300D type certificate , and it states oil pressure 30-40 psi .
My C 170B owner's manual states "normal" oil pressure 30-40 psi , and min 5psi at idle.

Do I have an issue with high oil pressure ?,..or an I being overly concerned ?

Any recommendations ?

[GAHorn]

I do not think you have a problem you cannot fix. Oil pressure up to 55-60 is allowed, according to Cory Thompson of TCM. (Early C-145's had problems with high pressure related to early valve lifters but all early designs have been replaced.) You should confirm you gauge's accuracy, and if you still have high pressure, inspect and/or adjust your oil pressure relief valve and spring. That device should "spill" excessive oil pressure back to the sump.

[jatkins]

THANKS George !

How do I "adjust the oil pressure reief valve "
looks like a spring and a plunger ?

John

[GAHorn]

AFTER you've determined your gauge is accurate...
I'm wondering if someone has assembled your engine with the incorrect or substitute spring.
The correct spring should be blue. (the color I'd call it is "baby blue")
The correct spring PN is 631706. <edit> superceded PN 637083, and when it is compressed 1.5 inches, it should have a pressure of approx. 6 lbs. (The exact measurements are: compressed to 1.56", the min/max pressures are 6.06 lbs to 6.31 lbs. A used spring may be returned to service with as little as 5.75 lbs. But do not accept a spring greater than the 6.31 lbs.)

A simple way to check yours is to remove it from the valve-plunger (twist it out like it was a bolt in a nut) and drop a length of safety wire thru a loose flat washer, then thru the spring itself, the other end tied to a flat washer (to provide a method of compressing the spring between the two washers by pulling the wire.) Using a fishing-scale tied to the loose end of the wire, compress the spring by using an opened wrench or other tool against the loose flat washer... to a length of about 1.5 inches, and read the fishing scale. It should be pretty close to 6 lbs. (no more than 6-1/3 lbs.)

Also be certain your plunger's excape to sump is not clogged with junk or gasket sealant.

[Bruce Fenstermacher]

Less spring pressure lowers max oil pressure. If there are any washers inside the acorn nut or inside the plunger remove them. Otherwise you have to replace the spring to lower the pressure. Problem is that chances are the brand new spirng you buy will perform the same as the one you have unless someone stretched your spring in order to increase the pressure.

My personal opinion is that though 60 lbs is the upper limit, I wouldn't worry a second about having 65 lbs of pressure.

BTW George you are correct about the blue spring being correct but I seem to remember the old spring could still be used. Going strickly from memory there are two acceptable springs for the small Continental the old one and the new blue one. Having had a low oil pressure problem with my Cub which uses the same parts, I've had the experiance of trying several examples of Continetal springs including a new blue one and found they all basically performed the same even though they didn't look the same. I seem to remember the Blue sping is shorter than the old one but probably a bit stiffer to make up for it.

[jatkins]

I am looking at page #18 of my Continental Service Parts Catalog , Models C-145 & O-300
page 18 shows crankcase cover assembly ( new style) ( I have a O-300D )
in Fig #10 , item # 21 is,.. Screw,Adjusting . How does one adjust this ?
Does it screw in to increase preassure and out to lower pressure ?
How many turns = how much psi ? any ideas ?
The spring pn on my page shows item #19 pn 635002

On the previous page #16 , fig 39 Crankcase Cover Assembly , the spring , item # 13
Shows as PN 637083.
My manual is dated November 1981.

Is your spring pn the most current pn ?

John

[GAHorn]

Good points, Bruce. I didn't mean to imply that a black spring (old spring) was unserviceable if it met the specifications.

The hazard of excessive oil pressure is remote, but the result is blown seals (crankshaft) and lifters that refuse to dump oil. This is only likely to occur with congealed oil due to cold temps. Once the engine warms up I don't think much problem will occur due to pumped-up lifters. I wouldn't want to lose an oil seal or, on a newer crank, the hollow-shaft plug, tho'. (Gotta admit, don't know what would be considered excessive in that regard, but at some point that would be reached.)

[GAHorn]

Both PN's should be good. It will depend on the source. Cessna uses the 637083 and TCM still lists both 631706 and 637083.

[Bruce Fenstermacher]

John,

I don't have the parts manual in front of me but am very familiar with both accessory cases available for the 0300. What you are most likely looking at is what I referred to as the acorn nut. If you could adjust this in and out you would in effect be adjusting the spring tension on the blow off plunger.

But you can not adjust this nut because it must be sealed and that is accomplished with a crush washer at its base against the accessory case itself. Any adjustment can be accomplished by adding washers inside the acorn nut which increases spring pressure and the blow off pressure. If there are no washers to remove there is no way to further reduce the pressure without finding a spring with less tension. Good luck with that. I have about 4 different springs and have found them to all perform the same in their stock configuration.

Of course, and I hesitate to mention this, many of us could imagine ways to reduce the spring pressure by using two crush washers under the acorn nut or reducing the length of the spring but I doubt the FAA would have the same imagination. And lets face it this is not your lawn mower we are talking about so don't invent a problem that doesn't exists and apply a cure that is worse than what is thought to be wrong.

[GAHorn]

While it may be a commonly accepted practice to insert washers in an effort to increase oil pressure, it is not a modification acceptable to the mfr'r. (And in most cases has little effect, if any, on oil pressure if the spring and plunger is proper/healthy to begin with. Falling oil pressure is most likely caused by wear/increased clearances at the main and rod bearings, and increasing the resistance at the relief valve will not cure that problem because it is at the end of the lubrication galleries.)

[jatkins]

Hi Bruce,
My part manual , shows 2 differant oil pressure relief set ups.
The first one is as you discribe, the 2nd one is " ( new style" )
and it includes pn 641075 screw, adjusting,... I was trying to see
what the adjustment involed, ( how much to turn the screw ).


John

[GAHorn]

John, I apologize. I missed your post advising about you having the new style cover. The previous pn's for the spring do not apply to you.

YOUR spring PN is indeed 635002. (The other parts won't even fit in your crankcase cover.)
Your adjusting screw adjusts inward to increase, outward to decrease pressure.

I should relate a personal experience I had with this type adjustment. (Not on this particular engine, however. It was a similar adjustment on a IO-520.) The left engine on a Baron experienced reduced pressure in-flight and the constant-speed prop changed rpm. It was during descent into Dallas Love Field and the oil pressure stayed up around 10 psi so I continued with reduced power.

When I shut down the prop feathered itself. I called for mx and they condemned the prop governor and changed it. They also found one small chip of copper/bronze in the oil screen, which they pronounced was from the failing governor.

After the governor was changed (and the oil as well) the engine was started and oil pressure was only 25 psi, so they increased the pressure with that adusting screw on the aft engine-cover. Each "flat" on the screw supposedly was worth 5 psi, if there's any correlation, which I doubt. They ran the pressure up to 40 psi and called it good.

On the next flight all was well. On the second flight the engine suffered low oil pressure again, reduced to 20 psi in cruise. I left it alone and completed the flight. (I reasoned that the prop hadn't wandered therefore the engine was getting oil.) We landed at Midland, TX and the local guy checked the screen and found nothing, so he adjusted that screw again and brought the pressure up to 30, which was all the way in. He told me the spring and plunger needed replacement and said it was OK to fly it back to home base at 30 psi.

We got home to Austin, and the spring and plunger was replaced and the pressure adjusted to 40 psi.

On the next takeoff, during climbout, passing thru about 3,000' the engine lost pressure and the gauge went to zero. I looked at the engine hoping against hope that this was all just a gauge problem when I heard/felt the engine "clunk" so I pulled the mixture, throttle, prop and it instantly feathered and stopped with no "slow-down" at all. When we landed the engine could not be turned over. It was exchanged for a reman. TCM was very critical of the record of adjustments on that oil pressure and it took quite some correspondence to get them to allow an exchange of that engine, due to the ruined crankshaft. (It never would have gone thru, in my opinion, had I not successfully argued that the old engine's crank would have been condemned anyway because it was not a VAR and wouldn't qualify for rebuild anyway. Since TCM would not deduct for a non-VAR crank, no penalty should accrue for a ruined non-VAR crank.)

How does this relate to your problem? FIRST check the accuracy of your gauge. A new gauge is not necessarily accurate, and you don't want to start messing with that pressure adjustment unless you know for a fact the pressure is excessive (which it is only slightly so...IF that gauge is accurate. You are going to be "ticked off" if you reduce your oil pressure and hundreds of hours later find that you'd done so based upon a faulty new gauge and had adjusted it to run your engine at 25 instead of 40. See what I mean?)

You can test the gauge at any instrument shop, or you can disconnect it from the engine and introduce a known pressure to the line using adaptor fittings and a regulated oil, air or nitrogen supply. You can also use another (calibrated) gauge temporarily installed/plumbed into the engine for a test-run on the ground for comparison.

Only after the panel gauge is calibrated or found to be within tolerance, should you attempt to adjust the oil pressure, in my opinion.

[GAHorn]

Both PN's should be good. It will depend on the source. Cessna uses the 637083 and TCM still lists both 631706 and 637083.

Who was it that questioned that PN and caused me to respond with this? (They apparently deleted their own message.)

I've rec'd a return call from the TCM rep who reversed himself and informs me that the 631706 number has indeed been superceded to the later 637083 number, but that either may be used when ordering. My bad for not checking my own revision.

(No change to the specificatons, only a re-numbering.)

[Bruce Fenstermacher]

Well John I learned there is a new accessory case style I haven't seen. George is correct of course that your gauge should be checked for accuracy first. But since you may be adjusting down just 5 pounds and seem to have an easy way to do it, I personally would just do it without checking the gauge) and look at the result. Lacking any other data I would adjust a quarter turn out and run the engine and see what effect it had and go from there.

George is also correct that this adjusting system is just a blow off system. It only regulates the maximum oil pressure. It has absolutely no effect on any pressure less than maximum. So bumping this maximum down to 60 will have no effect on any pressure lower than that. When the oil gets hot and thin and you see your pressure drop to say 40 when the max was 60 this means that the blow off valve is fully closed. More pressure on the spring will not close it further and raise the oil pressure. What is happening is oil is escaping out around the oil pump itself and or the main and rod bearings (and other places).

Off course low oil pressure is not what you describe as your issue. Your pressure is just a bit higher than the limit.

Now as for washers under the spring George is also correct that Continental does not prescribe to that method of adjustment. As I recall they say nothing about it other than spring replacement. Lycoming on the other hand does prescribe to using washers in their engines with virtually the same system and so it is common to see washers in Continentals.

George is not correct if he is saying washers under the spring will not adjust the maximum oil pressure higher or removing them will not lower it. They do have a good effect at this. He is correct as I've already described washers or any method of adjustment will have no effect on any pressure less than the maximum.

Here is my story. Rebuilt my Cub engine. 25 hours after rebuild the idle oil pressure was at or below minimum. Of course it couldn't be the new bearings and crank so I proceeded to install washers behind the spring. Made the max pressure go up till the oil was warm then it dropped to the same bad low levels. At one time I had the max pressure at 100 till the oil got warm and down the pressure came. For the next 20 hours I swapped out about 6 springs, 3 plungers, 3 accessory cases and who knows how many types and quantity of washers later I could not adjust the low oil pressure. BECAUSE THAT IS NOT WHAT THAT VALVE DOES. Tore down the engine to find shot bearings and crank.

Another thing I saw in those twenty hours of staring at the oil pressure and temperature gage is the correlation between oil temperature and oil pressure. 30 degrees of oil temperature can change pressure a lot once it is below the max threshold of the blow off valve. John with your oil at 150 which is a bit low (assuming your temp gage is correct) you are probably seeing your oil pressure stay at the max or close to it longer. Maybe that is why you noticed this issue at all. BTW I don't think oil thickness in the range of approved oils for our engines will have much effect on the max oil pressure only the time it stays at max. John is running 40 weight if he were to drop to a 30 weight his max pressure would be the same but perhaps just not as long as the oil gets warm and thins to it's rated viscosity.

[jatkins]

Thanks George,
I was the one asking about pn because the number was different from my book.
The posting is he 6th one in this.

Once I get home from FLL , I will get a calibrated gauge , and see where we really are ?

I am guess the pressure is high , as I was having oil misting out of my nose seal back in the summer.
The 3rd replacement seal is holding.

I guess as long as I can correct it ?? High oil pressure is WAY better than low oil pressure.

THANKS for the help / recommendations