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  • C85-12 oil pressure

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Technical Chat Tips and Advice, including flying the Luscombe

Moderator: HTB

 #8413  by Vic
 Mon Mar 10, 2014 7:34 am
Since I've only been flying G-BSYF since Decemeber it's always been a bit on the chilly side 5 degC, apart from my last couple of flights a week ago 10 degC and this weekend 18 deg C.
I was told to keep a close eye on oil pressure and any changes from normal. So with the recent change of OAT :) I would expect a change in running temp and oil pressure, so I'd just like to run these figures by you chaps to check all is well.

No oil pressure on 1st start of the day for approx 3 seconds since fitting a spin on filter (always used to show 60PSI straight away)

In the cruise at about 2150rpm OAT 5deg C oil temp 160F oil pressure 40PSI
In the cruise at about 2150rpm OAT 10deg C oil temp 170F oil pressure 38PSI
In the cruise at about 2150rpm OAT 18deg C oil temp 180F oil pressure 35PSI
below about 1700rpm about 20 PSI any temperature

I could get the 35 PSI back up to 40 PSI if I did a long decent with low power to cool the oil and then power up again.
 #8414  by PeterB
 Mon Mar 10, 2014 10:03 am

What you are seeing is exactly what I would expect from a less than new engine.

A really nice tight new engine will run a small amount of oil over the relief valve at pretty much all temperatures and RPMs. The pressure gauge stays fixed at pressure relief-valve (PRV) setting usually 48PSI. If the oil is really cold and thick, the pressure may run above the PRV setting for a few minutes until it gets warm.

As the engine wears and the clearances increase, the leakage rates around the journals steadily rise until the pump can no longer put out enough oil to maintain flow through all the little leakage paths and run oil over the PRV even at maximum rpm. Just to add to the process, the pump is also slowly wearing and putting out less oil per revolution. At this point, the oil pressure modulates with oil viscosity and pump flow (rpm).

The Continental manual gives a cruise RPM limit of 30-60psi and a minimum of 10psi at Idle so you are well within limits. As the weather warms up the oil temperature will get a little higher (and thus the viscosity will fall) and the oil pressure will reduce a bit.

If you are running 80 grade oil, the pressures will all go up a bit if you change to 100 grade, but it won't fundamentally change the fact that the clearance are getting larger. Sadly, thick oil films are (generally) less effective at lubrication than thin ones and the wear rates only increase with time.

Based on my experience of end-of-life engines, once you start to see pressures below 30psi in the cruise then it's time to have a long hard think about a rebuild. I'm sure there will be people who say they have been running at 25psi for years and it's all working fine, but I put quite a high price on my own life and welfare....

 #8415  by Vic
 Tue Mar 11, 2014 7:16 am
Many thanks Peter,

The engine was rebuilt about 13 years ago and has 420 hours on it, obviously the way it was rebuilt and how it has been used since will have a dramatic affect on its life expectancy. All I know is that it had an 0200 crank but other than that, I have no information on what was done. I was advised to use W100 which I have been and oil consumption is approx 0.25 quart / hour which seems quite high. I fitted the spin on filter about 6 hours ago and I'm going to cut it open and have a look. I know I've got a leaky exhaust valve on No.4 which I need to sort out. Strange since it was supposedly done at old Warden with new rings just before I bought it, but then the prop was supposedly re varnished and balanced and I found that to have bare patches of wood and the tracking was out by 10mm :shock:

Having flown behind Jabiru engines for the last 15 years, with unchanging oil pressure and little or no oil consumption its a bit worrying :?
 #8417  by Pete Bush
 Wed Mar 12, 2014 4:09 pm
Hi Vic,

It sounds like your engine is a little worn but I wouldn’t say the figures you are getting are anything to worry about at the moment. It may be due to the fact that it has done 400 odd hours with unfiltered oil and the rings may well be clogged with carbon and lead if it has been run on 100LL. A long climb on a 25c + day will tell a lot about a small Continental.

In my opinion running W100 in the winter months is not recommended, if you want to use W80 and W100 it is normal practice to use 80 in the winter and 100 in the summer. As our British weather is so diverse I tend to use a modern multigrade such as Total DM all year round, these oils have better corrosion inhibiting specs and anti scuff additives, they are also much better at handling the huge variations in running temps on air cooled engines. ... s-13-c.asp

You can have a good gander around when your number four cylinder comes off .

 #8418  by Vic
 Wed Mar 12, 2014 6:06 pm
Thanks Pete.
I used to use aeroshell 15w /50 in my Jabirus.

I'll use up what I have of the W100 which won't take long :lol: and go onto the semi synthetic. I think I mainly have a ring problem. It was always run on 100LL before I got it and the plugs had lead balls in them. It stood for a year, without running. I'll do a compression check and start with No.4 barrel off to see whats what. :scratch:

On the plus side, she is flying very nicely and I get very positive admiring comments from everyone who sees her :)
I'm working my way through all the snags that I keep finding and it gets better and better :wink:
 #8438  by Jonathan Holland
 Thu Mar 27, 2014 11:02 pm

I have exactly the same engine as you. Mine has done 600 hrs since it was top/bottom overhauled (no such thing as a Continental zero time in the UK IMHO) and it sounds to me like your pressures are absolutely fine. It's worth having a look at this:

I use 80 grade between Oct and March and 100 grade from April until it cools off in the Autumn.

Your prop tracking is rather concerning at 10 mm out, and you might want to look into that.

Also, are you getting much gunk coming out of the breather? If so, this might be the explanation for your oil usage. I installed an 'up and over' breather tube that allows vaporised oil to percolate back into the engine and my consumption has gone right down to an almost negligible amount.

Good luck.
 #8439  by Vic
 Sat Mar 29, 2014 7:53 pm
Thanks Johnathon, my readings look in the ball park looking at those figures.
The prop tracking, I sorted right away, I've never seen one that bad! Varnished and balanced too and has made it smoother - or is it my imagination :)

I have a new length of breather hose ready to install up and over. There is a bit of oil that comes out the breather but only when the sump is filled right up. When closer to 3 1/2 quarts it uses very little.
Unfortunately work has stopped play just lately so I haven't had a chance to tinker with it
 #8440  by Pete Bush
 Sun Mar 30, 2014 12:29 pm
Hi Vic,

It sound like your heavy oil consumption could just be down to the level in the sump. The small Continentals seem to throw out lots of oil through the breather if filled any where near max, I flew over 500 hours in an C85 Luscombe and didn’t fill above 3.25 qts on the dip stick, that engine used 1 qt every 25 hours when running multi grade oil at 38 PSI in the cruise.

If your hot differential compressions are good then I would say no worries.

 #8448  by Phil Laycock
 Sun Apr 06, 2014 10:55 am
A bit of thread drift , I know, but anyone need a quantity of W100 Aeroshell oil ? I have a half full 20 litre drum ( so thats just over 10 litres for the mathematically challenged ! ) that is just taking up space here.
It is listed on ebay ...... ... 1586.l2649
but there would be a discount to any Luscombe owner , of course ! :applause: