The International Cessna^® 170 Association

Hello All,

I have a troubling situation developing in my 52' 170B with a C-145 (675 SMOH) that I am hoping someone else has some experience with......actually I hope no one has this experience but you get the idea...

When I approach the airplane cold you can rotate the prop freely without any resistance, as you would normally on any other airplane. When you shutdown the airplane after it has been up to operating temperature the prop appears to stop more suddenly then normal. (ie there is no reversal near the very end like most small piston engines) Then when I try to pull the prop through it is very difficult to turn. If you work it gently a bit it seems to free up and then turns as normal. As you work it while its tight it makes a terrible squeaking noise that sounds to me like its hurting something so I don't do it anymore. And I'm not flying anymore either.....

I have found very little information on the web and even less when asking local mechanics other then doom and gloom. Speculation includes but is not limited to: Carbon buildup on wrist pins? Stuck rings? Bad main bearing. Unequal heating? etc etc....

No abnormal indications during flight Oil Press, CHT, Oil Temp, normal static RPM. I'm cutting the filter today and the oil is going out for analysis. I must say I am fearing the worst but if the engine has to come out I want to do it before the snow flies here in CT.

It's what I would consider a very well maintained airplane that I fly quite a bit... Almost 100 hours a year. I would be disappointed to say the very least if we need an overhaul after only 675 hours.....

Any experiences or thoughts to share would be very much appreciated....

Thanks,

Matt
52' C170B
Candlelight Farms Airport, CT
11N

Has your engine been modified with a light weight starter?

Well you are doing the right thing by getting the oil analyst done and filter checked . Then I would remove the starter and see if that makes any difference . I would then start removing other accessories one by one and see if the resistance is removed .

No lightweight starter here and It's the older pull type that dis engages when you release the handle. The only other accy back there is the alternator so I could have look at that but this really sounds and feels like an internal issue as there is zero play when you try to move the prop. If it was an accessory you would at least have some backlash in there from the geartrain that you could feel even if it was only a few hundredths.

I was able to find someone with some O-300 experience who is pointing me toward the valvetrain and one or more sticking valves. Going to have a close look there.

ML

I have had a stuck valve twice. Both times with around 500 STOP. the tightness and squeaking you describe were present both times. I would explore that area first.

Thanks Jim. I'm optimistic at this point that it won't be as serious as I had originally thought.

My engine is usually quite a bit tighter on shut down but it has never turned out to be a problem although like you I was initially quite concerned. Bill K

I've experienced several stuck valves over the years. I can't say that a tight engine was a symptom I noticed before hand.

I think it would be unlikely, even if every valve was tight, that you would notice drag turning the prop. The mechanical advantage through the prop your tuning and the crank then gears and finally the cam and cam lobes wouldn't allow you to feel it. Then of course the hydraulic lifters aren't solid so there would be movement. If the valves are that tight they would stick open and your engine wouldn't run right if at all.

That does not mean you won't find tight valves, I would be surprised correcting it would remove this symptom.

Can we assume that when the engine cools it loosens up?

That's a good point Bruce..

If you gently work the prop it loosens right up. Maybe a cm cw and a cm ccw and it's free. It's also loosens on its own when it cools a few minutes...... It's odd. Otherwise this engine runs great... Runs fine at the idle stop, purrs at cruise and makes good static rpm. Compressions were good last annual with nothing blowing by valves.

Maybe main bearings? That's would be bad news for certain.. It's going the shop tomorrow so I hope to narrow it down a little..

From first hand experience with a C-85 which is not the exact same as a C-145 I think close enough. If your main bearings are going bad the first thing you will see is a loss of oil pressure and maybe oil temperature higher than normal.

I once wiped out brand new main bearings in a C-85 in the first 45 hours. From fresh new bearings to worn all the way through the babbitt. Symptoms where low oil pressure and high oil temperatures. I changed the oil and inspected the screen several times in that 45 hours and did not see higher than normal metal which you would think I should have.

I would say you are right to be inquisitive. And at the cost of a crank your very justified to be cautious.

However, I'd say that if your oil pressure has not dropped and your oil temperature is running fine, and you don't find anything of significance in the filter, your fine.

Matt-
Not saying you do not have a problem with your engine but I see exactly the same thing happen in my C170 with an O-300D, C180 with a Continental 520 and C195 with a 300hp Jacobs radial. The DC-3 I fly does the same thing as well occasionally.

It does not happen every time I shut down and what I see is the prop just kinda jerks to a stop instead of softly stopping and rocking back and forth a few times. All of my planes have done this long term. However, I have never heard the "squeaking" noise you describe.

I do not think this is evidence of a problem. What I think is happening is the engine is quickly stopping on a compression stroke. Then right after stopping, the engine is still hot with no oil circulation and no cooling air flowing across the cylinders so clearances close up and bind the engine, which could cause the squeaking noise you describe. As you say, in a few minutes, the engine will loosen up and spin freely as it cools off.

What I do is never touch the prop until after I have tied down and unloaded the plane, giving the engine time to cool down and loosen up. I really don't think you would hurt anything by turning the prop when tight but I don't like to force anything.

I have never tried to correlate this condition to events just preceding shutdown such as a short, high power descent to landing and quick shutdown, or to a longer than normal taxi to shutdown, both conditions might contribute to higher than normal engine temps at shutdown which might increase the possibility of the prop jerking to a stop. I don't know, just thinking out loud.

Seldom ever diagnosed at first.

Rx first spend monies on Batteries , Starters.

Still difficult to do warm restart?

FRETTED CRANKCASE. It is possible.

Research articles rom 1980s - Continental C-145 known to sometimes develop fretted crankcase. Always always check the crankcase bolt torque at each annual.

Several C-152s developed this in busy busy flight schoosl. Highly experienced AP/IA head scratching for weeks before final diagnosis.
Professional mill each side of crankcase halves then line bore. Sure is precise, sounds expensive.

I hope not your circumstance.

Thanks all for the replies. I think the best course of action is to give it due diligence and not panic. So far the following easy checks. Oil analysis, filter inspection, bore scope jugs, check crankcase torques, and I think have a listen for where the squeak is with a stethoscope. I'll post what I find.

Most engines have a little axial play on the mains and thrust bearing. I.e.if you grab the prop and pull on it the crank should move in and out a few thousandths (feelable). When your engine is temporarily "seized", this would tell you if it's a main or not.

If it were a stuck valve, wouldn't the engine rotate one way but not the other (as the cam ramps down)? I'm going through it in my head, and having a hard time envisioning how a stuck valve would seize it both directions.

This still leaves a whole host of other possibilities. My money is on a cracked or stuck ring, but I wouldn't necessarily bet against any of the other possibilities either.

Good luck!
DEM

DEM, a stuck valve won't seize the engine. The valve train from the cam to the valve has much more force to move the valve open than the valve spring has to return it closed. As the valve gets tight in the guide it will stop in the open position against the spring. The valve train lash will be loose and no damage usually takes place.

I have had a stuck valve while in flight with the engine running. This being on a A-65 which has essentially the same hydraulic lifter, pushrod, rocker arm, valve and valve guide. I had reduced throttle and shock cooled the cylinder. On throttle advance the engine was running on 3 cylinders. The valve was stuck open. And it was really stuck requiring heat and brass ball pean hammer to drive it out of the valve guide.

I've never seen nor heard of a valve stuck closed on an engine in recent serviceable condition. Most stuck valves that actually stick in one spot are found open upon start up after seizing when the engine cooled after the last run.

So the engine will rotate either way the same with a stuck valve.

Stuck ring you won't hear or feel and likely low compression will be the result. Broken ring gouging into the cylinder will most certainly cause low compression and metal in the filter and screen.