Engine Guidance

[juredd1]

I am looking at a 170B with an 0-300 engine with around 900 SMOH. It was majored in 1980 and has 3 cylinders at 62/80 and 2 at 64/80, and 1 at 72/80. I am a bit nervous about this engine but there has been some reduction in price based up on the three lowest cylinders. Is this a plane that I should avoid or is it worth considering with the thought that a top will likely be needed in the near future. My IA says his cutoff for a top is 60/80 which puts at least three of those pretty darn close.


Any advice would be appreciated.

[marathonrunner]

TCM service bulletin SB03-3 has been out for almost a decade now. It describes the tools needed and the procedures to do a compression check and also the boroscopic inspection. I won't quote the whole bulletin but, you do need several things that are often overlooked. Those cylinders may or may not be needing a top in the near future. Keep in mind that a compression check is done at 80 P.S.I. and the engine operates at 800-1000 psi and is turning over at 2400 r.p.m.

For starters you do need a source of dry compressed air capable of supplying 125 p.s.i. and 15 cfm. You also need the Eastern Technology E2M differential compression checker. This is because ther is a test setting on it and you are able to use that to determine the lowest number permissable given certain conditions. Often this number is in the 40-43 range. If you use a standard other tester you need the master orifice tool to determine the low number to be used. Also you need to make sure the gauges are calibrated against a known gauge. I have seen gauges off by 20 pounds in some shops/trunks.

I have seen guys doing compression checks with the aircompressor roaring away and not even able to maintain 80 much less 125 psi. The borscopic inspection will give you an idea of the condition inside the cylinder and if there is any rust or pitting. You need someone who has looked at a lot of cylinders this way as often what may look bad to one person is fine.

The compression check needs to bedone when the engine is hot and as soon after shut down as practible. You need to rock the prop and stop on top dead center ass you are going in normal direction of rotation. tocking is permissable to obtain the highest reading.

I would also recommend removing the rocker covers. If there is a lot of rust inside then I would suspect that the internals have had a lot of moisture as well.
Remove the screen and if possible drain the oil through a t shirt or cotton cloth and let it dry or wash through with solvent and dry same with the screen and use a magnet to see if you get any steel. If you do you can be sure the cam and lifters are at some point of degradation.

Much as I hate to use the term "buyers inspection" since there is no such animal and, the best way to determine the condition is a detailed inspection or annual inspection, I have found on two recent occasions engines that once we found metal we removed a cylinder. One only needed some lifters replaced as they usually go first and the cam was fine. I drag a fine point awl across the cam lobes and if it catches it is not a good sign. IF not and the lifters are pitted I just replace them. On the other occasion the cam and lifters were so bad that one lifter became a Lycoming style as it could not be removed from the 0-470 normally and the case had to be split. If any lifters or cam is bad on the 0-300 you will have to split the case.

I only mention those last inspections because 1980 is a long time ago although I have seen a lot older engines that were running fine.

You can download the SB03-3 from the website www.tcmlink.com

Good luck and hope it works out

[juredd1]

Man that is a lot to take in for my simple mind. Don't get me wrong as I much appreciate the info. If I move forward with an inspection maybe I can convey that info to the inspector. Yea the 32 years since major seemed like a long time for this thing but like you said its possible that all is ok.

[hilltop170]

I didn't read all that other post but 900 hours in 32 years is not much, less than 30 hours/yr. The cylinders need a borescope inspection to determine if they are rusted in the least and all probably need to come off for a proper inspection. That is risky in itself as main bearings can be dislodged out of position with all cylinders off when crank is turned. You can expect internal oil pan corrosion as well.

I'm not an A&P but from my 42 years flying experience owning several airplanes, I would consider that engine to need a complete major overhaul, not a top. An overhaul is running about $25,000 these days using new cylinders.

[Bruce Fenstermacher]

Most telling is how many hours in the last two years or so. This engine in the right hands is likely to go another 2000 hours. I'd have to actually see and understand where the cylinder leaks are. For someone with low or no airplane owner experience relying on and paying a mechanic, it's hard to say.

What does your IA think you should do? How well do you know him and trust his judgement?

BTW there have been plenty of "good compression" engines that have crapped out on their owners. Aircraft ownership is a bit of a gamble.

Asking questions and learning before buying is your only hedge.

Good luck in the hunt.

[bagarre]

What does your IA mean by his cutoff is 60psi?

Is tat the time that he tries staking or lapping the valves, honing the cylinders or tries new rings? Or does he consider them junk and wants to buy new? The answer to that will tell you something about your IA.

There have been tons of cylinders replaced long before their time because of low compression.

When I had a cylinder come in at 50psi, my IA told me to go fly it for a few hours at near full power and lean it as much as you can. I did, we retested it hot and I showed 72psi.
This last time, we had to pull one off, clean it up and lap the valves. That took us from 58 to 68.

Mine has 700 hours since 1993 overhaul.

These engines don't like to be 'lugged' around the sky at low rpm. The rings don't seal, the valves load up and your compression drops. Most times, it just needs to be run hard for a few hours to burn stuff off.

If you have a mechanic that knows how to repair old airplanes, I wouldn't fear an old engine (but it is still a little bit of a gamble). If you have a mechanic that knows how to replace parts, it's going to be expensive either way.

Good luck!

[marathonrunner]

That SB addresses a lot of what you guys are saying. I would definitely check for steel doing what I already said about pulling rocker covers, draining oil through a cloth and solventing the screen too. Wouldn't hurt to stick a strong magnet in the hole the screen comes out of and see it anything sticks.

[GAHorn]

I have run two previous engines which had amazing amounts of rust in the upper-ends (above the ring area) for hundreds of hours and even the TCM Field Rep. advised me to continue operations. Those engines never gave any problems whatsoever.

My present engine has 925 hours on it and at last annual three of the cylinders tested in the low-40's for compression. The airplane had not flown in three months.
I flew the airplane for two, 1.5-hour flights and those cylinders are all now in the low 70's.

I do not think you should be too nervous about the airplane you are considering based upon cyls in the low 60's. Simply be advised and keep an "eye" on them in the future.

Go enjoy your 170.

[juredd1]

I apologize for the delay in responding. Long day of work the two hour trip to nearest big town. Just got settled in our hotel, our 9 month old boy has to get some tubes put in tomorrow.

Bruce,

I am not aware of the time on this engine in the past two years but will get that info but believe it has been flown regularly over the past 15 months. As for my IA's thoughts. He said that he would not be ready to perform a top even if it was 60/80, he would want me to run the plane for several hours in a short time and recheck. I recently sold my C152 and had compression issues for a couple years. I was interested in selling and only then did he say we gotta do a top otherwise he was willing to let me run it a while longer to see if it would get better. So he's not one to jump the gun on this type of stuff. I have used him for a about 6 years and have been pretty happy. My uncle flies an Aronica Champ and has been flying for 45 years and is pretty pleased with him.

bagarre,
What my IA said is that his cutoff would be 60psi in the instance of if I was trying to get rid of it. Not sure but he did say that even then he would want several hours on it in a short window to be that wouldn't improve it.


Thanks to everyone that responded.

[Bruce Fenstermacher]

Justin,

Sounds like you have a decent IA. 60/80 is a industry general standard that folks use to judge airworthiness but it is not a fast hard rule. This is why your IA would want to improve that number if you were selling, because, it raises less eyes from buyers and cast less doubt on your IA.

I personally use an arbitrary number 1200 hours for cylinder life rather than TBO for just the cylinders. This does not mean that the cylinders have no life after 1200 hours and it certainly doesn't mean they won't give trouble before, just that I would expect more trouble after than before that number. So 900 smoh assuming new cylinders at moh would be OK and I would expect some exhaust leakage that could easily be improved and that could explain the lower leak down checks.

As for the recent time on the engine it is a better indicator of condition as far as I'm concerned without borescoping cylinders or removing one to view the inside of the engine. If an engine had very little use for many years, but then in the last two years was run 50 hours a year or about 100 hours, and it had internal issues, they'd be likely showing in low oil pressure from rust destroyed bearings and metal particles from worn cam and cam followers. If filters are clean with that type of recent history, after a period of low usage the engine is more likely to be fine than not.

And don't apologize for any delay, life gets in the way of airplanes sometimes and life comes first always. Hope all works out for your boy.

[marathonrunner]

If there is a problem with a cylinder, you are going to notice it in your aircrafts performance far sooner than a compression check will reveal. I still say go with the TCM S.B. as a guideline

[Sixracer]

When doing the test I always open the carb and keep an ear near the ex. By taking the valve covers off you can also hear airflow in the crank case.
You can hear and detect:
1 ex valve leakage
2 intake valve leakage
3 ring/cylinder leakage
It is my opinion there is a lot more to the test than just the numbers on the paper. A savy test operator is part of the equation.
If it is valves then a slight touchup on the valve angles could fix the problem. When going that far I might consider a hone job on the cyls and put in a fresh set of rings and make sure all/everything meets specs.
I have an 0-300 D that had a 66 cylinder when I bought it. After flying about 25 hours and doing the next oil change it moved up to 74. I suspect it had a lifter problem that was holding the valve open a little during the test. I had added a 1 pint of Marvel Mystery oil when I got it home to my hanger. I suspect that loosened up the lifter plunger.

[Bruce Fenstermacher]

I suspect it had a lifter problem that was holding the valve open a little during the test. I had added a 1 pint of Marvel Mystery oil when I got it home to my hanger. I suspect that loosened up the lifter plunger.

Oh, NOW you did it. Brought out the MMO

---

The Continental SB is telling and of course a part of the equation. It lays rest, from an official point, the 60psi limit so many use. And is clear guidance, well that the path is not so clear.

Sixracer is right in that the most important factor is the savy test operator who understands the Continental SB and identifies very easily, by simply listening to the engine, what the issue is, what can be done and what should be done, which are often two different courses of action. All to many times in my opinion, cylinders have come appart when simply running the engine would have been a corrective action.

[blueldr]

When I first bought my first C-170B, the compression check numbers were pretty bad and I got the price lowered to make up for it. The damn thing would, however, make the static RPM on the runup check so I figured it was putting out enough power to fly and just kept on flying it. The oil would turn black almost immediately after an oil change. By the next annual it was almost up to miniums and stayed that way for about four years. When Superior came out with new cylinders for half the price of Continentals, I overhauled it.

[juredd1]

Thank you for the additional info and experiences. I would like to have my IA do the inspection but I am not sure I can afford to pay him for the 3 days it would take him away from his shop as the plane I am looking at is 12 hours away. I think I am just going to have to attempt to find one locally and hope they don't know the owner and are good. Anyone know if there is a tool to locate IA's in a certain area?

By the way the boy is doing good. Hopefully this will put an end to off and on ear infections for the last 9 weeks.