The International Cessna^® 170 Association

Looking for a general rule of thumb answer, and may there isn't one.

I notice on some comments such as "1st run engine" orsomething of that nature, does that mean this engine has never been O/H, noteven the top?

How many hours is a cylinder good for, 1 redo,2 redo's in general. At some point you got to toss them where does that come as far as hours.

Same thing as far as lapping the valves.

I understand these are very general and a lot goes into that determination, but is it as simple as, well it meets factory tolerences it's ok? I also understand that 500 hours on a engine for some people may take 10 years and for others it may be 1-2 years.

Would it be better if your engine has had a top done on it 1x to put new cylinders on or rework them again?

There may be no easy answere to any of the above, just looking for some of your thoughts.

Thanks
W.

W.J.Langholz wrote:Looking for a general rule of thumb answer, and may there isn't one.

I notice on some comments such as "1st run engine" orsomething of that nature, does that mean this engine has never been O/H, noteven the top?

How many hours is a cylinder good for, 1 redo,2 redo's in general. At some point you got to toss them where does that come as far as hours.

Same thing as far as lapping the valves.

I understand these are very general and a lot goes into that determination, but is it as simple as, well it meets factory tolerences it's ok? I also understand that 500 hours on a engine for some people may take 10 years and for others it may be 1-2 years.

Would it be better if your engine has had a top done on it 1x to put new cylinders on or rework them again?

There may be no easy answere to any of the above, just looking for some of your thoughts.

Thanks
W.

WOW Dubbia.....Looks like an answer from George......Mine would be......Depends....not the diaper

Brad, I know what George is goin say.....holy crap...where do I start!!!!!!

Looking back on my engine log, it had a major done in 1968. In April of 2008 it had 1600 and odd change. 40 years aprox 40hrs/yr. With 2 of us flying in the family we will be closer to 400/yr than 40. There are always trade offs but labor is labor putting on new or putting on rebuilt. Yes they have to be yellow tagged etc etc but how good is the reconditioned ones really going to be. I can buy into the 1x but beyond that is it worth it?

W.

Well I'll take a stab at a few of the questions.

A first run engine is just that, never overhauled. Doesn't really cover the cylinders but could. It is generally a vague description of the lower end. Generally speaking a first run engine with reasonable care will only have so much wear at TBO. More wear means more might have to be done to the case halves, crank, cam and lifters and other internals to bring them into tolerance if they are even out of tolerance. So an engine that has been overhauled or run well past TBO may have more wear and or more already done to it to bring it to tolerance. There is a limit to how much can be done to recondition these parts and so newer or 1st run engines might be worth more.

There is a lot that goes into how long a cylinder will last. What cylinder and how they are run can make a big difference. It is my opinion that cylinders generally will not make TBO. They seem to get between 800 and 1200 hours before there is some kind of failure. Of course some only make 200 hours and others go 2000. Depending on what fails cylinders can be reconditioned in the field by re-lapping valves and a few other things as long as they don't crack and the cylinder as well as other tolerances remain within serviceable specifications.

One of the failures of cylinders is cracking. How fast they crack can depend widely on design and how they are used. But generally speaking the more the cylinder expands and contracts as it is heated and cooled the closer it comes to cracking. So new cylinders generally last longer than overhauled cylinders and 1st run overhauled cylinders last longer that say 3rd run cylinders when they are overhauled.

Brand new cylinders today don't cost much more than overhauled cylinders for our engines and so generally speaking for most people it is a better deal to by new rather than overhaul.

I have heard a lot of different opinions from people who I respect on this question. I have a friend who owns a beautiful 1960's vintage Cessna 172 with 2400 hours since new and it has never had a cylinder removed. It runs great. He changes the oil and has rebuilt the mags and the carb several times. Engine baffleing is in very good shape btw. He has owned it forever and he is IA and "knows" his airplane. You say it has 1600 hours since major in 1968. If it is running well and not using alot of oil, I would change oil every 25 hours and do oil analysis. I would make sure mag and carb mx has been accomplished and I would also check your oil pressure guage with a calibrated one to make sure the reading is good. I learned about that the hard way. I overhauled my 0-300 about 4 years ago at Lycon in Visalia, Ca. I used new continental jugs and I have been happy with results. If you are going to be flying 400/year you might want to plan to o/h it sooner rather than later. I would only do it if you plan to keep the airplane because, like george says, you won't get the o/h money back when you sell.

W.J.Langholz wrote:Brad, I know what George is goin say.....holy crap...where do I start!!!!!!

Looking back on my engine log, it had a major done in 1968. In April of 2008 it had 1600 and odd change. 40 years aprox 40hrs/yr. With 2 of us flying in the family we will be closer to 400/yr than 40. There are always trade offs but labor is labor putting on new or putting on rebuilt. Yes they have to be yellow tagged etc etc but how good is the reconditioned ones really going to be. I can buy into the 1x but beyond that is it worth it?

W.

Dubbia,
I know what your saying, this late summer Dad and I had a real problem with his (35 year) overhaul.....(see the ugly crank thread)......every thing has been within standards....yet the #6rod bearing decided to go....no rhyme or reason...it just happens...yet when I tore the engine down the #4 piston had been bad for many years..(in my opinion) but still made compression.......go figure....The more you use an aircraft the better.....sitting just makes problems arise and makes it harder to find, and think about what can, or will happen......I'm pretty annal.....so you will have to take my post with a grain of salt ........Brad

Bruce
I would have to agree with you in general. Reading most ads for aircraft, looks like most go though a top o/h around the 8-900 hr mark. With out calling on everyone, you don't know for sure "what" they did for sure on the top but they cracked it open for a reason. Looks like most cylinders need something in that time frame. Doing some price checks a new cylinder kit goes for somewhere in the $700 range rebult ones from $3-400.00 range with the question of what are you really getting. So my thought was on a new one chances are, at 400 hours a year I may get 3 years again b/4 I need to do something. I have been pondering something a little bigger, 180,185 or 205/206 but my 172 frame and avionics are in great shape so it will be hard to give it up.

Brad
would you have fond the bad cylinder with a borescope? I did read the bad crank thread...wow! Gets me wondering?????

John
Never thought about double checking the oil gauge. I was up this morning for awhile. The oil preesure was reading 40psi. The B/P was 2989 and OAT 4 above and the o-300 was just a cookin. AT 3500', 2475rpm we were clicking away at 120-122mph, just me in the plane and 8-900 ft/min climb was not a problem.

This engine has me bewildered. It's been so cold it's hard to work out side to try and figure why it likes oil. If it gets up to 25-30 degrees sometime soon I'll do another compression check, last time it wasn't bad (within acceptable range) but wasn't ringing the bell either. I would like to wash the engine off real good to see if there are leaks anywhere but that hard too in the cold.I do get 2250on satic pressure run up and with a standard prop thats about what I should see. The engine pops off right away even when it sets for a week or so.
I'm going to ask some of my friends to see if anyone has a borescope to use and take a peek inside at the cylinder walls, maybe something will show up there. Could have some stuck or coked up rings and just hasn't showed up until now that the plane is being flown alot, that's why I was asking about the "Lucas" gas treatment or the .......MMO....ooopppps I said it again something to loosen up the rings ...maybe. I did the "Mag off" hand crank through and listened at the oil filler tub and intake and exhaust but didn't hear anything unusual(but that might be one of those experience things, more like lack of)
So here I am back at looking at just replacing the cylinders and being done with it, or maybe I'm just too picky, it is a 51 year old plane.....just fly it right

W.

W.

If I'm understanding bits and pieces of a few of your posts your engine was overhauled in 1968 and had an average of 40 hours per year till you bought it and it is at 1600 SOH right now. And right know the engine is running fine but you think it is using a bit more oil than it should but you didn't say how much.

You are wondering what to expect of this engine and what route to take once you decide to overhaul it. And you plan to fly your engine 400 hours a year.

Well the first thing that comes to mind is that pretty soon your cylinders are going to start nickle and diming you with little problems. A low cylinder here then one there. Nothing wrong with just repairing them as needed but at some point you going to get tired of the bird in the shop rather than the air.

You are probably getting blow by and the valves are lose in the cylinders allowing for the higher oil consumption. I'd expect this at this amount of hours.

I personally would keep flying this engine until such time as the repairs get to be a pain or you don't feel comfortable. If you are truly flying it 400 hours it won't take long for one or the other to happen. If you are flying that much I'd overhaul the entire engine and use new cylinders because in no time flat you will be looking at more work otherwise.

If you were only flying 50 hours a year I might consider a top overhaul and stretch the overhaul another 5 or 6 years. I would approach this by having the cylinders removed and the inside case, cam, rods and followers inspected as best they can be. BTW this inspection can't be more than a visual inspection and tugging and pulling on the rods looking for obvious bearing wear. A rod or two may also be pulled to view the bearing wear and view those rod journals. If all looked acceptable then proceed with new cylinders. If not then perform the major overhaul.

A lot of this is up to your style of flying and your comfort level. If you are always on the go and don't want the plane in the shop then generally new parts and a fresher overhaul is a better deal. If you fly around the pattern and not much farther and have time to kill with the plane in the shop then overhauled cylinders and and older overhaul maybe perfectly fine as well.

If you are in a position financially to do your overhaul, you might want to go ahead now while you have a wider selection of new cylinders to decide upon. (Who knows how much longer new cyls from Superior will be available. Of course, there's also the problem that if you actually select Superior cyls....how long will any warranty be supported?)

Otherwise, if you are inclined to operate your engine beyond TBO, then some rebuilt cyls are a good option. The only problem I have with rebuilt cyls that come from the "pile" at the rebuilder is... you have no idea how many times they've been rebuilt, so you are at the mercy of fortune as to how long they will last before suffering a crack. (The more "cycles" they have, the more likely to suffer a cracked head. Rebuilds may also have been repaired by welding, and while that may get some more hours out of a cylinder, it is also notoriously poor as a repair technique as far as longevity goes.)

In other words, if you intend to keep this engine in service for more than a few years, new cyls are the way to go, in my opinion. I hate changing the same cylinder every few years.

As for inspecting the bearings by disassembling a rod and actually looking at a bearing.... I don't like that at all. I think that disturbing a bearing that is broken in and showing no signs of trouble....is asking for trouble, and it does no good for a conn-rod bolt to reuse it after it's been torqued, run a thousand hours, then removed to inspect a bearing in mid-life. Besides, what can you expect to find by looking at a bearing? If it's healthy, now you've gone and disturbed it, and you've lost it's crush. Installing a new bearing will now upset the relationship to the other bearings, and will introduce a new risk of breaking in that new bearing. If the bearing was already in trouble, you'd have already seen that evidence by finding copper in the filter/screen or a scorched area of the conn-rod, or you'd see bearing mat'l already extruding at the edges.
Therefore, when viewing the lower end of a mid-life engine, look for scorched big-ends of conn-rods, but don't disturb them.
The other problem with removing cylinders at mid-life is that the main bearings will temporarily lose their "crush". You do NOT want to rotate that engine while main bearings are not held by their crush, or you risk re-locating the bearing and subsequent failure. ("Crush" is a term used to refer to the compression-by-torque of main-bolts. In these engines the thru-bolts which hold the case-halves together, also help hold cylinders to the case. So removing a cylinder, also removes the torque which originally assembled the engine. IF that torque is changed while re-installing the cylinder, then one of two possibilities exist: 1- More torque will result in more crush and the bearing is upset in it's relationship to the crankshaft, and it may be damaged and/or gall. 2- Less torque may result in a bearing that becomes loose in it's bed, and if it subsequently rotates within the bearing cap/bed, it will burn and damage the crankshaft, and potentially lead to engine failure. The same relationship occurs with a conn-rod bearing. I hate disturbing a healthy bearing just for a "look-see".
The thru-bolts usually are held from rotating while torqueing from the opposite side. If they rotate they may damage the oil sealing O-rings at the case halves. Now you have a leaker. (It's my preference to attempt to install replacement cylinders using the upper range of specified torque to avoid loose main bearings, while holding the opposite thru-bolt nut to avoid rotating the bolt. It's also my preference, when selecting rebuilt cylinders, to specify that my own cyls be rebuilt, rather than to exchange them. I would likely do this when the engine is high-time and it's overhaul is upcoming within the next couple of years, .. or if I were planning on selling the airplane.)

Keep in mind that oil is cheap. If you are merely seeing oil consumption without any other sign of trouble, you can do a lot of flying before the cost of a cylinder is consumed.

Bruce and George

Just got home from a very long work day but had a chance to read both of your post. Very good advise from both of you so thank you very much. George your's is a little headie so I'll have to re-read it again when I'm not brain dead. I followed most of it but I sure it will make more sense in the morning. Wish I had more free time, I would love to go back to school and become and A&P .......maybe when I retire eh.

Brad
just out of curiosity, had you change the cylinder where the bad spot was on the crank? Just wondering from George post if that spun the bearing.

Thanks again everyone.....buy oil...clean belly...buy more oil....wax on wax off

W.

W.J.Langholz wrote:
Brad
just out of curiosity, had you change the cylinder where the bad spot was on the crank? Just wondering from George post if that spun the bearing.

Thanks again everyone.....buy oil...clean belly...buy more oil....wax on wax off

W.

Dubbia,
Nothing had ever been changed.... and the Cylinder looks "OK" (Harrison Real chrome in "72") This whole thing has been kinda a mystery to me..... Brad

George
Would it be correct that there are 4 stud bolts and 2 through bolts per cylinder?
W.

Is this a trick-question?

In these engines the thru-bolts which hold the case-halves together, also help hold cylinders to the case. So removing a cylinder, also removes the torque which originally assembled the engine. IF that torque is changed while re-installing the cylinder,

George

Are you making fun of me.....

I have not had one of these engines apart so I don't know. Are all the bolts that hold a cylinder on, though bolts? How do you determine the torque at which they are at now prior to taking it apart?
W.

The answer W is that you can't confirm the torque before dissassembly. Just like you can't confirm for sure you have good bearings before you dissassemble a connecting rod for example.

GAHORN wrote:....If it's healthy, now you've gone and disturbed it....

The trick with the through connecting rods, two for each cylinder in our case, is not to turn them in the process. The through bolts have o-rings on them in the center sealing the through bolt holes in the case halves. If the seal is broken you are likely to get oil seeping around the through bolts at the cylinder base. Of course though it is not current procedure many also sealed the through bolt holes with silk just like the case rest of the case halves.

I believe what George was saying is he likes to use the upper limit of the torque range when reassembling cylinders and I believe he wants to do that to insure that the bearings are held at least as tight as they were before. Of course in doing so I will point out that you could be over "crushing" the bearings from where they were before assembly.

W. as you know there is risk to everything including doing nothing. Rest assured there have been hundreds of thousands of cylinders removed and reinstalled mid engine life successfully and I feel comfortable saying quite a few connecting rods as well. Just use some caution and reason. The trick is to inspect just enough, but not to much.

And remember, "Curiosity killed the cat", of course "Diligence is the mother of good fortune".